This document discusses several major trends in electricity generation and use, including electric vehicle charging, home automation, and battery storage systems. It provides details on types of electric vehicles, charging levels and times, battery technologies, and relevant codes. Home automation is defined as homes that can communicate and control devices remotely. Various communication protocols for home automation are summarized, with X-10 and UPB discussed as examples.

1. Electrical Trends:
Electric Vehicle Charging,
Home Automation and
Powerwalls
Roger Messenger, PhD, PE

2. Believe it or Not, Times are Changing
Wikipedia InspirationSeek.com
Deleece Cook

3. Including How We Generate
and Use Electricity
PVMagazine.com
Forbes.com

4. Major Trends in Electricity Generation
•Inefficient to Efficient
•Fossil to Renewable
•Dirty to Clean
•Predictable to Less Predictable Sources
•Some Storage to More Storage
•More HVDC Transmission Lines
•More Control and Data Analysis

5. Growth of Worldwide Energy Production
by Source
Note that this
graph shows
ENERGY
production – Not
installed capacity.
Will renewable
kWh production
exceed nuclear
kWh production
in 2019?
30
10
3
100
300
103
1
Quads
1989 1994 1999 2004 2009 2014 2019 2024
Year
Crude Oil Coal Nat Gas Nuclear Hydro Other ren
?

6. Major Changes in Grid Use that will Impact
Engineering and Contracting
• Electric Vehicles
• All Electric
• Plug-in Hybrids
• Energy Efficiency
• Lighting
• Refrigeration
• Almost Everything
• Implementation of Smart Grid and Smart Home Technology
• Distributed Generation and Energy Storage

7. Part I – Electric Vehicles

8. Electric Vehicle Charging
NEC 625.2 Recognizes the Following Vehicles as EVs:
General – “an automotive-type vehicle for on-road use,” such as
• Passenger Automobiles
• Buses
• Trucks
• Vans
• Neighborhood Electric Vehicles
• Electric Motorcycles
• Electric Golf Carts and Forklifts are not included in NEC 625.2
• EVs must Use Rechargeable Batteries.

9. Why does a Hybrid get better mileage?
• Hybrid vehicles have a combination of gasoline engine and electric
drive motor.
• They have a 12 V battery for lights, starter, etc. and a high-voltage
battery pack to supply the drive motor.
• In economy mode, when the vehicle is stopped, the engine stops.
About 15% of normal driving involves the engine idling.
• When starting up, the electric motor gives the engine a boost.
• Dynamic braking involves converting energy of motion to electricity to
partially recharge the batteries.
• All battery charging comes from either gasoline-driven alternator or
dynamic braking. No external charger.

10. Two Types of EVs
• All Electric (EV) has only an electric motor with no gasoline backup.
• Range varies between about 100 and 400 miles.
• Battery storage capacity between about 30 kWh and 100 kWh.
• Most batteries are lithium-based.
• Charging times depend upon level of charging (to be discussed later).
• Plug-in Hybrid (PHEV) is a hybrid with larger battery pack that can be
charged by plugging into an electrical outlet .
• Range is usually up to about 400 miles.
• Electric-only range is up to about 50 miles with typical 18 kWh battery pack.
• Additional hybrid gasoline mileage range is about 350 miles.
• Lithium-based batteries in nearly all.
• Charging times vary depending upon charging level, but since electric range is
less, charging time with comparable chargers is also less.

11. Fuel Cost per 1000 miles
It’s interesting to compare electric operating cost vs gasoline operating
cost for a vehicle. Let’s compare a few vehicle types. Assume gasoline
at $2.75/gal and electricity at $0.15/kWh.
1. Full-size SUV @ 20 mpg
1000 miles will use 50 gal @ $2.75 = $137.50
2. Small SUV @ 30 mpg
1000 miles will use 33.3 gal @ $2.75 = $91.67

12. Fuel Cost per 1000 miles
3. Hybrid Sedan @ 55 mpg
1000 miles will use 18.2 gal @ $2.75 = $50.00
4. EV or PHEV Sedan operating
electric only @ 4 mi/kWh 1000 miles will
use 250 kWh @ $0.15 = $37.50
5. PHEV Sedan operating 500 mi
on electric and 500 mi on gasoline
will use $27.50 in gasoline and $18.75 in electric = total of $46.25

