Solar notes

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Solar cells- principle, types construction and calculations – Pavithran, MNU, FET – 2017
Photovoltaic effect:- The generation of voltage across the PN junction in a semiconductor due to the
absorption of light radiation is called photovoltaic effect. The Devices based on this effect is called
photovoltaic device.
Note: Semiconductors are materials, which become electrically conductive when supplied with light or
heat, but which operate as insulators at low temperatures
Solar power is the conversion of sunlight into electricity, either directly using photovoltaics (PV), or
indirectly using concentrated solar power (CSP). CSP systems use lenses or mirrors and tracking systems
to focus a large area of sunlight into a small beam. PV converts light into electric current using
the photoelectric effect.
Materials for Solar cell
Solar cells are composed of various semiconducting materials
1. Crystalline silicon
2. Cadmium telluride
3. Copper indium diselenide
4. Gallium arsenide
5. Indium phosphide
6. Zinc sulphide
Silicon is a semiconductor material (solar cell – made of SiO2). When it is doped with the impurities
gallium and arsenic its ability to capture the sun's energy and convert it into electricity is improved
considerably. ... When the sunlight hits the solar cell,the energy excites electrons that leave behind holes.
The photons (light particles) produce an electrical current as they strike the surface of the thin silicon
wafers. A single solar cell produces only about 1/2 (.5) of a volt. However, a typical 12 volt panel about
25 inches by 54 inches will contain 36 cells wired in series to produce about 17 volts peak output.
Solar panels installed on your roof work best during daylight hours. When the sun is shining directly
onto them, sunlight can be converted into electricity. Your solar panel efficiency drops at night because
there is no sunlight to convert to electricity and solar panels can't generate power in darkness.
If power is needed at night or on those winter days, energy can be conserved by utilizing a utility grid or
a battery bank. The utility grid can be used simultaneously with solar power and the battery
bank can convert solar energy into electricity throughout the day to be stored for later use

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Gallium arsenide (GaAs) is an important semiconductor material for high-cost, high-efficiency solar
cells and is used for single-crystalline thin film solar cells and for multi-junction solar cells.
To calculate the energy you will use over time, just multiply the power consumption by the hours of
intended use. The 20W TV in this example, on for 2 hours, will take 20 x 2 = 40WH from the battery.
Repeat this for all the appliances you wish to use, then add the results to establish total consumption like
below.
Divide your daily kWh requirement (see question No. 1) by the number of daily peak sunlight hours. This
gives you the amount of energy your panels need to produce every hour in kilowatt-hours. Multiply this
number by 1,000 to convert your hourlypower generation need to watts
Let's say that this number is 1100 kWh per month. Now divide your average monthly usage by 30 to get
your average daily kWh usage. In this case, it would be about 37kWh per day. Assuming that you use
about 37 kWh per day, you would needabout 37 solar panels to meet 100% of your average energy
needs.
Solar photovoltaic system or Solar power system is one of renewable energysystem which
uses PV modules to convert sunlight into electricity.
Typically, homeowners use about 900 kWh a month on average. So, take 900 kWh and divide by the
amount of kWh one solar panel produces over the course of a month (30kWh), and you get a 30 panel
installation. 30 panels x 250 watts per panel equals a 7,500 watt system (7.5kW).
Homeowners can often power most household appliances using between 3000 and 6500 watts. If your
home has a smaller furnace and city water,you can generally expect that 3000-5000 watts will cover your
needs. If you have a larger furnace and/or a well pump, you will likely need a 5000 to 6500 watt
generator.
So, a 2,000 square foot home would be allowed a solar array of 4,000 watts. Depending on the type
of panel that you choose, a system of this size would be anywhere from 12-18 solar panels. Keep in
mind, this formula to estimate consumption varies depending on who provides your electricity.
As you can see above,the average solar panel these days is about 3.25 feet by 5.4 feet (about 17.5 square
feet) and puts out about 265 watts of electricity. That makes it simpler to see how many panels you can fit
and how much electricity the system would be rated for, except for one thing: solar panel setback.

