Solar pv design1012

Residential Solar PV Design 101
What is in this guide?
In this guide you will learn
1. Solar Basics: Power, Energy, Current, Resistance, Circuits,
Irradiance, Irradiation, Azmith, Horizontal Tilt, Declincation, Voc,
Vmp, Imp, Isc, Temperature and Voltage Relationship, Irradiance
and Current Relationship
2. How to Size an Array based customer constraints and site
constraints and how to estimate power production.
3. How to Design and Size Grid-Connected Solar PV Inverter, Size
Strings and Size Conductor
Who is the guide for?
This guide is for electrical contractors, roofing contractors, general
contractors, engineers, managers, salespeople, career changers or
anyone who is looking to enter the solar PV industry and needs a basic
technical understanding of how the technology works.
Who is HeatSpring?
Heatspring Learning Institute provides world class industry certified
training to building professionals interesting in geothermal heat pumps,
solar photovoltaic, solar thermal and energy auditing. We have trained
over 4,500 professionals since 2007.
What is HeatSpring Magazine?
HeatSpring Magazine is a trade magazine that provides tips, information,
and resources to all professionals interested in the marketing, sales,
design and installation of geothermal heat pumps, solar pv, solar thermal
and energy efficiency.

HeatSpring Magazine: http://blog.heatspring.com
HeatSpring Solar Training:
http://www.heatspring.com/categories/solar-photovoltaic-pv-training-courses
2

About the Author
Chris Williams is the Chief Marketing Officer at HeatSpring
Magazine.
He writes at Cleantechies, Alternative Energy Stocks and
Renewable Energy World. He's a clean energy jack-of-all
trades. He has installed over 300kW of solar PV systems,
tens of residential and commercial solar hot water systems
and 50 tons of geothermal equipment. Chris is an IGSPHA
Certified Geothermal Installer and will be sitting for his
NABCEP in September.
If you have any questions….
If you read this guide and have any questions or want to get more article,
whitepapers or information there are a few ways to keep in touch.
Subscribe to HeatSpring Magazine with your RSS subscribers.
Ask HeatSpring a question on our facebook page.
Ask me, Chris Williams, @topherwilliams, a question on twitter.
Subscribe to HeatSpring TV, our video podcast to
get updates on interviews with industry experts on best practices.
Join our linkedin group to connect with HeatSpring alumni and
other professionals.
Email. You can email directly: cwilliams@heatspring.com
Phone. If you have an in-depth question call me at 917 767 820

3

Table of Contents
1.
SOLAR
PV
THE
BASICS..................................................................................................................... 4

CALCULATING
AND
CORRECTING
SOLAR
RESOURCE..........................................................................................8

PART
2
HOW
TO
DESIGN
A
SOLAR
PV
ARRAY
SIZE
AND
ESTIMATE
POWER

PRODUCTION ........................................................................................................................................16

1.
CUSTOMER
CONSTRAINTS................................................................................................................................ 17

2.
SITE
CONSTRAINTS............................................................................................................................................ 20

ROOF
CHARACTERISTICS
TO
CONSIDER
AND
GATHER .................................................................................... 20

COMMERCIAL
CONSIDERATIONS ......................................................................................................................... 23

REMEMBER
TO
COLLECT
FROM
A
SITE
VISIT:.................................................................................................... 26

EXAMPLE:
HOUSTON,
TEXAS
HOUSE ................................................................................................................. 26

3.
DETERMINING
LOCATION
IRRADIANCE......................................................................................................... 36

1.
CITY
IRRADIATION............................................................................................................................................. 36

2.
ADJUST
CITY
IRRADIATION
FOR
ROOF
IRRADIATION
AND
ESTIMATING
POWER
PRODUCTION. ....... 37

CONCLUSIONS
ON
POWER
PRODUCTION............................................................................................................ 40

PART
3
–
HOW
TO
DESIGN
A
SOLAR
PV
INVERTER,
SIZE
STRINGS,
AND
SIZE

CONDUCTORS........................................................................................................................................42

1.
INVERT
SIZING
AND
SELECTING...................................................................................................................... 42

2
–
L OW
MANY
MODULES
CAN
WE
FIT
ON
THE
ROOF? ................................................................................... 45

3.
HOW
DO
WE
SIZE
THE
STRINGS?..................................................................................................................... 46

WHAT
IS
THE
MAXIMUM
VOLTAGE
ALLOWED
FOR
THE
SYSTEM?
HOW
MANY
MODULES
WE
CAN

CONNECT
IN
SERIES?.............................................................................................................................................. 47

HOW
DO
WE
CALCULATE
THE
MINIMUM
NUMBER
OF
MODULES
IN
A
STRING?........................................... 49

4.
HOW
DO
WE
SIZE
CONDUCTORS?.................................................................................................................... 52

1.
STANDARD
AMPACITY
TABLES
BASED
ON
CONTINUOUS
OR
MAXIMUM
CURRENT. ............................... 54

2.
DERATING
WIRE
FOR
CONDITIONS
OF
USE.................................................................................................. 55

1.
STANDARD
CONDUCTOR
AMPACITY .............................................................................................................. 55

2.
ADJUSTED
FOR
CONDUIT
FILL......................................................................................................................... 56

3.
ADJUSTING
FOR
TEMPERATURE
RATING ...................................................................................................... 57

ABOUT
THE
AUTHOR .........................................................................................................................61

4

1.
Solar
PV
The
Basics

Weʼve created tutorials on solar thermal design 101 and geothermal design, but
we havenʼt paid the same attention to solar PV yet.
This is the first in a series of posts weʼll publish on the basics of designing and
installing residential solar PV systems. The goal of the series will be to get the
basics covered. If youʼre an experienced installer, none of this information will be
new to you. If youʼre brand new to solar, it will be helpful. But keep in mind, weʼll
be skimming the surface. Please leave any and all questions in the comments
section and Iʼll address them.
Weʼre going to begin with the basic terms. This is very important for design
because you need to understand the concepts before you start applying real
numbers to a design. It will also help with sales because it will help you explain
some basic terms to curious customers.
Power
Power is an AMOUNT of energy. Itʼs the measurement of energy, measured in
kilowatts (kW). Power is measured in an instant. Most of the sizing done in solar
PV design; conductors, inverters, fuses, the size of the solar rates is based on
how much power will be passing through a specific component of the system.
Because power is measured in an instant, it can vary widely over time and from
minute to minute.
Power (watts) = current (Amps) X voltage (volts)
Energy
Energy is the is the actual work done by power. It is measure in kilowatt-hours
(kWh). Consumers pay for kWh. Itʼs a measure of power over time.

