Information about using your EV battery to power your house

RonR Registered Users Posts: 7 ✭✭
There has been some rumbling (Ford for example) about using your EV battery as a power backup for your house. Does anyone have any detailed info?
I'm in Seattle with a badly oriented roof. Short of rotating the house by 90 degrees it is difficult to justify going to solar no matter how many panels I add but .....  adding the combined benefit of solar charging an EV and using the same EV to provide emergency backup power makes it all more compelling. 
On top of that, I believe the UK has started some pilot programs where they use private EV's as national grid battery storage. I'm guessing the concept is that during low demand all connected EV's are charged to max and at high demand connected EV's are discharged to some agreed level. Seems like it might be a nice way to flatten the demand curve.


  • BB.
    BB. Super Moderators, Administrators Posts: 33,500 admin
    Looking into the engineering details--This article has some information (February 2021 article):

    Does V2G work in all electric cars?

    Unfortunately, not yet. The car needs to be compatible with a CHAdeMO DC connector. Only the Nissan Leaf and Nissan e-NV200 are V2G compatible at the moment. But it won’t be long before other manufacturers are joining the V2G bandwagon. 

    So--There is a charging interface that does support bi-directional energy flow (high voltage DC, I guess).

    And there is a software platform (for UK presently?) that uses the Internet to manage vehicle/charger/grid interfaces. (case study)

    Some cost estimates:

    It looks like a functioning system... Claim in the future, a vehicle owner can have a 5 year return on investment (for charging dock/installation?) based on buying power cheap (off peak) and selling expensive (on peak).

    I am not a huge fan of using storage systems (only) to balance the grid... There is no energy being created here--Just shuffling around between Grid (use it or lose it electric model) vs expanded storage + Grid (or V2G -- Vehicle to grid).

    If I use 95% efficiency between each energy transformation step (could be a bit better, could be worse--Just a model), each full cycle (grid to dock to car to doc to grid) would be overall (just a SWAG):
    • 0.95 G2D * 0.95 D2B * 0.95 B2D * 0.95 D2G = 0.81 "in home" energy cycle efficiency
    The present electrical transmission system overall efficiency is around 50%... So one loses 50% from generator (fossil fuel plant, solar, wind, etc.) to home... There is also some loss from Home back to Utility Grid--50% (if energy is transmitted long distances... Maybe 90% efficient if Home A to pole transformer to Home B load... Or
    • 0.81 "V2G" efficiency cycle * 0.90 V2N to neighbor home = 0.73 = 73% SWAG "best case" energy efficiency using local storage for "local" grid balancing.
    • 0.81 V2G * 0.50 Long distance V2E "to everywhere" long distance = 0.41 = 41% SWAG "worst case" energy efficiency using local storage for "country wide" grid balancing
    Remember, at this point, no energy was created, just destroyed--Conversion and transmission losses. And actual energy usage (heating, lights, charging somebody else's non V2G car, etc.).

    Still need to generate that electricity in the first place.

    We have already seen that Wind and Sun are both highly variable (can have days or weeks of dead calm. And winter in northern countries, solar does not harvest much).

    We either need days to weeks of "storage" or energy generation that is not weather/location dependent...

    A Nissan Leaf has a 40 or 62 kWH battery bank (2022 model):

    The average home uses around 500 to 1,000 kWH per month (north American average). The average EV owner will want a minimum 30% reserve for driving (per Kaluza case study):
    • 500 kWH per month home (assuming natural gas, propane for heat or very conservation minded all electric home)
    • 500 kWH / 30 days per month = 16.7 kWH per day
    • 40 kWH Leaf standard battery * 0.70 available for V2G grid storage = 28 kWH of "grid storage"
    • 28 kWH storage / 16.7 kWH per day average home = 1.67 Days of "available" V2G storage with Leaf--Assuming car is not charged to 100% by next day for driving
    So if "every home" had V2G Leaf--At best this will take care of 1 to 2 days of energy storage for "variable sources" (Wind/Sun) and variable loads (cold weather heating, hot weather A/C)...

    Today, the grid is designed to operate for years (exgeration) in with any loads... Winter heating, summer A/C, charging the small number of EVs today (something like 1%? of vehicles?)... How much does each major class of energy consumer use?