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To calculate the energy you will use over time, just multiply the power consumption by the hours of
intended use. The 20W TV in this example, on for 2 hours, will take 20 x 2 = 40WH from the battery.
Repeat this for all the appliances you wish to use, then add the results to establish total consumption like
below.
A 6kW Solar Kit requires up to 450 square feet of space. 6kW or 6 kilowatts is 6,000watts of DC direct
current power. This could produce an estimated 400 to 1,000 kilowatt hours (kWh) of alternating current
(AC) power per month, assuming at least 5 sun hours per day with the solar array facing South.
The PV Modules gather solar energy in the form of sunlight and convert it into direct current (DC)
electricity. An inverter can convert this DC power into alternating current (AC power, which is the type
of electricity used in your home). PV Modules are joined together to form a PV Solar Panel system.
Solar cells, also called photovoltaic (PV) cells by scientists, convert sunlight directly into
electricity. PV gets its name from the process of converting light (photons) to electricity (voltage), which
is called the PV effect.
Solar power is arguably the cleanest, most reliable form of renewable energyavailable, and it can be
used in several forms to help power your home or business.Solar-powered photovoltaic
(PV) panels convert the sun's rays into electricity by exciting electrons in silicon cells using the photons
of light from the sun.
Photovoltaic panels can use direct or indirect sunlight to generate power, though they are most effective
in direct sunlight. Solar panels will still work even when the light is reflected or partially blocked by
clouds. Rain actually helps to keep yourpanels operating efficiently by washing away any dust or dirt.
As with nighttime hours, the efficiency of solar panels decreases on cloudy dayssince less
sunlight can pass through the clouds to reach your solar roofs; however, this does not mean that zero
power is being produced – just a lot less. Maximum sunlight is necessary in order to provide optimal
performance.
An off-grid or standalone photovoltaic system is when your solar photovoltaicsystem is not connected to
the utility grid and you are producing your own electricity via solar, wind, microhydro, generator, etc.
These systems will generally have a battery bank in order to store the electricity for use when needed.
The term off-the-grid (OTG) can refer to living in a self-sufficient manner without reliance on one or
more public utilities. ... Off-the-grid homes are autonomous; they do not rely on municipal water supply,
sewer, natural gas, electrical power grid, or similar utility services.

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When sunlight hits the solar photovoltaic (PV) panels, electricity (or 'solar energy') is produced. The
electricity then runs from the solar panels through an inverter. The inverter turns the power from direct
current (DC) into alternating current (AC), which you can then use for electronic appliances in your
home.
A grid-connected photovoltaic power system, or grid-connected PV power system is an electricity
generating solar PV power system that is connected to the utility grid. A grid-connected PV system
consists of solar panels, one or several inverters, a power conditioning unit and grid connection
equipment.
Meaning/Usage: Going back to work after a break. Explanation: "Back" is goingback to
something. Grinding something is hard work, so going back to the "grind" is going back to hard work.
"Ok everyone break time is over, get back to the grind." "Back to the grind for me.
Physical Size. Length and Width – Although length and width varies slightly, most companies are
manufacturing solar panels in standard sizes. The most typical size used for residential installations is 65
inches by 39 inches, while the common size for commercial applications is 77 inches by 39 inches
All solar panels are rated by the DC power produced in standard test conditions. A typical solar panel
produces about 200 watts of electricity based on the efficiency and size of what's installed. For example,
if you have 25 panels installed, you may have an output of about 5 kilowatts (kW).
Simply put, a solar panel works by allowing photons, or particles of light, to knock electrons free from
atoms, generating a flow of electricity. Solar panels actually comprise many, smaller units called
photovoltaic cells. (Photovoltaic simply means they convert sunlight into electricity.)
Solar panel refers to a panel designed to absorb the sun's rays as a source of energy for generating
electricity or heating.A photovoltaic (in short PV) module is a packaged, connect assembly of typically
6×10 solar cells
A photovoltaic array is the complete power-generating unit, consisting of any number of PV modules
and panels. Figure 1. Photovoltaic cells, modules, panels and arrays. The performance of PV modules
and arrays are generally rated according to their maximum DC power output (watts) under Standard Test
Conditions (STC).