5

Power (kW) X Time (hours) = Energy (kWh)
Current
Electricity is the flow of negatively charged electrons. The current is the amount
of negatively charged electrons in a specific part of a circuit.
Many people find it useful to use a water analogy when discussing electrical
terms. In the water example, itʼs useful to think of a dam with a pipe at the bottom
that water can flow out of. The amount of water that can pass through a slice of
the pipe, in other words the area of the cross section of the pipe, is analogous to
electric current.
Voltage
Voltage is a measure of the ʻforceʼ or ʻpressureʼ of the electric current in a circuit.
Itʼs measured in volts. Electrons of the same material WANT to
be homogeneous, i.e. they want to be evenly spread out. Thus, if one area has
less electrons then another, the electrons will move in an attempt to equalize.
This flow is what created a voltage potential and causes electrons to move.
To use the water example with a dam. If the size of the pipe at the bottom of a
dam is a measure of current, the height of the dam is a measure of voltage. The
higher the water is one on side of the dam versus the other, the more pressure
there is.
Resistance

6

Electrical Resistance is the resistance of the flow of electricity through a
conductor. It does not reduce the current flow of electrons (how many electrons
there are in the circuit) but it does reduce the voltage (how fast theyʼre going,
remember the dam example). It is measured in ohms.
Voltage Drop (volts) = Current (amps) X Resistance (ohms)
Series Circuit
A series circuit is when one negative and positive of each power source or
appliance, are connected together.
Remember, CURRENT is constant and Voltage ADDS in series circuits.
Parallel Circuit
In a parallel circuit, all of the positives are connected together and the negative
are connected together, each separate.
In parallel circuits, CURRENT ADDS and voltage stays constant.

7

AC Current
AC refers to alternating current. It refers to electrical systems where the voltage
and current are constantly changing between positive and negative. A complete
“cycle” is completed when when the current reaches returns to either the peak, or
trough of the wave. Frequency is measured in Hertz (Hz) and is measured in
number of cycles per second. The power in the US is operated at 60 Hz.
DC Current

8

DC means direct current. DC is the type of electricity where the voltage and
current stay constant over time. Typical DC applications are batteries, solar
modules, and wind turbines.
Calculating and Correcting Solar Resource
Irradiance
Irradiance is the amount of solar radiation falling on a particular area at any given
time. It is a RATE. Itʼs a measure of POWER, in that itʼs an instantaneous term
that does not consider time. Remember the difference between power and
energy.
It is measured in watts per square meter.
Irradiation
Irradiation is a measure of solar energy, the amount of irradiance that falls on a
location over time.
Irradiation is measured in kWh / square meter / day.

9

Irradiation was formerly called insolation.
Solar Energy in the US
The below pictures shows that amount of solar irradiation that falls on the various
surfaces across the US depending on average local weather circumstances.
Horizontal Tilt

10

The tilt angle from the sun is the angle from the horizon to the sun. Solar PV
modules will produce the most energy when the sun is shining directly onto them,
from a 90 degree angle. Thus, all else equal, for fixed PV modules the best tilt
angle will be the same as the latitude of the site. For example, if the PV site is at
44 N, the best tilt will be 44 degrees. However, most roofs and and commercial
racking are not at 44 degrees, so you must apply correction factors for projects
that are not at perfect tilts. We will discuss this in a later article.
Azimuth
The azimuth is the number of degrees from true south that the sun, or another
object, is facing. Itʼs used when designing a solar PV system because due south
will provide the best production, all else equal, over the course of a year. Weʼre
not going to get into tracking systems in this series so all of our arrays will be
fixed. However, if the object is not directly south, you will need to apply correction
factors that we will get to in later articles.
Magnetic Declination
Keep in mind that if youʼre doing site visits with a magnetic compass you will
need to correct your magnetic readings to find truth south. The process is simple.
Determine your declination by look at diagram like the one below and
determining your location.

11

If youʼre location has a eastern declination, youʼll need to add the numbers to
reading. If from the west, subtract.
EAST Subtract. If youʼre compass reading was 190 degrees and you lived in San
Francisco, about 17 degrees east, you would need to subtract 17 degrees to find
true south. Youʼre TRUE SOUTH reading is 173 degrees.
WEST Ad. If you live in Belfast, Maine (about 19 degrees west) and your
compass reading was 165 degrees, you would need to subtract 19 degrees to
get TRUE SOUTH of 184 degrees.
Solar Module Terms: The below terms are terms you will need to understand
when sizing your system.

12

Voc: Volts open circuit is the maximum voltage a solar module can ever make
when it has no load on it. Voc is used when sizing solar arrays along with
temperature coefficients to determine worst case voltage scenarios.
Vmp: Volts maximum power is the reading of the maximum volts a module can
produce when under load under standing testing condition, STC, irradiance levels
(1000 W / M2) . If you look at the below curve, the Vmp would be somewhere in
curve on the right in the bend. It will be on the place in the curve the creates the
most power (volts times amps). The number is actually rather to difficult to
calculate exactly and can change rapidly from second to second as the current
changes.
Isc: Amps short circuits it the maximum amount of amps that a solar module
could produce. You will find Isc on the x axis of the above graph where there is
no voltage and thus no power being produced.

