Electric Car + Off Grid Solar Home

Electric Car + Off Grid Solar Home

So, the big problem with off grid solar is that battery packs are very expensive. But with the onset of electric cars, people may be investing in large and expensive batteries anyway. Seems like this could be the perfect solution for someone looking to eliminate all ties with the utility company without buying a special battery just for home solar.

I saw some postings online about the Nissan Leaf being able to feed energy from the battery back to the grid at night or even during the day (during high demand). Is this something that Tesla is looking at offering or do they already have such a system available?

Does this idea appeal to anyone else? Why or why not?

DHrivnak | 29 August 2012

I would love that option if available. The Tesla Roadster can power my house for a good 2 days which is longer than we have ever been without power.

evanstumpges | 29 August 2012

According to the EIA, the average residential home uses 11,496kWh of electricity per year (31.5kWh per day). Right now I'm living in an apartment and only consume about 3.5kWh per day and I suspect other eco conscious homeowners with PV installations use quite a bit less than the average. Seems like even a lower capacity EV like the Leaf, Focus, and Fit could meet the residential power requirements of many people for at least a few days. Heck, with a Tesla pack running essential residential electrical devices during a few days of bad weather probably wouldn't put much a dent in the pack's capacity...

For a while, I've been trying to think of a way to make an off-grid residential installation more attractive, but this just hit me today and it seems like a no brainer to me. Not sure about everyone else out there, but the transmission, distribution, fees, and taxes in my electric bill cost more than the electricity. I'm sure grid tie systems cut down a bit on these extra costs, but they still exist and you are still dependent on the utility companies.

Brian H | 29 August 2012

Sole dependence on car and solar: what happens when it's dark AND the car is away? Freezer defrosts, electronics all reset, etc.

Volker.Berlin | 30 August 2012
Timo | 31 August 2012

I think there will be used Roadster battery packs on sale soon (as in couple of years). Maybe is you get one of those instead of full car to complement your solar setup. Assuming of course that Tesla doesn't want them back when swapping to new one.

evanstumpges | 31 August 2012


Good point. In 3-4 years there will probably be some used EV packs on the market that could be cost effective for home solar.

@ Brian

I see your point about the freezer defrosting. Might still need at least a small battery pack or generator to kick in at times.

Docrob | 1 September 2012

Brian, you just don't make it sole dependence. The average off grid home is set up for very low background consumption, eg standby devices on wall switches, solar hot water etc. this means that the loads while the car and people are away is very low. Therefore you can have a much smaller home battery bank which buffers the very small load whilst away which is essentially the fridge only, efficient ones use 0.5-1kwh per day, therefore using the 3-5 day rule you would need a battery bank ~5kwh max. A normal off grid home would have a battery bank of 20-40 kwh. Then the cars battery is added to the system exactly when it is needed, when people are home, then all the added loads with people present are buffered by the much larger vehicle battery. V2H will not do away with the need for storage for off grid homes but it can significantly reduce the size and therefore cost of the required battery bank.

jerry3 | 1 September 2012

Docrob -- 3-5 day rule you would need a battery bank ~5kwh max

The UPS to run my equipment at home is bigger than that :-)

Brian H | 1 September 2012

What's a freezer's load?

evanstumpges | 2 September 2012


Refrigerator/freezer load can vary a lot depending on how efficient it is. The fridge at my old house was quite old and had an average load of about 80W. A quick search on Lowes for new Energy Star units showed models in the range of 40W average load.

For the sake of argument, let's say your efficient solar home needs 50W to keep the fridge and any other essential electronics running. If you want a backup battery to last 3 days in the even your car isn't available and there is no solar energy available for some reason, here's what you'd need.

3[days] * 120[hours/day] * 50[Watts] = 18,000Wh = 18kWh

Unfortunately, this is still a fairly large and expensive battery pack. But an old Nissan Leaf pack or equivalent might fit the bill in a few years.

Even then it might still make more sense to have a small natural gas generator as a backup power source. A small unit can be had for under $2,000.

Brian H | 2 September 2012

I was thinking of a large freezer chest, suitable for holding a butchered deer.

jerry3 | 2 September 2012


It's thought that LPG would be more available in a disaster. Natural gas lines are often cut.