13. Charging Levels
• Level 1 – Uses 120 V outlet and charges at about 1200 W.
• Maximum charge rate on 15A, 120V Circuit is 12 A, or 1440 W.
• Level 2 – Uses 208 or 240 V single phase outlet and charges at
somewhere in the 3,000 to 7,000 W range.
• 3,000 W @ 208 V uses 14.42 A, so needs 2P20A OCP.
• 7,000 W @ 208 V uses 33.65 A, so needs 2P40A OCP.
• 3,000 W @ 240 V uses 12.5 A, so needs 2P20A OCP.
• 7,000 W @ 240 V uses 29.2 A, so needs 2P40A OCP.
• Level 3 – Charges in the 125,000 W range and usually
connected to a 3-phase service. This results in
347 A @ 208 V or 157 A @ 460 V. So not a residential option.
• Higher Charge Rates are on the Horizon.

14. Charging Times
The range of most of the presently available EVs is about 3 to 4 miles
per kWh of charge in the batteries, depending upon how the vehicle is
driven. We can thus conclude that:
• A level 1 charger with a 1200 W charging rate will increase battery
charge by 1.2 kWh per hour. Since 1.2 kWh x 4 mi/kWh = 4.8 miles,
regardless of whether an EV or a PHEV, if it gets 4 mi/kWh, then an
hour of level 1 charging will add about 5 miles to the electric range.
• A level 2 charger at 5000 W charging rate will increase battery charge
by 5 kWh per hour. This amounts to 20 miles per hour of charging.
• A level 3 charger at 125,000 W will add 125 kWh per hour, which, at
present, is greater than the capacity of any automobile battery packs.

15. Desirable Charger Features
Basic
• Compatibility
• Communication
• Charging Time
• Installation Cost
• Safety
• Code Compliant
Advanced
•Bidirectional
• Adaptable
• Portable
Large Pins for Power
Small Pins for Communication and
Proximity Detection.
Proximity Detection prevents driving
away when vehicle is plugged in.

16. EV Battery Types – Capacity vs Storage
• Capacity refers to available power in kW. High power needed for
starting gasoline engine.
• So 12 V lead-acid shallow discharge battery used for starting, headlights,
radio, etc.
• Battery is recharged as soon as engine is started. If battery is discharged too
far, battery life is shortened.
• Storage refers to energy storage capacity in kWh. Mileage of drive
motor measured in miles per kWh.
• Fast charging (heavy braking) or fast discharging (heavy acceleration)
of drive batteries results in lower mi/kWh. High voltage storage used
to provide power to drive motor.
• Deep discharge of storage batteries is OK.

17. Lead Acid and Lithium Ion Batteries
• Lead Acid
• Less expensive, but store fewer kWh per pound and capable of fewer charge-
discharge cycles.
• Can be overcharged somewhat, but if so, hydrogen is produced.
• Lead-acid chargers need to meet certain ventilation requirements per NEC
• Lithium Ion
• More costly, but last longer, more charge-discharge cycles, deeper discharge
and about twice as many kWh/lb as in lead-acid.
• Important to avoid charging beyond 100%, so charging circuitry more
complicated. All cells must be charged equally.
• Can be used for capacity or storage. Capacity needed for acceleration and
storage needed for driving range.
• No flammable gases given off during charging, so no ventilation
requirements.

18. Voltage Conversion
• AC voltage is converted from low to high or high to low or simply
isolated at the same voltage by means of a transformer.
• Low to high voltage involves high to low current change, since power out
cannot exceed power in.
• High to low voltage involves low to high current change for same reason.
• DC voltage is converted by means of a DC/DC converter that can
convert low voltage to high voltage or vice versa.
• This is how HV batteries are charged and how energy is transferred from HV
to LV.

19. Some Applicable Codes, Standards & Abbreviations
• NEC 625 Electric Vehicle Charging System
• NEMA L6-30 30A, 250V, 3-wire twist lock connector
• NEMA 14-30 30A, 125/250V 4-wire connector
• NEMA 14-50 50A, 125/250V 4-wire connector
• NEMA 6-50 50A, 125/250V 3-wire connector
• SAE J1772 Standard for Electric Vehicle Charging Equipment
• OCPP Open Charge Point Protocol – Enables communication among
chargers
• UL 2202 Standard for EV Charging System (buy UL 2202 for $1000)
• UL 2231 Standard for Personnel Protection Systems for EV Supply Circuits
https://www.greencarreports.com/news/1050948_what-is-evse-and-why-does-your-electric-car-charger-need-it
https://en.wikipedia.org/wiki/Open_Charge_Point_Protocol