13

Imp: Amps max power, like volts max power, is the current point on the power
curve when the module is producing maximum power.
Youʼll find the above material on the back of every individual solar PV module
and it is standard information that manufacturers and distributors will tell about
their product. Below is a product description for two Sharp modules from AEE
Solar. All the data is public and available on AEEʼs website.
Temperature and Voltage: Itʼs important to understand the relationship between
temperature and voltage in solar modules for design purposes. While
temperature does have a slight impact on current, itʼs considered to be negligible.
However, temperature has a large impact on voltage. When you are determining
the maximum number of solar modules in a string, based on the inverters
acceptable voltage window, you will need to take into account expected lowest
temperature ranges that can increase voltage.

14

Irradiance and Current:
Irradiance and current also have a direct relationship. The amount of irradiance
falling on a solar PV module will directly impact the current that module is
producing. This is key for understand when performing designs, and
troubleshooting systems.

15

This is it for basics. The next post will be on sizing the array based on the
customers needs.
Please let me know if you have any questions or if I was unclear about any of
these terms.

16

Part
2
How
to
Design
a
Solar
PV
Array
Size
and
Estimate
Power

Production

This is the 2nd article in a series about how to design solar PV projects. We
started with solar 101, the basics. If youʼre brand new or need to brush up on the
basics, please read it first. It discusses electrical theory, key solar terms needed
to design any system and the relationship between irradiance, temperature,
amperage and voltage among other things.
This section is dedicated to sizing an array based on customer needs and site
characteristics – it also discusses estimating power production. The main focus is
residential applications, but Iʼll also highlight slight differences in commercial
projects.
The goal of the article is to provide a basic process for you to understand how to
size an array and provide you with further resources youʼll need to continue your
learning. There will be some overlap in this discussion with more advanced
topics, like string and conductor sizing that will be covered in future articles, and
how the design will impact the financial returns of a system, which will be
discussed in a future article on Solar PV financing. If you need to read on up
renewable energy finance, you can start with Finance 101 for Renewable Energy
Professionals.
First, let me outline what weʼll talk about, then I will go into each part with more
detail and depth.
Below is the process for designing a solar PV array.
In the field, most of the power production estimating is done with software.
However, Iʼd argue that itʼs still important to understand the theory behind power
production estimates and the variables that impact power production so you can
make sure to gather the correct information when performing a site visit.

17

1. Customer Constraints. What about a specific customer will impact the size of
an array? The most common restraints are:
 Energy Usage
 Client Budget
2. Site Constraints. What about the client site will limit array size? These are
the most common details about a site you need to gather and weʼll discuss how
these variables impact the size of an array:
 Local Shading
 Horizontal Shading
 Available Roof Space and Roof Characteristics (dimensions, tilt, azimuth)
 Module Size and Racking Considerations
3. Determining Irradiation. In order to compute power production, you need to
understand how much energy is hitting your specific area.
 Measured in kWh/M2/day or Sun hours per day
4. Estimating power production based on irradiation, customer constraints,
and site characteristics.
 Sun hours per day adjust for site characteristics
 Power production estimates based on solar resource and the amount of modules
you can fit on the roof.
1. Customer Constraints.

A. Energy Usage
A possible constraint on the size of a solar project is the clientʼs energy usage.
Because of how net-metering programs are set up, typically it does not make
sense to produce more then 100% of a clientʼs annual energy usage. However,
because most property owners use so much power, and the power density of
solar PV is so low, itʼs rare to have an array that can produce 100% of the power
with solar power. Itʼs typical that the solar fraction of a project (total power used /
power supplied by solar) is less then 30%.

18

Commercial Considerations
For a commercial client you will need to understand their demand charges and
usage charges. In order to understand if the solar array will reduce their demand
charges you need to understand the load profile of the building and when exactly
their demand is the highest to see if solar will shave that demand. For example,
do they have the highest amount of demand in the summer or winter? What time
of day, early morning, afternoon, evening?
We will not go into depth on demand charges for this post. However, WE WILL
discuss the impact of different electric rates, demand and usage charges in the
solar PV financing article because itʼs critical to understand the value of the
power that a solar project produces. Right now, weʼre just concerned with pure
design.
If you need to learn more about what demand charges are, Iʼve found these are
good resources:
 Understanding demand charges
 Demand Charges Explained
What you need to collect about energy usage:
 Yearly average kWh used by the client
 Cost of power
 The value of a kWh of solar is directly related to the cost of the power it offsets.
On a site visit make sure to get a few months of electric bills.
Example
Letʼs assume a customer uses lives in Houston, TX and uses 550 kWh of AC
power on average per month and wants a solar system that will produce 100% of
the power they use in a year. How large would you need to design the system?
You need to reverse engineer the problem, hereʼs how:

19

1. 550 kWh/month / 30 days per month = 18.33 kWh per day
2. Calculate and Adjust Irradiation based on site characteristics. According to PV
Watts, Houston gets an average of 4.79 sun hours per day. For now, letʼs
assume the roof is directly south and at 30 degrees (the latitude of Houston) so it
can harvest 100% of the 4.79 sun hours per day. See section 4 for how we
adjust irradiation based on a roofs characteristics
3. 18.33 kWh per day / 4.79 adjusted sun hours per day in Houston = 3.83 kW AC
needed in production. Now we need to convert to DC
4. 3.83 kW AC / 80% (to make up for the inefficiency of converting to DC to AC.
80% is a rule of thumb. You will read more about this in the next part of this
series when we talk about string and conductor selection, inverter selection and
derating) = 4.78kW DC
If the customer wanted to produce 100% of their power from solar energy in
Houston and they had a perfect roof, they would need a 4.78kW DC system.
Weʼll discuss what happens if there roof is not perfect below.
B. Customer Budget
One of the most common client constraints is budget for the system, if they are
purchasing with cash. If they are leasing the system, this will not be so much of
an issue. Learn more about solar leases, prepaid leases and how to sell a solar
lease here.
If your installed cost is $5.00/watt, a 4.78 kW system will cost you $23,900. If the
customers budgets is only $15,000, you could only install a 3 kW DC system.
Things to remember:
 Know if itʼs a cash or lease sale. Learn more about lease sales in our free course
about solar lease.
 If itʼs a cash customer, make sure you understand what their budget is. Make
sure you understand if they are purchasing cash, or with a home equity line of
credit or wrapped into a mortgage for new construction.