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So the equation now reads 140.875 Amp-hours/20.42 Amps, which equals about 6.9 hours. So in this
simple example, one 245 watt solar panel will take just under 7 hours to charge one 12 Volt batter with a
load that is using half of the battery's capacity.
The grid connect inverter converts the DC electricity produced by the solar panels into 240 V
AC electricity, which can then be used by the property/household. If agrid connect system is producing
more power than is being consumed, the surplus is fed into the mains power grid.
Calculate the current in amps by dividing power in watts by the voltage in volts. For example, if the solar
panel is rated at 175 watts and the maximum power voltage, Vmp, is given as 23.6 volts, then calculate
the current as 175 watts divided by 23.6 volts, which is equal to 7.42 amps.
The following are the different types of solar cells.
 Amorphous Silicon solar cell (a-Si)
 Biohybrid solar cell.
 Buried contact solar cell.
 Cadmium telluride solar cell (CdTe)
 Concentrated PV cell (CVP and HCVP)
 Copper indium gallium selenide solar cells (CI(G)S)
 Crystalline silicon solar cell (c-Si)
Solar cells can be classified into first, second and third generation cells. The first generation cells—also
called conventional, traditional or wafer-based cells—are made of crystalline silicon, the commercially
predominant PV technology, that includes materials such as polysilicon and monocrystalline silicon.
Monocrystalline silicon differs from other allotropic forms, such as the non-crystalline amorphous
silicon—used in thin-film solar cells—and polycrystalline silicon, that consists of small crystals, also
known as crystallites.
Advantages. The process used to make polycrystalline silicon is simpler and cost less. The amount of
waste silicon is less compared to monocrystalline. Polycrystalline solar panels tend to have slightly
lower heat tolerance thanmonocrystalline solar panels.
Amorphous silicon (a-Si or a-Si:H) solar cells belong to the category of silicon thin-film, where one or
several layers of photovoltaic material are deposited onto a substrate. Some types of thin-film solar cells

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have a huge potential. These technologies are expected to grow rapidly in the coming years as they
mature.
Crystalline silicon is the dominant semiconducting material used in photovoltaic technology for the
production of solar cells. These cells are assembled into solar panels as part of a photovoltaic system to
generate solar power from sunlight.
Polysilicon is a hyper pure form of silicon and is the earth's second most abundant element. Due to its
semiconductor-like material properties, polysilicon is used as feedstock material in most solar energy
applications. Polysilicon is an initial building block for the process of manufacturing silicon based Solar
PV.
Concentrator photovoltaics (CPV) (also known as Concentration Photovoltaics) is a photovoltaic
technology that generates electricity from sunlight. Contrary to conventional photovoltaic systems, it
uses lenses and curved mirrors to focus sunlight onto small, but highly efficient, multi-junction (MJ) solar
cells.
That is, of all the power contained in sunlight falling on an ideal solar cell (about 1000 W/m²),
only 33.7% of that could ever be turned into electricity (337 W/m²). The most popular solar cell material,
silicon, has a less favorable band gap of 1.1 eV, resulting in a maximum efficiency of about 32%.
The current and power output of photovoltaic solar panels are approximately proportional to the sun’s
intensity. At a given intensity, a solar panel's output current and operating voltage are determined by the
characteristics of the load. If that load is a battery, the battery's internal resistance will dictate the
module's operating voltage.
A solar panel, which is rated at 17 volts will put out less than its rated power when used in a battery
system. That’s because the working voltage will be between 12 and 15 volts. Because wattage (or power)
is the product of volts multiplied by the amps, the module output will be reduced. For example, a 50-watt
solar panel working at 13.0 volts will products 39.0 watts (13.0 volts x 3.0 amps = 39.0 watts). This is
important to remember when sizing a PV system.
An I-V curve (see image on right) is simply all of a solar panel's possible operating points
(voltage/current combinations) at a given cell temperature and light intensity. Increases in cell
temperature increase a solar panel’s current slightly, but significantly decrease voltage output.
Basic System Components

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The following diagram shows the major components in a typical basic solar power system.
The solar panel converts sunlight into DC electricity to charge the battery. This DC electricity is fed to
the battery via a solar regulator which ensures the battery is charged properly and not damaged. DC
appliances can be powered directly from the battery, but AC appliances require an inverter to convert the
DC electricity into 240 Volt AC power. Some DC appliances can be connected to the regulator to take
advantage of the Low Voltage Disconnect and protect your battery.
The solar panelshown in that article contains 4 cells, and each of them can produce 0.45 volts and 100
milliamps, or 45 milliwatts. Each cell measures 2 inches by 0.5 inches.