20. Some Charger Configurations
Coulomb technologies
CT-500 EV charging station
Level 1 charger with J1772
Connector for Vehicle
Level 2 charger on
outdoor pedestal

21. Service Calculation for Adding a Charger
• Follow NEC 220.83 for existing single family and NEC 220.84 for existing
multifamily.
• For other sites, other sections of NEC apply.
220.83 Example for 1500 sq ft dwelling – no new A/C or heat – only charger
General Lighting @ 3 VA/sq ft 4,500 VA
A/C @ 3,600 VA 3,600 VA
Washer, Dryer, Referigerator, 5 Appl ckts 16,000 VA
Elect Water Htr, Elect Range, 5.5 kW Level 2 charger 22,000 VA
First 8 kVA @ 100% 8,000 VA
Remainder @ 40% 15,240 VA
Diversified Load 23,240 VA
120/240 V service size 96.8 A
So as long as the
existing service is at
least 100A, the charger
can be added, provided
there is room for a
2P30A CB in the MDP.
Otherwise may need to
tap at service.

22. Alternate Service Calculation for Adding a Charger
• NEC 220.87 allows determination of dwelling unit feeder load based
upon measurement of maximum peak demand over past 12 months.
• Since it is rare for a dwelling unit to be served by a demand meter, it
is also acceptable to measure demand for a month under load
conditions consistent with maximum use. Both line currents need to
be monitored and the larger is to be used.
• Add proposed new load to 125% of max monthly demand and
compare with 220.83 calculation.
• For previous example, if the 220.87 analysis results in kVA < 23.24,
then the smaller kVA may be used.

23. Where to Tie in EV Branch Circuit?
• If convenient and if an OCP space is available, use MDP.
• If inconvenient and if the Main Disconnect is closer to the proposed
location of the charger, then determine max distance from tap to charger
circuit OCP/disconnect.
• From known charger size, determine branch circuit OCP and branch circuit
wiring (125% of load).
• Compare charger conductor ampacity with Main OCP.
• Follow rules of NEC 240.21(B)(2). Charger branch circuit OCP allowed to be
within 25 ft of tap if ratio of charger conductor ampacity to Main OCP is at
least 0.333. This will work in most cases. Ampacity of #8 of 55 A allows for
175 A Main Breaker. #6 allows for 225A Main.
• Larger Main Breaker will reduce distance to branch OCP to 10 ft.

24. Have You Ever Seen Something Like This?
Customer meter
& Main Disc
Gutter for feeders to units
Possible location for EV
Branch ckt disconnect/OCP
Garage on other side
of wall
Almost perfect
Set-up for tap
At Main OCP.
All Mains are
150 A & load
Calc with EV
Level 2 about
120 A.

25. NEC 625 – Electric Vehicle Charging Stations
• 625.2 defines EV Storage Battery as one that is sealed and does not
release gas.
• Fastened in place – can be removed without a tool.
• Fixed in place – need a tool to remove.
• 625.4 Nominal AC system voltages 120, 120/240, 120/208, 240, 277/480,
480, 347/600, 600 and 1000 V. DC voltages up to 1000 allowed.
• 625.5 Electric Vehicle Supply Equipment (EVSE) and Wireless Power
Transfer Equipment (WPTE) must be listed.
• Part II gives manufacturing specifications for listing.

26. More NEC 625
• 625.40 Each outlet installed for charging EVs shall be on a single branch
circuit.
• 625.41 Branch circuit OCP must be rated at 125% of equipment rating.
Feeders rated at 125% of continuous load + 100% of noncontinuous.
Different meanings for residential & non-residential.
• 625.43 If equipment rated more than 60A or more than 150V to ground,
need readily accessible lockable disconnect. (208 or 240 is normally 120V
to ground.)
• 625.44 gives rules for connecting Portable, Stationary or Fixed equipment
to the premises. Nonlocking, grounding receptacles are specified.
• 625.48 covers Interactive Systems with references to 702 & 705. UL 1741
applies to these systems. WATCH THIS ITEM CAREFULLY!
• 625.50 requires charger to be more than 18” above floor.

27. A little More NEC 625
• 625.52 gives lots of information on the amount of ventilation needed
if a charger is marked as needing ventilation (such as for lead-acid).
• Part IV of 625 is new. It covers Wireless Power Transfer Equipment.
• NEC 626 covers Electric Truck Parking Spaces.