20

2. Site Constraints

Site constraints are the second most common attribute that limit the size of a
solar array, behind a customers budget. Answering the question “how many
panels can fit on the roof” is a major limiting factor of a project. However,
remember that itʼs not just how many panels can you physically fit on the roof, but
how many can be on the roof and produce maximum power.
**NOTE: Iʼm not going over structural aspects in this part of the series and that
will be discussed in a future post. Remember, simply becasue there is room on
the roof doesnʼt mean you can install solar. The roof needs to be able to hold the
additional load.
Roof Characteristics to Consider and Gather
 Total Roof Area: When performing a residential or commercial site visit itʼs good
practice to measure the whole plane on the roof where you plan to install the
array, then begin to work backwards and eliminate space that is shaded or
unsuitable for panels.
 Local Shading. Local shading is shading that occurs on the roof. Common
examples include: chimneys, stink pipes, eaves, shading from another part of the
roof.
 A good rule of thumb for local shading is donʼt place modules anywhere that is
closer then 3x the height of the obstacle from the object. If a stink pipe is 12
inches, donʼt place any module north, east or west of it closer then 36 inches
away. You can still place module south of the local shading areas.
 When doing a site visit make sure to mark the locations of all local shading
elements. Also, note if there is an attic or cathedral ceilings. If an attic,
sometimes pipes and other items can be moved easily.
 Horizontal Shading. Horizontal shading is most often caused trees, but can also
be from buildings. It is shading that occurs off the roof that impacts the amount of
irradiance hitting the roof.

21

 Itʼs best to have no shading between the hours of 9am and 3pm for the whole
year. If this is the case, you will not need to adjust your irradiation numbers for
shading.
 If you have any shading between 9am and 3pm during any point in the year you
will need to adjust the irradiation numbers that we will discuss step 4.
 Here are two examples of a nearly perfect roof and a roof with some shading.
The “solar access” percentage is what we care about, and this is the number that
will adjust irradiation values. This percentage is a measure of the amount of sun
light youʼve lost due to shading. If itʼs 95%, youʼve lose 5% production from the
best case scenario due to shading.
 Key to remember: Trees Grow. If youʼre building an area that has some shading,
when you perform your power production estimates it will be good to assume
your shading will increase by a small amount each year, letʼs say .5%.
 Key to remember: Some states have rebate programs that say a roof must solar
access of at least 80%.
A great roof: On average this roof only loses 4% product due to shading

22

An okay roof: This roof will lose 20% product due to shading.

23

Commercial Considerations
Commercial projects seem more open then residential applications because you
can orient the modules how you wish, but there some considerations that are
more critical to watch for on commercial projects:
 Local shading becomes much more important. Make sure to have
a DETAILED roof plan that shows the dimensions of the roof, and everything else
on the roof that will impact where you can place modules; drains, the footprint
AND HEIGHT of the AC units, skylights, height of knee walls, and all other
equipment.

24

 Examples below of skylights, knee-walls, AC units, and existing conduit.

25

26

 Edge of the roof. 6 feet from the edge is common
 Double check with your fire department about array layout. Fire AHJs are
becoming more and more stringent with where modules can be placed because
they will need access to the roof in the case of a fire.
Remember to collect from a site visit:
1. Raw roof dimensions
2. Location and height of all other obstacles
3. Shading analysis with a Sun Eye
4. Tilt of the roof if residential. If commercial, this will be based on the racking you
use
5. Azimuth of the building. This means, where is the building facing. Itʼs best for the
roof to be facing directly south. On residential roofs, you tend to not have a
choice. On commercial, you have more freedom to point the array where you
wish.
Example: Houston, Texas House
The process for determining how many modules can fit onto a residential roof are
the following.

27

1. Measure the raw roof. This is an example house in Houston, TX.

28

2. Locate all other obstacles. The above roof is perfect, but letʼs assume that
there is a chimney on the top left of the roof.

29

3. Perform a shading analysis. Mark any areas that have less then 80% solar
access. The above roof does not have any shading, but if there was a tree on the
left hand side you would need to get on the roof and use a sun eye to determine
how far the shading goes onto the roof. Mark the section of the roof where the
shading stops!
4. Determine the unusable space created by local obstacles and shading on the
roof. Remember to use 3x the height of the obstalces as the closest distance a
module should be to said obstacle.

30

5. Determine how many modules can fit in the adjusted usable space based on
the size of the module and racking.
Youʼll need 3 things
1. The amount of usable space on your roof
2. The dimensions of your module
3. Needed space for racking
A few other key tips to keep in mind.