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Detailed Component Description
Solar Panels
Solar panels are classified according to their rated power output in Watts. This rating is the amount of
power the solar panel would be expected to produce in 1 peak sun hour. Different geographical locations
receive different quantities of average peak sun hours per day. In Australia, the figures range from as low
as 3 in Tasmania to over 6 in areas of QLD, NT and WA.
As an example, in areas of the Hunter Valley in NSW, the yearly average is around 5.6. The monthly
figures for this area range from below 4.0 in June to above 6.5 in December. This means that an 80W
solar panel would ideally produce around 320W per day in June and around 520W per day in December,
but based on the average figure of 5.6, it would produce a yearly average of around 450W per
day....without taking losses into account.
Solar panels can be wired in series or in parallel to increase voltage or current respectively. The rated
terminal voltage of a 12 Volt solar panel is usually around 17.0 Volts, but through the use of a regulator,
this voltage is reduced to around 13 to 15 Volts as required for battery charging.
Solar panel output is affected by the cell operating temperature. Panels are rated at a nominal temperature
of 25 degrees Celcius. The output of a typical solar panel can be expected to vary by 2.5% for every 5
degrees variation in temperature. As the temperature increases, the output decreases. With this in mind, it
is worth noting that, if the panels are very cool due to cloud cover, and the sun bursts through the cloud, it
is possible to exceed the rated output of the panel. Keep this in mind when sizing your solar regulator.
Solar Regulators
The purpose of solar regulators, or charge controllers as they are also called, is to regulate the current
from the solar panels to prevent the batteries from overcharging. Overcharging causes gassing and loss of
electrolyte resulting in damage to the batteries.
A solar regulator is used to sense when the batteries are fully charged and to stop, or decrease,the amount
of current flowing to the battery.
Most solar regulators also include a Low Voltage Disconnect feature,which will switch off the supply to

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the load if the battery voltage falls below the cut-off voltage. This prevents the battery from permanent
damage and reduced life expectancy.
A solar regulator also prevents the battery from backfeeding into the solar panel at night and, hence,
flattening the battery.
Solar regulators are rated by the amount of current they can receive from the solar panels.
See section below for information on correctly sizing a solar regulator.
Inverters
An inverter is a device which converts the DC power in a battery to 240V AC electricity. Inverters come
in two basic output designs, pure sine wave and modified sine wave (squarewave).
Most AC devices will work fine on the modified sinewave inverter, but there are some exceptions.
Devices such as laser printers can be damaged when run on modified sinewave power. Motors and power
supplies usually run warmer and less efficiently, and some things, like fans, amplifiers, and cheap
fluorescent lights, give off an audible buzz on modified sinewave power. However, modified sinewave
inverters make the conversion from DC to AC very efficiently, and they are relatively inexpensive.
Pure sine wave inverters provide AC power that is virtually identical to, and often cleaner than, power
from the grid.
Inverters are generally rated by the amount of AC power they can supply continuously. Manufacturers
generally also provide 5 second and 1/2 hour surge figures. The surge figures give an idea of how much
power can be supplied by the inverter for 5 seconds and 1/2 an hour before the inverter's overload
protection trips and cuts the power.
Deep Cycle Solar Batteries
Deep cycle batteries that are used in solar power systems are designed to be discharged over a long period
of time (e.g. 100 hours) and recharged hundreds or thousands of times, unlike conventional car batteries
which are designed to provide a large amount of current for a short amount of time.
To ensure long battery life, deep cycle batteries should not be discharged beyond 70% of their capacity.

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i.e 30 % capacity remaining. Discharging beyond this level will significantly reduce the life of the
batteries.
Deep cycle batteries are rated in Ampere Hours (Ah). This rating also includes a discharge rate, usually at
20 or 100 hours. This rating specifies the amount of current in Amps that the battery can supply over the
specified number of hours.
As an example, a battery rated at 120Ah at the 100 hour rate can supply a total of 120A over a period of
100 hours. This would equate to 1.2A per hour. Due to internal heating at higher discharge rates, the same
battery could supply 110Ah at the 20 hour rate, or 5.5A per hour for 20 hours. In practice, this battery
could run a 60W 12VDC TV for over 20 hours before being completely drained.
There are many factors that can affect the performance and life of a battery bank. It is highly
recommended that you speak with an experienced solar power system installer or solar battery provider
prior to making any significant battery purchase.
Solar Regulator Sizing Information
A solar regulator must be able to handle the maximum current that can be produced by the solar panels.
Reflected sunlight and specific temperature conditions can increase the output current of a solar panel by
as much as 25% above it's rated output current. The solar regulator must be sized to handle the increased
current.
Solar regulators often short the solar panel input when regulating. This does not damage the solar panel,
but it does mean that the solar regulator must be sized to handle 125% of the solar panel's rated short
circuit current.
Example:
A BP Solar 80W solar panel has a rated output current of 4.55 Amps and a rated short circuit current of
4.8 Amps.
Minimum solar regulator size for a single BP Solar 80W panel would be: 4.8 Amps x 1.25 = 6 Amps.