28. Charger Configurations
• Basic one-way charger
• Bidirectional Charger
External
One-Way
Charging
Circuitry
AC in
AC to
vehicle
comm &
proximity
to and from
vehicle
On-board
AC to DC
comm & control
To Drive
Batteries
External
Bi-directional
Charging
Circuitry
AC in
or out
AC to and
from vehicle
comm &
proximity
to and from
vehicle
On-board
AC to DC &
DC to AC
comm & control
To Drive
Batteries

29. Home
Automation

30. Definition of a Smart Home
“The term commonly used to define a residence that
has appliances, lighting, heating, air conditioning, TVs,
computers, entertainment audio and video systems,
security, and camera systems that are capable of
communicating with one another and can be controlled
remotely by a time schedule, from any room in the
home, as well as remotely from any location in the
world by phone or internet.”
Smart Home USA (https://www.smarthomeusa.com/smarthome/)

31. Code References
• NEC does not have either “smart homes” or “home automation” in
the index.
• So many of the present smart home devices are being installed by the
owners.
• Thus, in considering installation, all devices used in smart homes
should have some appropriate listing, just like any other appliances.
• In some cases, sections of NEC 750 (Energy Management Systems)
• Or maybe NEC 760 (Fire Alarm Systems)
• Or Article 800 (Communication Systems).

32. Device Communication Methods & Protocols
• Some devices communicate via wires, such as power circuits.
• Some devices use wireless communication.
• Some devices operate on house power.
• Others are battery-operated.
• Others use battery-backup with house power.
• Today there are at least 15 communication protocols on the market.
• So the key to system compatibility and reliability may depend upon
the protocol chosen.

33. Summary of Communication Protocols
X-10 is the oldest communication protocol
• Systems usually had a central control box plugged into the wall.
• Signals from the control box were transmitted to receiving devices via
the building wiring system via power line carrier.
• Each receiving device had an “address” for receiving signals from the
control box.
• Normally these were ON-OFF commands sent to lights or other
plugged in appliances, such as fans.
• Some systems also had dimmer options.

34. Example of X-10 and UPB Control Signal
• X-10 has 4 V control signal at  300 kHz on top of power line waveform.
• X-10 can control up to 4 devices on-off with dim possibility.
• UPB has 40V control signal on top of power line waveform.
• UPB can control up to 64,000 device addresses and is 30x faster than X-10.
• Need for powerline connection limits versatility.
• In each case, control signal is isolated from power line by a high pass filter.
120V
4V
X-10

35. Power Line Frequency Removed by Filter
• Control signal bursts can be shortened or lengthened.
• Spacing between bursts can be varied.
• Encoding of burst length and separation defines address of device and
control function to be accomplished (ON-OFF-DIM).
• Encoding is standardized to accommodate numerous device receivers.
Filtered X-10 or UPB control signal

36. Z-Wave Protocol
• Used by over 250 “Z-Wave Alliance Companies.”
• Over 1200 different products certified by the alliance.
• 2-way wireless protocol.
• Operates in 900 MHz band.
• Devices run on batteries and communicate with each other.
• This means one device can pass along “message” to another device.
• Supports 232 devices.
• Lower power than wi-fi and longer range than Bluetooth.
• Easy to install – multiple manufacturers may be on single system.
https://internetofthingsagenda.techtarget.com/definition/Z-Wave

37. Example Z-Wave System
Device
5
Device
6
Device
7
Device
8
Device
9
Device
11
Device
10
HUB
Device
1
Device
3
Device
2
Internet
Connection
Device
13
Device
12
Device
14
Device
4
One Hub as Controller
Maximum of 4 hops from
Device to Hub
Maximum Device
separation depends
upon Device Series. 700
Series allows 328 ft
If a Device fails, signal is
rerouted to Hub.
Hub has Network ID for
isolation from other
systems.
https://internetofthingsagenda.techtarget.com/definition/Z-Wave

38. Other Protocols
• ZigBee
• Wireless Home Automation protocol very similar to Z-Wave
• Best of all devices from system are from one manufacturer
• Low cost, low power, good range, fast mesh network.
• EnOcean
• Wireless protocol using FSK modulation of primary frequency
• Can be implemented on a wide variety of RF transceiver systems
• Designed to improve performance of systems.
• Insteon
• Capable of power line and wireless for multiple message pathways
• Devices “join” home network as soon as they are powered up
• Dual-band mesh network is compatible with X-10 network.
https://www.electronichouse.com/smart-home/home-automation-protocols-what-technology-is-right-for-you