31

 Itʼs good to make sure the modules do not overhang the ridge. Itʼs good for the
space between the ridge of the roof and the array and the bottom to be equal, if
possible.
 It looks best if you can space the array on both sides equally as well, but
sometimes this is not possible.
 Rectangles, including squares, always look the best.
 Remember Unirac racking will take 1 inches between all modules but not the top,
bottom or either side. Prosolar is also very common. Other brands are coming
along including ZEP Solar and other brand specific raking, like Westinghouse
Solar. Just know your racking dimensions.
Here is the module weʼre going to use:

32

33

6. Result: 20 Modules Will Fit on the Roof.
Height: The height of the array is 119 inches (59 X 2 + 1 inches for the racking)
Width of the top row: 279 inches (39 inches wide X 7 modules + 6 inches for
each space)
Width of the bottom row: 519 inches (39 inches wide X 13 modules + 12 inches
for spacing)

34

This may not be the exact amount of modules for the final design depending on
what our string sizing calculations comes out as OR if we choose to use micro-
inverters or an AC module. But you get the idea of the process.
7. Gathering Roof Characteristics
The two other things you need to collect about the roof that will be needed for
power production estimates are the tilt roof and itʼs azmith. We will discuss power
production estimates next.
The tilt of our sample roof is 30 degrees, or a 7 pitch.
The true azimuth of the building is 132 degrees. The magnetic reading of where
the building was facing was 140 degrees. HOWEVER, we must adjust magnetic
south to true south. Houston has a declination of 8 degrees EAST. EAST
Subtracts, you remember that.
140 degrees magnetic – 8 degrees declination east = 132 degrees.

35

36

3. Determining Location Irradiance
Now that we understand the basic process for determine how many modules can
fit on the roof, collecting data about shading, and where the roof is facing.
Hereʼs the general process.
1. Determine the amount of sun falling in your city
2. Determine how much of that sun is falling on your specific roof
3. Determine how much sun falling on the roof the modules can harvest, based on
how many modules you have and their power rating.
1. City Irradiation.
This is not an official term but itʼs how I think about it. First what weʼre looking for
is how much sun, on average, is falling in the city where my roof is located. What
youʼre looking for is called IRRADIATION, formerly called Insolation with an “o”.
Here are some good resources to look up the irradiation in your city:
• Whole Solar Sun Hours Map
• PV Watts
REMEMBER, an easier way of thinking about the term “kWh / M2 / day” is “Sun
Hours Per Day” Or how many hours of direct sunlight (at STC) are falling. The
reason I like sun hours per day is it makes calculating power make more sense to
me. If I have a 1 kW array that gets 5 sun hours, Iʼve produced 5kW (1kW X 5
hours)
According to PV Watts, Houston gets an average of 4.79 Sun Hours per day.

37

2. Adjust City Irradiation for Roof Irradiation and Estimating Power
Production.

38

In order to calculate the irradiation that falls onto the roof we need to correct the
local information for the conditions of the specific roof. If you remember from
solar design 101, solar modules are most efficient (produce the most power)
when they are perpendicular to the sun. Note, I wonʼt be discussing tracking
arrays in this article. Here are the best conditions for a fixed tilt array.
 Azimuth = Directly South at 180 degrees. Only in the northern hemisphere
 Tilt Angle = Latitude of the Site. Houstonʼs latitude is 28 degrees north, so 28
degrees is the best tilt of the roof.
If the array has a different tilt and azimuth then from the above, we need to adjust
the city irradiation numbers to get an accurate power product estimation for the
specific roof. Here is an example of a table used for locations that are 30 degree
north.
Notice from the above graph that at 180 degrees south and 30 degrees tilt angle,
the correction factor is 1, or 100%. Itʼs useful to analyze this graph to get an
understanding of the implications of different site conditions. This is useful for
marketing purposes to determine good sites from bad sites.

39

 If the building was facing directly WEST, it would only lose 17%, but if it faces
directly EAST, it will lose 22%.
 Also note what happens when the module is at 0 degrees, flat, it only loses 13%.
Mainly due to the fact that Houston is close to the equator so the summers are
long.
Solmetic also has an amazing tool that will tell you the optimal tilt and azimuth for
a building in a specific location. Then you input the specific characteristics of your
roof and it will tell you how much to adjust your irradiation numbers by.
This is data for Houston
Here is a link to the Solmetric tool

40

According to Solmetric, the optimal tilt for Houston is 28 degrees, the azmith is
178 degrees. You can find this at the top of the graph.
If you look at the bottom, you can find our roofʼs characteristics, is says that a
roof with a tilt of 30 degrees tilt at 148 degrees will get access to 97.8% of the
sun.
Example with roof adjusted irradiation
Multiply Houston Irradiation, 4.79, by the roof correction factor 97.8% to equal
4.68 sun hours per day.
We would then use the roof adjusted irradiation numbers in our power production
estimates. For the amount of module that fit can fit on the roof, 20 in hour case.
Note that 20 is not taking into account customer budget.
1. 20 modules X 205 watts per module (find this on the modules specs) = 4100
watts DC rated power
2. 4,100 watts X 4.68 average sun hours per day (roof adjust irradiation) = 19, 188
kWh DC produced per day on average.
3. 19.18 kWh DC X 80% (to convert from DC to AC) = 15,350 watts-hours AC
average daily production
4. 15.35kWh per day X 30 days per month = 460 kWh AC production per month.
Conclusions on Power Production

Thatʼs a step-by-step guide for sizing a solar array and estimating power
production. The process is slightly different and there is more to consider for light
commercial applications. I will dedicate a specific post to commercial array sizing
and power production in the future.
To wrap up what we discussed.

41

1. Client specific constraints: budget and energy usage
2. How a roofʼs constraints impact a solar arrayʼs size: Local and horizontal
shading, roof dimensions
3. How to determine and adjust irradiation numbers based on the roofʼs
characteristics; tilt and azimuth.
4. How to estimate power production based on the irradiation reaching a roof and
the number of modules on it.
In this article, we used a rule of thumb 80% derate factor to convert from DC to
AC. In the next article, we will dive deeper into inverter sizing, string sizing and
conductor sizing, all of which will directly impact this 80% number.
If you have any questions or comments, please leave them in the comment
stream.