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It is recommended that the regulator selected is even slightly larger than this figure to ensure that it is not
constantly operating at 100% of its rating, particularly in regions with higher ambient temperatures.
Sample Sizing Calculation
In order for you to size the system correctly, you need to note the power rating of each appliance that will
be drawing power from the system.
For this example, we will calculate the power requirements for a campervan with:
 2 x 15W 12VDC Fluorescent Lights
 1 x 60W 12VDC Water Pump
 1 x 48W 12VDC Fridge
 1 x 50W 240VAC TV
 1 x 600W 240VAC Microwave
(Note that a 600W microwave will consume approximately 900W of power)
1. Calculate Loads
Calculate total DC and AC loads:
DC Loads
 Lighting - 2 x 15W DC Lights - each used 2 hours per day = 60Wh/day
 Pump - 1 x 60W DC Pump - used 1/4 hour per day = 15Wh/day
 Fridge - 1 x 48W Fridge - runs 8 hours per day = 384Wh/day
Total for DC Loads = 459Wh/day
AC Loads
 Television - 1 x 50W - used 2 hours per day = 100Wh/day
 Microwave - 1 x 900W - used 15 min per day = 225Wh/day
Total for AC Loads = 325Wh/day
Allowing for inverter efficiency of 85% = 382Wh per day (ie. 325 / 0.85)
Total for AC and DC Loads = 841Wh per day

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2. Calculate Required Solar Input
In Central to Northern NSW expect a usable average of around 5 peak sun hours per day.
Required solar panel input = (841Wh / 5h) * 1.4 = 235W
Note: The 1.4 used in this formula is a factor we have found that can be used to simplify the calculations
for basic systems.
To ensure that adequate power is produced in the winter months, use a figure of around 4.0 to 4.5 peak
sun hours per day instead of 5.
3. Select Solar Panels
Select solar panels to provide a minimum of 235W. Always best to go bigger if possible:
 2 x 123W solar panels chosen which, when connected in parallel, will provide 246W or 14.32 Amps.
4. Select Solar Regulators
The rated short circuit current of the 123W solar panels is 8.1 Amps each, giving a total of 16.2 Amps.
Select a solar regulator that is more than capable of handling the total short circuit current: 16.2 x 1.25 =
20.25 Amps
 Steca 30Amp regulator chosen.
Note that, as described in the notes above, you must allow 25% extra capacity in the regulator rating as
solar panels can exceed their rated output in particular cool sunny conditions. A 30A regulator will allow
for an additional panel in the future.
5. Select Inverter
Select an inverter that is more than capable of supplying the maximum anticipated combined AC load
required. In this example, maximum load would occur if the microwave and TV were running at the same
time. Load in this case would be 900W + 50W = 950W.

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Note that this calculation assumes that the inverter selected has a suitable surge rating to cope with the
start-up surges of the microwave or other loads. A 1000W inverter would appear to be suitable, but a
1200W - 1500W inverter would be recommended.
 1200Watt pure sine wave inverter chosen.
Note: A pure sinewave inverteris the preferred choice, but if the budget is tight,a modified sine wave unit
could be used.
6. Select Battery
Select a battery, or a matched combination of batteries, that is capable of supplying the total power usage
without being discharged more than 70%.
In most cases it is recommended that the batteries are sized such that they have around 3 to 4 days back-
up capacity. This allows for days with low sunlight and reduces the daily depth of discharge resulting in
longer battery life.
With 3 days storage capacity, the battery sizing would be as follows:
 Ah Required = (841Wh * 3 / 12V) / 0.7 * 1.1 = 330Ah.
Note: The 1.1 is used in this formula as batteries are generally only about 90% efficient.
Notes:
The appliance ratings used in the above examples may not be accurate. They have been used for example
purposes only. Check the ratings on your appliances before performing any calculations.
This calculation demonstrates one simplified method of calculating the solar power requirements for a
campervan or similar set-up. When sizing a larger system, such as a system for a house, there are many
other factors that need to be taken into account to ensure the system performs as required. These include,
but are not limited to, solar panel output tolerance, battery temperatures and discharge rates, system
autonomy ie. catering for days without adequate sunlight, etc. Seek the advice and assistance of a BCSE
accredited designer before constructing a larger renewable energy system.