39. Other Protocols (continued)
• Thread
• Low-power network that can support more than 250 devices
• Uses same frequency and radio chips as Zigbee for mesh communication
• Cloud connection possible.
• Bluetooth
• High data bandwidth, but uses less power than Wi-Fi
• Limited range means limited use with certain devices such as motion sensors.
• No central hub required.
• KNX
• Open protocol that has been in use for decades
• Operates on power line, twisted pair, infrared, Ethernet and RF
• All units are smart units so no Hub needed plus added safety built in.
www.iotforall.com/smart-home-protocols/

40. Selecting a Smart Thermostat
Wide range of functions available, so explore possibilities
• Both heat and cool? Reverse cycle heat pump or auxiliary heat?
• How many zones? Zone control?
• How many stages?
• Switch manually or automatically between heat and cool?
• Variable or multi-speed fan and/or compressor?
• Performance monitor?
• Failure mode monitor? (condensate drain clog?)
• Local or remote control?
• Peak load or energy management tie-in?
• Humidity control, occupancy sensor, voice recognition, learning ability, . . .
• Compatible with existing system or planned system?

41. Lighting Control Options to Consider
• Plug-in modules and wall switch modules
• Small, easy-to-use-lose remote controls
• Wireless or wired technologies
• Manual or automatic Hue control
• Lighting system on motion sensor or sensors that
communicate with each other as person or vehicle enters
• Combine with light sensor for daylight/dimming control

42. Smart Irrigation Control
• Can override automatic controls from a distance
• Can adapt for weather by adapting to
• Grass/vegetation type
• Soil type
• Shading as it varies during the year
• Seasonal and daily Temperatures
• One of the popular systems likes to communicate via Wi-Fi
• So be sure to check the communications protocol for compatibility

43. Appliance Control
• Did I remember to turn off the . . . . ?
• Ask your phone (provided that you have it with you),
• Then turn it off if necessary.
• What about the refrigerator that tells the owner that it’s turned off or
that it’s not maintaining it’s temperature setting?
• What about the water heater that keeps track of energy usage and
reports any unusual usage, such as what might occur if a faucet is
leaking?
• And, of course, what about the TV that could be turned on remotely
or randomly to simulate occupancy?

44. Security Cameras
DESIGN CONSIDERATIONS
• Need to specify desired system function that may include
• Real time display only or include memory?
• If memory, how long?
• How many cameras?
• Simultaneous or sequential display?
• Communication method – wired or wireless?
• Communication protocol – one-way or two-way?
• Fixed or moving cameras?
• Indoor, outdoor or both?
• Adjustable field of view and focus? Manual or Auto?
• Availability of power for night vision or moving camera?
Cameras Recorder (a)
Monitors
(b)
Good reference: Samsung Networked Surveillance System Design Guide

45. Surveillance System Components
• Camera(s)
• Need to choose from at least 7 different formats
• Each format has different resolution.
• More resolution requires more bandwidth & usually more power
• High resolution may reduce frame rate from 30 fps to 20 fps
• Types include box, zoom, dome, infrared and PTZ (up/down & side-to-side pan)
• Typical data compression formats for reducing bandwidth requirements
• MJPEG, MPEG-4 & H.264
• Frame recorded & next frames keep unchanged elements and revise changed
elements.
• MJPEG has least compression and H.264 has most since it also compresses the
initial frame of a sequence.
Product reference: https://www.backstreet-surveillance.com/video-samples-page.html

46. Other Possibly Desirable Camera Features
• Wide Dynamic Range for use with scenes having high contrast
• Local Data Storage as backup to central recorder
• Additional power for illumination (IR or visible)
• Intelligent Video Analysis for
• Motion detection
• Face detection
• Tamper attempt detection
• Crossing a user-defined virtual line
• Enter and exit events
• Appear/Disappear events
• Network Video Recorders (NVRs)
• Network Video Recorders (NVR) channels in multiples of 2.
• System logon access often based upon HTTPS protocol.

47. Smart Door Locks
• Garage door openers have been around a long time.
• Remote vehicle locks have also been around.
• So why not do the same thing with occupancies?
• Maybe even have additional communication options?
• Link to smart phone
• Link to lighting system. Unlock door and lights come
on
• Cound be inside lights for convenience, and,or
• Outside lights for additional security.
• Battery operated, so NEC requirements, but part of
overall smart home systems.