    Very roughly 18% for Commercial, 33% Industrial, 26% Transportation, 22% Residential.

    So, if 100% of transportation was V2G (including aircraft?), it would just cover residential usage... And leave ~51% of the loads uncovered.

    And in any case, still need "something" to charge the battery banks.

    Looking at using GT Solar (most efficient, cheapest, no storage) for your home. Say 500 kWH per month or 16.7 kWH per day average usage. If you had a south facing array, your sun harvest would be:

    Seattle (south facing array)
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 42° angle from vertical:
    (For best year-round performance)


    Seattle (East facing array)
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 42° angle:
    (For best year-round performance)


    You lose around 1 hour of sun per day in Summer with an East facing array... And about 0.5 hour per day in Winter... Not great. Add all months together and divide by 12 months a year, and we get 2.89 hours of sun year round average. To make 500 kWH per month average power usage via GT solar:

    • 16,700 WH per day * 1/0.77 panel+GT inverter deratings * 1/2.89 hours of sun per day (over year) = 7,505 Watt solar array to "break even" for 500 kWH per month in Seattle.
    • 7,505 Watt array * $5 per Watt (rough guess) for GT solar = $37,525 for GT solar on East facing roof (again, pure guess--Prior to last year's inflation)
    And note that V2G and GT solar do not provide any backup power if grid goes down.

    And if you added hardware (not that expensive in commercial volume), you would only have backup power to your home if your car was plugged in... Drive car to work, no battery to run home AC system. And your car (during a power failure) would probably have something like 50-30% battery capacity to do your driving that day (charge at work?)

    Anyway--An interesting (at least to me) thought exercise. The numbers above are very rough estimates and could be way off for V2G energy cycle efficiency... But it at least gives us something to hang our hat on.

    GT Solar for an East facing home (with no shading from trees/buildings/power lines) is not great at 2.89 hours of sun per day--And only use can decide if it it worth the expense to your family/home. And you need to check with your electric company on their Net Metering plans and policies for your utility... As time goes on, the polices are becoming less "GT Customer friendly"--I.e., less GT Customer subsidies from the rest of the rate payers/state tax breaks.

    In the end, we still need to generate electricity somehow... And until (if ever) electrical storage becomes cost effective/practicable for weeks/months of energy storage (like fossil fuels today--Tank farms, pile of coal, Nuclear, hydro, etc.)--The needs still need to be addressed.

    Your thoughts?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.
    BB. Super Moderators, Administrators Posts: 33,500 admin
    California and GM looking for V2G. Handwaving at moment?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RonR
    RonR Registered Users Posts: 7 ✭✭
    Thanx for the very well thought out evaluation (FYI, any call to any Seattle solar vendor will always result in it being a great idea with ROI in less than 8 years and don't you want to be good to the earth. None return a call after you ask any tough researched questions). Yup, marginal, at best, in Seattle but potentially useful if it provides some other features.

    I came up with a back up duration of 4 to 6 days but I assumed it was an emergency situation and many big drains (microwave, misc lighting, water heater, etc.) would be turned off in favor of the furnace and fridge. In the Seattle area, outages are 90% during the winter with about .5 2 day outages per year. I use gas for heating and cooking. It is still the most cost effective way to do it, even in a low electrical cost area like Seattle.

    You'r "no new energy generated" is very on point. But, there is a little more to it. If peaks can be flattened, by use of local batteries, then there would be less use of costly and generally less efficient and more polluting peak energy sources like gas fired turbines. Also, small local batteries (such as in an EV) really only have enough energy to flatten the energy consumed in that house or neighborhood so it would not be burdened by the inefficiencies of distribution to the entire network.

    I think it is a viable next step. Thanks for the links. I plan to do a little more research and potentially do a panel install this summer in anticipation of the V2G concept (and before the federal rebate goes away). We had a state solar rebate but it was capped and fully utilized last year. I think the combined rebates is what local vendors are still selling (at least until you mention that the state rebate is gone and they don't call back).

  • BB.
    BB. Super Moderators, Administrators Posts: 33,500 admin
    Hi Ron,

    Actually peaker Combine Cycle Natural Gas Turbines are pretty efficient... And natively less CO2 per kWH compared to Coal (I think--There are CO2 scrubbers out there--So don't know how many/end results).