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Fig.1. Current, Voltage and Power Curves
A Current (I) versus Voltage (V) Curve of a PV / Solar Module (â€œI-Vâ€• Curve) shows the possible
combinations of its current and voltage outputs. A typical I-V curve for a 12 V Module is shown at Fig. 1
above.
The power in a DC electrical circuit is the product of the voltage and the current. Mathematically,
Power (P) in Watts (W) = The Current (I) in Amperes (A) X the Voltage (V) in Volts (V) i.e. W = V X A
A Solar (PV) Cell or a Panel / Module produces its maximum current when there is no resistance in the
circuit, i.e. when there is a short circuit between its Positive and Negative terminals. This maximum
current is known as the Short Circuit Current and is abbreviated as Isc. When the Cell / Panel (Module) is
shorted, the voltage in the circuit is zero.
Conversely, the maximum voltage occurs when there is a break in the circuit. This is called the Open
Circuit Voltage (Voc). Under this condition, the resistance is infinitely high and there is no current, since
the circuit is incomplete. Typical value ofthe open-circuit voltage is located about 0.5 â€“ 0.6 V for
Crystalline Cells and 0.6 â€“ 0.9 V for Amorphous Cells. These two extremes in load resistance, and
the whole range of conditions in between them, are depicted on the I-V Curve. Current, expressed in
Amps, is on the vertical Y-axis. Voltage, in Volts, is on the horizontal X-axis.
The power available from a photovoltaic device at any point along the curve is just the product of Current

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(I) in Amps (A) and Voltage (V) in Volts (V) at that point and is expressed in Watts. At the short circuit
current point, the power output is zero, since the voltage is zero. At the open circuit voltage point, the
power output is also zero, but this time it is because the current is zero.
There is a point on the knee of the I-V Curve where the maximum power output is located and this point
is called the Maximum Power Point (MPP). The voltage and current at this Maximum Power Point are
designated as Vmp and Imp.
The values of Vmp and Imp can be estimated from Voc and Isc as follows: Vmp = (0.75 â€“ 0.9) Voc
Imp = (0.85 â€“ 0.95) Isc
The rated power of the PV / Solar Module in Watts (Pmax) is derived from the above values of voltage
Vmp and current Imp at this Maximum Power Point (MPP):
Rated power in Watts, Pmax = Vmp x Imp
Example of I-V Curve and Ratings of a 12 V Solar (PV) Panel
Fig.2. Example of I-V Curve and Ratings of a 12 V PV / Solar Panel
The I-V Curve for a typical 12 Volt PV / Solar Panel is shown at Fig.2 above

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This Maximum Power Point in the example curve given above is where Vmp is 17 Volts, and the current
Imp is 2.5 amps. Therefore, the rated or the maximum power Wmax in watts is 17 Volts times 2.5 Amps,
or 42.5 Watts.
A typical 6"x6" cell puts about ½ Volt, and about 7 Amps DC Power.
Watts = Volts x Amps
So one gets about 0.5 Volts * 7 Amps = about a maximum of 3.5 Watts from the cell.
If you put 4 of the cells together, you get about 14 watts per square foot, or about 0.014 kw per square
foot (with 1000 watts per kilowatt). This is more or less the maximum power you might expect to get
with noon-time sun during the summer. You will get a lot less with morning or evening sun.
Solar panels are made up of multiple solar cells. If the cells are wired in parallel, one increases the
amps. If the cells are wired in series, one increases the volts.
The Open Circuit Voltage and Short Circuit Amps that the cell puts out are more or less theoretical
maximums. You find that the greater the power draw, the lower the volts that the cell puts out. Thus,
there is a tradeoff between Amps and Volts. Most panels now have a "Peak Power"rating.
A typical 12V Car battery actually is about 12.6V when fully charged. But, it is generally charged at
about 14V.
So...
Older solar panels were often rated to put out 12V, but actually had a peak output of about 18-20V.
More modern (consumer grade) panels usually have power ratings of about 30-50V. Usually one uses a
charge controller to isolate the system or batteries, and the charge controller can automatically adjust to
the "Peak Power"output of the panel.
I have some panels that are about 3' wide, 4.5' tall, with an output of about 200W, maximum voltage of
68V, and Maximum Power Point (MPPT) of about 56V.
AC vs DC.