42

Part
3
–
How
to
Design
a
Solar
PV
Inverter,
Size
Strings,
and
Size

Conductors

This is the 3 part in a series on residential solar PV design. The goal is to provide
a solid foundation for new system designers and installers.
The goal of the article is to convey the basic process for sizing an inverter,
strings and the conductors. You may not be an expert at the end of the post, but
youʼll have a better understanding of how to do these things.
As always, having specific numbers is the most useful for examples – so weʼll
continue with the example from part 2 on sizing an array and estimate power
production. The house was located in Houston, TX and the roof, given local
shading conditions, has enough room on the roof for 20, 205 watt modules.
(see part 2 to see how we got this number)
Here is the spec sheet on the Sanyo HIT 205 module weʼll use for the example.
So, the largest possible size of the array we can fit on the roof at STC is 4,100
watts. We can go lower then this, but not higher.
1. Invert Sizing and Selecting
Given that we know how many modules can fit on the roof, how do we use this
data to size the inverter? The size of the inverter is driven by answering 2
questions:
1 – What is the capacity of the existing electrical service?
Per NEC 690.64B2 (2008) 705.12 D2 (2011), an existing electrical service is only
allowed to backfed up to 120% of the rated capacity.

43

What does this mean with a typical home?
100 amp service X 20% = 20 amp backfed breaker allowed
20 amp X 80% (for continuous load, weʼll talk about this below) = 16 amp
continuous inverter output current
16 amps X 240 volts (or 208 volts, depending on the homes location) =
3840 watts. This is the maximum allowed AC power output of the inverter.
There are a few ways of getting around this, by upgrading the service, performing
a line-side tap, and it can sometimes be accomplished with subpanels. However,
for this example, letʼs keep it simple.
If the existing service only had room for a 20amp breaker, we would not be able
to have an inverter that has a rated AC continuous output that would exceed the
16 amp (see example above) or 3840 watts AC.
Per NEC 690.8 A3 the maximum AC ouput current from an inverter is defined as
the manufacturers continued rated output current.
Max Current (inverter AC circuits) = continuous current output.
For our example, weʼll assume that the existing electrical service can supply an
additional 25 amp back-fed breaker, 20 amps continuous allowed. This limits our
choice of inverter to either a PVI 3000 or PVI 4000 inverter based on the
electrical service capacity, as the PVI 5000 has a continue output current at 208
VAC of 20.7 amps.

44

Figure 1 – A Sampling of Solectria Residential Inverter Specs

45

2 – How many modules can we fit on the roof?
From our example, we know that we can fit 20, 205 watt Sanyo modules on the
roof.
Here is the spec sheet for the module
Figure 2 – Spec sheet for Sanyo 205 Module

46

First, we need to guess the size of the inverter. Itʼs a good rule of thumb to size
the inverter, based on the rated AC continuous output, to be 80% smaller then
the rated STC output of the array. The reason for this is that there is a lot of
inefficiency from the array to the inverter, so if we undersize the inverter, the
array is more likely to hitting the upper limit of the input ranges of the inverter and
will more likely be operating within the MPPT operating range of the inverter.
For example, for our array size at 4,100 watts DC STC, weʼve guessed that the
inverter would have a AC continuous output range of 80% of 4.1kW, or 3,280
watts AC.
Youʼll notice that the naming of Solectria inverters (PVI 3000, 4000, 5000) also
seem to match this relationship between the DC rated power of an array (the
name of the inverter) and the AC continuous output of the inverter (2700W,
3400W, 4300W, respectively)
We will choose the Solectria PVI 4000 for our example from our choices between
the PVI 3000 and 4000
3. How do we size the strings?
Right now, we have concluded two things. First, the inverter weʼd like to use the
PVI 4000 based on the number of modules that can fit on the roof and how their
capacity relates to the inverter. Second, we know the number of max modules
we can fit on the roof. Now, we must begin string sizing.
String sizing is the number of modules that we will connect in series and parallel
before connecting them to the inverter. The size of our strings will determine the
voltage and amperage that is inputted into the inverter.
When string sizing, our goals are:
1. Make sure we NEVER supply the inverter with too much voltage, which will kill it
–> Maximum string length

47

2. Make sure that we can ALWAYS supply the inverter with enough voltage to turn
on, given the array is receiving full sun –> Minimum string length
What is the maximum voltage allowed for the system? How many modules
we can connect in series?
NEC 690.7 specifies that our worst-case voltage, the highest voltages that the
DC array can create, must fall within the limits of the inverter.
The exact definition states that: The Voc of each module times the number of
modules in a string, correct for lowest expected ambient temperature in the
arrayʼs location.
For the PVI 4000, maximum acceptable voltage is 600 VDC.
To calculate the maximum number of modules allowed, we need a few pieces of
data
 Voc at STC for the module at 77F/25C = 50.3 volts
 The temperature coefficient for the module. Typically given in volts per degree C
or % voltage per degree C. You will find all this data on any module spec sheet =
-.14V/C
 The lowest and highest temperatures seen in the specific jurisdiction. Below is
the data for Houston from weather.com = 9F or -13C

48

49

Here are the calculations for the max system string size. The goal in determining
the maximum system voltage is to make sure that power production from the
array will never kill the inverter.
1. Temperature coefficient. -13C lowest temperature – 25C STC = -38C change
from STC
2. -38C X -.14V/C = 5.32 voltage increase. (negative times a negative is a positive)
3. 50.3 volts + 5.32 = 55.62 is the highest voltage we will ever expect to see from
each module, and this is the voltage we will use to determine the maximum
number of modules in a string.
4. 600VDC (highest acceptable inverter voltage) / 55.62 = 10.78 modules.
5. We round down to 10 modules, because we cannot go over 600 volts.
6. Maximum system voltage (MSV) = 10 modules X 55.62 = 556 volts
How do we calculate the minimum number of modules in a string?
The goal of calculating the minimum number of modules in a string is to make
sure that in the worst case scenario, when the array is extremely hot, the system
will still produce enough voltage to turn the inverter ON. Thus, weʼre looking to
understand the lowest possible voltage the system will create.
Hereʼs what we need:
1. Vmp of the module. Operating voltage of the module under load = 40.7 volts
2. Temperature coefficient correction factor for the module from STC = -.14V/C
3. The highest temperature recorded for the location youʼre installing = 106 degrees
4. The bottom range of acceptable voltage for the MPPT range for the inverter =
200 V DC
5. Ambient air correction factors from the conduit that the electric wire will be in.
This can be looked up at NEC Table 310.15 B2C. Based on how far the conduit
is off the roof, it will give us the temperature that we need to ADD to the highest