    In the end, peaker plants are expensive to run (i.e., Natural Gas tends to be more expensive)... And in our area (California), it looks like peaker plants are in financial trouble--Only "make money" if generating electricity.

    For you... Just some example math... Turns out, for example, a 300 Watt desktop computer running 10 hours per day vs a "low power" laptop for 10 Watts @ 10 Hours per day. And a 1,200 Watt microwave running 20 minutes (1/3rd hour per day):
    • 300 Watt desktop * 10 hours per day = 3,000 WH per day = 3 kWH per day
    • 3 kWH per day * 30 days per month = 90 kWH per month
    • 10 Watt desktop * 10 hours per day = 100 WH per day = 0.1 kWH per day
    • 0.1 kWH per day * 30 day per month = 3 kWH per month
    • 1,200 Watt microwave * 1/3rd hour per day = 400 WH per day = 0.4 kWH per day
    • 0.4 kWH per day * 30 days = 12 kWH per month
    • 120 Watt full size refrigerator * 24 hours per day * 0.50 compressor duty cycle = 1,440 WH per day = 1.44 kWH per day
    • 1.44 kWH per ay * 30 days = 43.2 kWH per month
    With an "all natural gas" home--You can probably get under 200 kWH per month pretty easily--(takes work to stay that low year in and year out)--Electrical loads just keep growing...

    Just an example where loads that run many hours (or 24 hours) per day can be huge energy usage hogs... And the "big appliances" like a microwave that only runs 20 minutes per day. Need to looks at the total power and energy usage for each major home load.

    Using a Kill-a-Watt type meter for measuring single appliance loads is a big help.

    And if you have large connected loads like 240 VAC A/C system, electric stove/water heater/well pump/etc... Then you can look at "whole house monitors:

    The above are just examples of equipment for you to begin your search--Lots of options these days.

    Say you can "live" on 200 kWH per month (100 kWH per month is suggested for a very energy efficient smaller home/family). Just some numbers:
    • 200,000 WH per month / 30 days = 6,667 WH per day (medium sized off grid solar power system)
    • 6,667 WH per day * 1/0.52 off grid system AC efficiency * 1/2.25 hours per day (Feb "break even") = 5,698 Watt array (Feb break even)
    Battery bank... 2 days storage, 50% max planned discharge (Lead Acid batteries):
    • 6,667 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max discharge * 1/48 volt battery bank = 654 AH @ 48 VDC lead acid battery bank
    Double check minimum rate of charge (10% minimum rate of charge to 13%+ for off grid full time system):
    • 654 AH * 58.0 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 4,926 Watt minimum array for full time off grid suggested.
    Figure out if you can run your home on a small genset (Honda eu2200i or similar "2 kWatt" genset). For example, we ran my in-law's home on a eu2000i genset for 4 days on about 3 gallons per day... 2x refrigerators & 1x freezer, and a koi pond air pump. A few LED lights, and really no other loads (used gas stove and oven to cook... Did not use central heat--Our area does not get cold during "wild fire seasons" (or utility lighting fire season from poor maintenance with high voltage lines).

    10 gallons of gas will run your "emergency loads" nicely for ~3 days with a smaller genset... More gas (20+ gallons of storage + fuel stabilizer). "Pickel" genset after fire season/usage (dump fuel, run until dies, drain carburetor, put a teaspoon of oil down sparkplug hole)--Several gensets stored for upwards of 10 years so far--Started just fine when needed.

    Put fuel stabilizer in gas cans, recycle gasoline to cars and refile with fresh fuel once a year (or if you can store propane--Another option).

    If you get install a Hybrid inverter system (both GT Function, and Off Grid AC inverter function)--The inverter will feed "excess" solar energy back to home and too grid--Save some energy bucks there (More expensive system, have a full size battery ban that lasts 5-10 years or so)....

    For me--I have GT Solar (no backup power) and a 2 kWatt and 3 kWatt gensets (backup genset if needed).

    So far, don't have enough power outages to justify full Hybrid solar system (including battery bank). Just runs the gensets rarely (one home, 8 hours in 10 years--Second home was 4 days in 10+ years). Gensets plus stored fuel (or natural gas fueled gensets) made more sense.