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If you are just running lights and such from a solar panel, then it is easy enough to set up a DC system,
and use DC light bulbs. No inverter to turn on and off, and no inverter loss.
However,as most of the appliances in the USA require 110V/220V AC current, one can also attach an
AC/DC inverter to provide the correct voltage. And, of course, people typically choose 60 HZ for USA
and 50 HZ for Europe.
You can run the system entirely "off grid" meaning that one has to generate 100% of one's own power
needs, and setup one's own batteries for backup power. Or,one can run the system "on grid", essentially
using the power company's system as a super-battery. One sends the surplus power to the grid, and can
take extra power from the grid as needed. No batteries are required, although some systems use them for
backup power.
Traditionally the inverters have been centralized, but some companies such as Enphase have an option of
connecting mini-inverters directly to each panel, in effect converting the DC panels to AC panels.
Here is a good picture of the construction of a solar cell/panel. The top most layers listed are actually part
of the panel, and outside of the cells.
The electrical power generated by a photovoltaic cell, ( PV ) has two components: Voltage ( V ) and
Current ( I ). The output power generated by the PV cell is measured in Watts, ( P ) that the cell produces
is the product of the cell’s output current times its output voltage. In other words, P = V x I.
The voltage output of the photovoltaic cell remains fairly constant over a wide range of input light
intensities because of the cells photovoltaic effect, just as long as there is some light. The output current,
however, varies in direct proportion to the amount of sunlight entering the PV cell. The more light
entering the cell, the more current it produces up to its maximum. The solar cell’s output voltage remains
fairly stable from low to bright sunlight.
For the purposes of this tutorial here, we will consider a standard 4″ by 4″ (100mm X 100mm) poly-
crystalline silicon photovoltaic cell. Mono-crystalline or amorphous silicon cells are available. The
absolute value of the voltage information will differ slightly, but their general performance tends to
remain the same for all types of silicon PV cells for the amount of sunshine it receives on a sunny day. So
how does a solar cell work.
A poly-crystalline silicon solar cell has an open circuit voltage of about 0.57 Volts at 25°C. Open circuit
voltage means that the cell is not connected to any electrical load and is therefore not generating any

18
current. When connected to a load, for example a battery, the output voltage of the individual cell will
drop to about 0.46 Volts at 25°C as the generated current flows. It will remain around this 0.46 V level
regardless of the sun’s intensity or the amount of current the cell produces.
This decrease in output voltage is caused by internal resistance losses within the cell’s structure as well as
voltage drops across the metallic conductors deposited on the cell’s surface to collect the current.
Ambient temperature also has an affect on the PV’s cell’s voltage. The higher the temperature is, the
lower the cell’s output voltage becomes as it heats up, which is strange seeing that they spend all day sat
in the sun.
Photovoltaic Cell Current
While the voltage produced by a silicon photovoltaic cell is fairly constant, its output current on the other
hand varies considerably. The amount of usable output current that a cell generates depends on how
intense the sunlight is shinning onto the cell’s surface, and also the voltage difference between the cell
and the load.
Under normal operating conditions a poly-crystalline cell is rated at about 2.87 Amperes of current. This
value can increase considerably on a very cold, very clear, very bright and very snowy winter’s afternoon.
Also altitude is another factor that affects the PV cell’s output current. The higher you are, the less
atmospheric conditions there is above and the more sunlight the cell will receive, assuming no clouds or
snow. So expect to see current gains if used well above sea level.
Connecting Individual Cells into Modules
When individual photovoltaic cells are assembled together into modules or panels they are generally
wired in series. That is the positive connection or pole of one PV cell is connected to the negative
connection or pole of the next cell, and so on until all the cells in the panel are connected together in what
is called a series string
When individual photovoltaic cells are assembled together into modules or panels they are generally
wired in series. That is the positive connection or pole of one PV cell is connected to the negative
connection or pole of the next cell, and so on until all the cells in the panel are connected together in what
is called a series string.
This series wiring is done to raise the voltage of the panel. We said earlier that a single cell has a voltage
potential of about 0.46 Volts. This is not enough voltage to do any usable work in a 12 Volt system. But if