48. Demand Control
• Has been around for a long time via utility commands to water
heaters, swimming pool pumps and a few other large energy users.
• Now utility could “ask” an occupancy to cut out xx kW of load and the
occupancy could decide what to turn off and for how long.
• In fact, the occupancy control could discuss payment with the utility
and arrive at a mutually acceptable value for kW demand.
• There are several methods of physically implementing demand
control, usually involving installation of a control box tied into a hub
or directly to the utility.

49. Energy Management
• 750.20 lists loads not subject to EMS control, including
• Fire Pumps
• Health Care Facilities
• Emergency Systems
• Legally required Standby Systems
• Critical operations Power Systems
• 750.30(A) prohibits overriding load shedding controls associated with 750.20
loads
NEC 750 Definition of Energy Management System
“A system consisting of anyof the following: a monitor(s), communications
equipment,a controller(s) a timer(s) or other device(s) that monitors and/or
controls an electrical load or a power production or storage source.

50. 750.30(B) lists loads for which EMS is not allowed
to disconnect power.
• Elevators and other equipment designed for moving people
• Positive mechanical ventilation for hazardous (classified) locations
• Ventilation used to exhaust hazardous gas or reclassify an area
• Emergency lighting circuits
• Health care facilities essential electrical systems

51. Some Energy Management Myths
• Timers on electric water heaters save lots of energy.
• Replacement of water heaters with tankless water heaters saves $$.
• Turning fluorescent lights on and off uses more energy than leaving
them on.
• Swimming Pool pumps need to run continuously.
• Swimming Pool pumps need to be run at least 8 hr/day.
• Surge suppressors save energy.
• Power factor correction capacitors on residences save energy.
• Lighting dimmers do not save energy.

52. Some Energy Management Facts
• Using LED bulbs saves energy.
• Make sure dimmable bulbs + LED dimmers.
• Minimizing run time for swimming pool pumps saves energy.
• Installing smaller HP or variable speed pool pumps saves energy.
• The higher the SEER, the more the A/C energy savings.
• Expensive refrigerators are not necessarily the most efficient.
• Using less hot water saves energy.
• Reducing water heater temperature saves energy.
• Energy Star appliances save energy.
• Variable speed control on certain loads can save energy.
• Motion sensors, such as on garage lights, can save lots of energy.

53. PowerWalls and Energy Storage
• Small energy storage systems have been around a long time.
• Historically, three types of PV systems have been developed that
employ storage:
• Stand-alone systems
• DC-coupled grid-connected systems
• AC-coupled grid-connected systems
• But these systems have been a bit costly
• And also sort of complicated
• They required on-site programming
• So most people ended up with this
M
Utility
MAIN
DISC
MDP

54. Then along came Mr Musk and his Tesla Battery
M
Utility
MAIN
DISC
W/FT
LugsInterruptible
Loads
Automatic
Relay
MDP
with
main
CB
Uninterruptible
Loads Combiner
Panel
PV
Inverter
PV
Array
Power
Wall
rectifier
inverter
• Utility + sun results in fully-
charged PowerWall and power
to MDP and to interruptible
loads and maybe back to utility.
• Utility + no sun results in only
utility power to all loads.
• No utility results in no power to
interruptible loads
• No utility + sun results in PV
power + maybe some PW
power to UI loads
• No utility + no sun means PW
has to supply UI loads.

55. Sizing the PV and the Storage
• Determine max daily total load from max monthly kWh bill.
• Ex. If max monthly kWh is 1200, then 120030 = 40.
• Estimate desired uninterruptible load by analyzing daily kWh of each
desired load.
• Ex. Leave out hot water & A/C & dryer – maybe about 20. This leaves about
20 kWh/day for all other loads.
• So it would be nice to make 20 kWh/day with PV.
• Ex. On average a PV array will produce about 1500 kWh/kW/yr, so producing
20 kWh/day will require about a 5 kW PV array, which is a typical residential
size. Note that NREL SAM enables relatively reliable calculation.
• Then it would be nice to store 20 kWh/day
• So select a storage unit (PowerWall) with about 20 kWh of useable storage.

56. What Will All This Do To The Grid?
Generation
Sub-station
(Distribution)
Transmission
xfmr
load load load
xfmr
load load load
xfmr
big
load
Generation
Sub-station
(Distribution)
Transmission
xfmr
load load load
medium
storage
big
storage
big
renewable
medium
renewable
xfmr
big
load
baby
storage
baby
storage
Bidirectional
Link baby
PV
baby
PV
small
PV
small
storage
Future
Grid
Today’s
Grid