50

temperature to derate module performance. Why? wires heat up more when
sitting in conduit rather then outside air.
Here are the calculations.
1. The conduit will be placed 2 inches above the roof. Thus, we must add 40F to the
output temperature of 106 to find the temperature we will derate the modules by
40 + 106 = 146. 146F = 81C
2. 81C – 25C (STC) = 56C above STC. Remember, voltage is indirectly related to
temperature. Higher temperature equals lower voltage. Thus, the hottest
conditions the array will ever see is 56C higher then the STC voltage.
3. 56C X -.14V/C = 7.85 DECREASE in voltage per module
4. 40.7Vmp – 7.85v = 32.84 Vmp (at 149F) What this means is that is the array is
under load (being used) and itʼs the hottest that itʼs ever been in Houston (109F)
and the conduit is 2 inches above the roof, we can expect that each module will
be producing 32.84 volts.
5. 200VDC (the minimum volts needed to turn on a PVI 4000) / 32.84Vmp = 6.09
modules. For this we need to ROUND UP (if we go down the inverter wonʼt turn
on) so our conclusion will be 7 modules.
Conclusion on Voltages
With a Sanyo 205 module, we can have between 7 and 10 modules given the
voltage ranges of a PVI 4000 inverter.
However, now we need to make a table to figure out how many strings to have
and the proper number of strings to produce enough POWER (watts) for the
inverter.

51

We could select either 2 strings of 10 modules or 3 strings of 7 because both will
produce enough DC power to power our inverter.
We will select 2 strings of 10 modules for two reasons.
 Our roof only has enough space for 20 modules so 21 will not fit on it.
 All else equal, itʼs better to have fewer strings and more modules per string
because higher voltages = less voltage drop because less amperage will be
flowing for the same amount of power.
The conclusion from Solectriaʼs string inverter tool match our findings done by
hand

52

String Sizing Tools are Readily Available and Free
All of these calculations are typically done with software or with an inverter
manufacturerʼs string sizing tool. Here are three free options:
 Solectria String Sizer
 Advanced Energy String Sizer
 Fronius
However, itʼs good to understand the theory behind their calculations.
4. How do we size conductors?

53

After weʼve selected the size of the array and the inverter we need to size the
conductors that will be used.
The purpose of conductor sizing is to make sure conductors can hold
the amperage that they need to help. The ampacity rating of a conductor is the
current it can safely conduct without overheating. The reason conductors cannot
overheat is because the insulation on the outside will melt and the faults will be
more likely to occur.
 Current causes heat in conductor due to resistance of the wire
 Bigger wires = lower resistance
 Lower resistance = less heat.
 Too much heat = insulation melting = faults, arcs, death and fire.
 Insulation rating determine ampacity
Most people, including myself, find this extremely confusing at first. So before we
start talking about conductor sizing, letʼs take a look at the problem from 30,000
feet to understand logically what is happening:
1. Understand how much the conductor NEEDS TO CARRY. The first thing we
need to understand is how many amps need to flow through a section of wire.
When looking at solar PV project they come into two main group, solar PV source
circuits (those from after the modules and before the inverter) and non-solar PV
source circuits (those coming after inverter)
2. Understand how much the conductor CAN CARRY based on itʼs rated ampacity
AND conditions of use. Weʼll talk about how to adjust a wire based on conditions
of use and itʼs rated capacity.
3. Thus, for every conductor sizing example, we should always be asking ourselves,
HOW MUCH does the conductor NEED TO CARRY and HOW MUCH can the
conductor carry. In all cases, the conductor MUST be able to carry more then it
must carry.

54

With that in mind, there are four areas that we need to consider when sizing
conductors.
1. Standard ampactiy tables based on a) continuous or b) maximum current.
2. Derating wire for conditions of use
3. Fuse or OCPD protector rating
4. Terminal Rating
1. Standard Ampacity Tables based on continuous or maximum current.
Determining ampacity requirements based on continuous or maximum current.
Remember this is calculating HOW MUCH a conductor needs to carry.
 Isc = Rated short circuit current which is the maximum current flow when the
positive and negative are connected together at STC. Our module has an Isc of
5.54A
 Maximum Current. NEC 690.8A Circuits that are supplied by solar PV modules
(anything before the inverter) can deliver output current that is HIGHER than their
rated short circuit currents. Rated short circuit is at 1000W/M2 irradiance. Real
conditions can see 1250 W/M2. –> Thus Isc X 1.25 = Maximum solar pv source
circuit current
 Continuous Current. NEC 690.8B1 and 210.19A1. Continuous loads can only be
loaded to 80% of itʼs capacity. Solar PV array output AND inverter output are
always considered to be continuous since they last for more then 3 hours. Thus,
10amps (max Isc) x 1.25 = 12.5 amp conductor.
To understand which needs to be applied to what circuits, itʼs easiest
to separate between solar PV circuits (before the inverter) and non-solar PV
circuits (after the inverter)
 Solar Generated Circuits = Isc X 1.25 (high current) X 1.25 (continuous load) =
Isc X 1.56 = the required conductor ampacity for a solar source circuit
 In our example, the Isc is 5.54 amps and we have 2 strings. Thus, our conductors
must be able to carry 5.54 X 1.56 X 2 = 17.28 amps.