19
we add the voltages together of say 36 cells by series wiring them, then we have a working voltage 16.7
Volts, and that’s more than enough to charge a 12 Volt battery.
The operational voltage of a typical 12 Volt lead acid battery ranges from between 10.5 volts to 14 volts.
The battery’s exact voltage depends on its state of charge,ambient temperature, and whether the battery is
being charged or discharged at the time. It is this battery voltage curve that the PV panels are designed to
fit and so MUST provide a greater voltage than the battery possesses. If the PV panel cannot do this, then
it cannot transfer electrons to the battery and therefore it cannot recharge the battery.
The output current generated by a solar panel of 36 cells in total remains the same as the current produced
by one single cell, about 3 Amperes. The series wiring technique causes the voltages to be added together,
but the current remains the same. We could parallel connect all the 36 cells but this would add their
currents together rather than their voltages. The result of this would be a solar panel that produces 108
Amperes of electric current, (36 x 3) but at only 0.46 Volts, too low.
o How Many Cells Do We Need?
Most photovoltaic (PV) panel manufacturers make 12 Volt solar panels for battery charging with 32, 36,
or 48 cells in the series string. They are all rated at about the same current, being composed of the same
basic cell. The difference between these panels is one of voltage. The question for us to answer here is
how their output voltages relate to the voltages we require for our 12V charging system.
32 Cells in Series
This size of photovoltaic panel has the lowest voltage rating of only 14.7 Volts (0.46 Volts times 32
cells). This is because it has the fewest number of PV cells in its series string. This panel design closely
matches the charging curve of a standard 12 Volt lead acid battery. As the battery charges-up, its terminal
voltage rises. When this battery is almost full its voltage is about the same as the PV cell’s at around 14.7
volts. The 32 cell module simply hasn’t enough voltage to continue charging the battery when its full so
cannot overcharge the average, small, lead acid battery.
The applications suitable for these small 32 cell solar panels are in RV’s, boats, garden lighting and
summer cabins. These applications are characterized by their intermittent use and relatively small battery
charging capacity. In these these types of low power applications, a 32 cell panel can be used with or
without a charge current regulator as the batteries will not become overcharged if left connect to the panel
during long periods of non-use.

20
36 Cells in Series
This size of photovoltaic panel has an output voltage of about 16.7 Volts (0.46 times 36 cells). This is
enough output voltage to be able to continue to charge a lead acid battery even though it may be already
fully recharged. The 36 cell panel is suitable for a home based 12 Volt alternative energy system with
high battery capacities as it has the higher output voltage necessary to recharge deep cycle lead acid
batteries.
However, a 36 cell solar panel will require some form of charge regulation to prevent overcharging the
battery during periods of high solar intensities or when battery usage is at its lowest.
A 36 cell solar panel tends to be more cost effective in a typical home power application because it can
produce a good amount of current or high voltages at elevated temperatures. The higher voltage produced
by the 36 series wired cells will more effectively recharges a large deep cycle lead acid batteries.
High ambient temperatures will cause the voltage of any PV panel to reduce slightly, but the 36 cell panel
has more than enough voltage surplus to still be an effective battery charger even at high ambient
temperatures.
48 Cells in Series
A 48 cell panel is the big daddy of the PV industry. 48 individual photovoltaic cells connected in series
produces an output voltage of about 22 volts. These large PV panels have sufficient output current
capacity to charge a 12 Volt system, regardless of the battery’s voltage or high temperature. However,
these large panels do require some form of charge regulation in just about every application. They have
the sufficient voltage necessary to raise a solar system’s voltage, while charging full batteries, to well
over 16 volts. This over voltage is high enough to ruin any electronic equipment rated at 12 VDC so some
form of protection is needed.
Generally, a 48 cell solar module has very specific applications where high power and currents are
required such as in pumping water or were long cable runs will have appreciable voltage losses even if
large diameter cables are used. Another disadvantage of this PV panel is its size and additional cost
compared to 32 and 36 PV cell panels. On the plus side, a 48 cell panel will perform better in very hot
areas and areas with very low levels of sunlight throughout the year.

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