    You can go with a nice integrated genset for $6,000+ natural gas (weekly auto test, full auto start/run if power failure, automatic transfer switch). Or a simple manual genset + some sort of manual transfer switch and just store genset until needed... I am cheap, went with the second option.

    So far, I did not use natural gas as fuel--I was preparing for earthquakes (natural gas may be shutdown) and the stored gasoline can be used for cooking or car to bug out if needed.

    With California wild fires and more "precautionary" power outages--Perhaps need to revisit Natural Gas as genset fuel source.

    One friend used a Onan RV Genset as his "all in one" home backup.

    Anyway--Lots of options.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MJSullivan56
    MJSullivan56 Registered Users Posts: 42 ✭✭
    the easiest “personal” solution would be 240v output from your EV to the generator input of your inverter. Easy peasy. 
  • BB.
    BB. Super Moderators, Administrators Posts: 33,500 admin
    Yes, that is the "easiest"... However, depending on the size of the inverter (3-4 kWatt minimum for a "house type" inverter taking space and adds to the weight ~50 lbs or more) of the vehicle. Since weight and vehicle aerodynamics are critical to reduce drag/rolling resistance--I doubt any car mfg. will add that to a car. It would not be worth the money ($1,500 or more) added to price of the new EV) and loss of vehicle efficiency to haul an AC inverter around that should be--Instead--Installed at the home for V2G/Emergency Backup power, etc...

    Instead, have the DC of the car sent to a "Hybrid" AC inverter (GT and OG capable) mounted in the garage... And the EV car is simply the "battery bank" for the system.

    But even then, what is it that people expect from such an installation. Grid Balancing (some cost savings to owner) and emergency backup power--As long as the EV is parked in the garage... Take the car out to work, out shopping, etc.--No battery bank, no backup power for 10 hours a day. Or--Need a 1/2 size or full size battery bank at home too for 24x7 backup power coverage--A home battery bank is probably 1/2 the price or less ($$$/kWH of storage) for a "simpler" fixed battery bank at home vs the specialized (and expensive $$$/kWH) EV battery bank (cooling/heating/compact/lightweight/small size/etc. needed for the EV.

    Nothing comes "for free".

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    you can also see 
    pass the test on the welcome screen, and there is a wealth of knowledge there. 

    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • MJSullivan56
    MJSullivan56 Registered Users Posts: 42 ✭✭
    Well the Ford Lighting has 120Vac out for powering onsite tools, so clearly they felt it was worth adding a transformer to the mix. Hmmm… it Just might be possible to swap it out for a 240Vac transformer…
  • RonR
    RonR Registered Users Posts: 7 ✭✭
    check out:

    It looks like the US is a little behind the power curve for effective use of the national EV fleet.

    My use case is to improve the economics / fully utilize a solar panel install (GT) and an EV. The economics of doing this in the Seattle area are weak since there is little sun and electricity is already pretty cheap. So why not use your EV battery for home backup if the incremental cost is low?

    I wanted to avoid having a generator parked in some corner that only occasionally get's used but needs regular maintenance as well as annual (if not bi-annual) drain and replacement of gas.

    It looks like the UK is trending down the path of having class 2 type EV residential charge points that support bi-directional power flow (DC, the inverter is in the charge point or home). How sweet, a tree falls on the power line, you fly through the night using your EV battery and small home inverter, a little recharge from solar the next day or recharge from the charge point at work, then back home for another night using the EV. Yup, no energy while the car is gone but it is an emergency not normal operation. If you don't use a microwave (or other high load devices) a fairly small inverter (possibly 1000W) can keep your furnace, fridge and lights going.

    Seems like this is not yet a thing in the US. The incremental cost, on the manufacturing side, appears to be fairly minimal. Sadly, hacking into your EV to get to the battery would be problematic since the DC voltage is likely not compatible with any inverter you could find and the BMS would likely be confused or non-functional (so you could over discharge and damage the battery or only discharge some of the cells causing serious balance issues). Don't even think about using your warranty if you hack into your battery.

    I only bring this up since the federal subsidy decreases next year so if this is something likely to happen in the next year or so it would be worthwhile installing panels this year.