55

 Inverter AC circuits = Rated current X 1.25 (for continuous use) = require
conductor amapacity. Note: many inverter manufactures will specify simply the
“continuous AC output current”, so you donʼt need to perform this calculation. For
the PVI4000, thatʼs 16.3 amps.
2. Derating Wire for Conditions of Use

Now that we understand how to calculate HOW MUCH conductors need to carry,
we need to select a conductor that can carry that current in the conditions where
it will be used. All else equal, the hotter the surrounding air that conductors are
placed in, the less amperage they can safely carry and still meet our ampactiy
ratings for safety.
There are three main things to consider:
1. The rated capacity of the wire at testing conditions
2. The effects of temperature where it will be used
3. The effect of conduit fill
1. Standard Conductor Ampacity
The NEC has tables of ampacity for different conductors depending on size and
the insulation used. The standard ambient temperature is assumed to be 30C.
 Table 310.16 is for conductors in conduit or earth
 Table 310.17 is for conductors in free air.

56

Below is a sampling from 310.16
Example: From our example, what is the smallest size wire that can be
used from a combiner box that combines 2 strings of Sanyo 205 modules
in parallel?
Min Amapacity = Isc X Number of Strings X 1.56 = 5.54 X 2 X 1.56 X =
17.28 amps
This can be satisfied with by a AWG 14 THWN-2 Conductor
Why? We NEED to carry 17.28 amps. AWG14 CAN CARRY 25 amps. 25
> 17.28
2. Adjusted for Conduit Fill

57

If there are more than 3 current carrying conductors in a raceway or cable, the
conductor ampacity must be derated for conditions of use per NEC 100, this
excludes grounding conductors per NEC 310.15B5
The impact of conduit fill is essentially the same as the increase in temperature,
more conductor in a conduit, and where that conduit is, will impact how hot it gets
in that conduit and thus how much the conductor can carry.
Table 310.15B2a gives factors for derating. Some value from 310.15 are below.
From our example, we only have 2 strings ( 4 home runs) from the array.
Letʼs assume that we have 40 modules (10 modules per string, 4 strings),
that we are combining, so we have to combine 2 arrays. 8 source circuits
into 4.
What would be the minimum amapcity in that situation?
Number of conductors = 4
Min ampacity needed for two conductors = 17.28
Adjusted ampacity for conduit fill = 17.28 / .80 = 21.6
14AWG still works because it has a maximum capacity of 25 amps.
3. Adjusting for Temperature Rating
The conductors are tested and their ampacity is rated for 30C. Thus, if the
conductors will be used in any condition that may be higher then 30C, we need to
reduce their ampacity ratings. NEC provides table to perform these calculations
in table 310-16 and 310-17.

58

Below is a sample from 310-16.
Example. Our system will be installed in Houston, TX, the highest
temperature is 106F. Can we still use 14AWG to combine our source
circuits?
14AWG Standard Amapacity = 25 amps.
Adjusted Ampacity = 25 amps X .87 = 21.75 amps.
This means that given it 106 outside, 14AWG is rated to carry 21.75
amps. We need to carry 17.28 amps. So 14AWG still works.
The temperature derating also needs to be adjusted based on the distance that
the conduit is above the roof.
Per NEC 310.15B2C, the below determines the temperature that needs to be
ADDED to the highest ambient temperature based on where the conduit is
placed.

59

From our above example, if the conduit was being run 1 inch above the
roof, we would need to ad 40F to the ambient temperature of 106F to
equal 146F.
This means that a 14AWG with a rated capacity of 25 amps will have an
ampacity of 25amps X .58 = 14.5 amps.
Letʼs walk through a full example to make sure we have all these concepts in
order.
What we need to make sure is that corrected amapacity of the conductors (the
rated capacity derated for use) is GREATER then the maximum amount of
current that will be flowing through the conductor.
We have 2 strings of 10, 205 watt modules
 Minimum Ampacity = 2 parallel strings X Isc X 1.59
 5.54 X 2 X 1.56 X = 17.28 amps
 17.28 amps is REQUIRED
We have 2 conductor pairs, 4 home runs running to the DC disconnect and
interview. Conduit filled is .80
The highest ambient temperature is 109F/43C

60

The homeruns will be in conduit that is 1/2 inches above the roof. This adds 33C
to the 43C ambient temperature equaling. Our temperature derating is now 76C
or .41
Thus, the equation is Conductor Ampacity X conduit fill derate X ambient
temperature derate.
• 14AWG is rated for 25 amps.
• 25 amps X .80 X .41 = 8.2 amps.
• Under these conditions 14AWG is only rated to carry 8.2 amps. WE MUST
CARRY 17.28 amps.
• We would increase our conductor size to #8 AWG
• 8AWG – 55 amps X .80 X .41 = 18.04 amps.
• 18.04 conductor ampacity > 17.28 minimum ampacity needed.

61

About
the
Author
Chris Williams is the Chief Marketing Officer at HeatSpring
Magazine.
He writes at Cleantechies, Alternative Energy Stocks and
Renewable Energy World. He's a clean energy jack-of-all
trades. He has installed over 300kW of solar PV systems,
tens of residential and commercial solar hot water systems
and 50 tons of geothermal equipment. Chris is an IGSPHA
Certified Geothermal Installer and will be sitting for his
NABCEP in September.
If you have any questions….
If you read this guide and have any questions or want to get more article,
whitepapers or information there are a few ways to keep in touch.
Subscribe to HeatSpring Magazine with your RSS subscribers.
Ask HeatSpring a question on our facebook page.
Ask me, Chris Williams, @topherwilliams, a question on twitter.
Subscribe to HeatSpring TV, our video podcast to
get updates on interviews with industry experts on best practices.
Join our linkedin group to connect with HeatSpring alumni and
other professionals.
Email. You can email directly: cwilliams@heatspring.com
Phone. If you have an in-depth question call me at 617 702 2676

62

Solar pv design1012

Recommended

Recommended

More Related Content

Viewers also liked

Viewers also liked (17)

Similar to Solar pv design1012

Similar to Solar pv design1012 (20)

Recently uploaded

Recently uploaded (20)

Solar pv design1012