Timo | 3 September 2012

@Brian H, freezer size doesn't actually matter much if the insulation is good and it is already at target temperature. Then it is just matter of maintaining the temperature, and even huge freezers can be very good at maintaining the temperature.

Because cold air is more dense than hot, chest freezers usually are better than the other kind of freezers at keeping the cold in (cold air doesn't escape when you open the door).

Fast net browse gave around 50-60W power requirement for large chest freezers.

@evanstumpges, last time I checked one day has only 24 hours, not 120 (unless you are having one of those "endless Mondays" -weeks). Where did you get 120 there?

evanstumpges | 3 September 2012


Wow, glad I posted my math there for you to catch that glaring mistake... The 120 actually came from the number of hours in 5 days (24 * 5 = 120), which is the duration I looked at initially. No idea how it worked itself into the hours per day slot. My bad. Corrected math is as follows:

3[days] * 24[hours/day] * 50[Watts] = 3,600Wh = 3.6kWh


5[days] * 24[hours/day] * 50[Watts] = 6,000Wh = 6kWh

Much more reasonable values. The 3.5kWh Li-Ion pack for the Iowa State Solar car cost about $3,700 so still not a drop in the bucket but for me that's not out of the question.

RNB | 18 September 2012

Why is being disconnected from the grid a good thing?
Why eliminate all ties?

The case for being grid tied is a strong one. One can sell power to the utility and have emergency backup when the sun don't shine. It also saves money by not having to buy batteries, additional equipment, and proper storage facilities.

I am qualified to talk about this:
Potential 3rd row of panels is for Ecar

Teoatawki | 18 September 2012

Why is being disconnected from the grid a good thing?

1) Some places even in North America are too far from the power grid to connect up economically.

2) So law enforcement can't tell how many grow lights you have in your basement pot farm.

RNB | 18 September 2012

Why would being tied into the grid tell LEO about grow lights if you produce your own power? They do not know what you use.

Being too far from power sources does not make it a good thing, it makes it needed. It does not make it good for me.

Vawlkus | 18 September 2012

For when the apocalypse comes of course.

Brian H | 18 September 2012

Mad Max is/will be hell on solar panels ...


stevenmaifert | 18 September 2012

Teoatawki Item 2: True, but 100 panels on a log cabin in the woods might still raise a suspicion :)

Being tied to the grid is fine until the grid goes down. Without a back-up like in my link above, you are still in the dark because your inverter, which senses power on the grid, will automatically shut down and disconnect from the grid for the safety of power company workers.

While it's true most utility companies are required to compensate their solar customers for excess generation, don't get solar with the idea of making much money on that. For example, in 2011 I had 2,993 kWh of excess generation. San Diego Gas & Electric sold that power for a minimum of 15 cents/kWh. They paid me .03699 cents/kWh. Wouldn't we all like to have profit margins like that. I'm not complaining. I purposely over sized my system in anticipation of a Model S purchase. Once the car comes, excess generation won't be an issue.

Docrob | 21 September 2012

"Why eliminate all ties?"
For some it is ideological or others financial. Friends of mine were building on a rural block, the nearest power lines were 2km from the house, the quote for power lines to the house? $2,500 per 100m or $50,000 total, of course you then still have to buy the power for ever from the utility, $50,000 buys a very decent off grid solar array with a mid size wind turbine for winter which, once bought, supplies power essentially free for the remainder of its life.

Vawlkus | 24 September 2012

Hope he has Satelite TV, cause he's not getting cable :P

Ellipsys | 11 May 2015

Using the EV battery to power the home at the time of peak consumer power usage would help the grid balance loading.

There must be many 2car families that use one car only for short periods usually later in the day once or twice per week as I do.

If they could invest in an EV and use the battery energy storage during peak loading when electricity is charged at 46cents per kWh and off peak cost is 9cents per kWh.

The saving over 12 months would warrant paying the extra cost of the EV and would benefit the electricity grid.

rlwrw | 11 May 2015

Theoretically, if one generates a lot of excess power, one can apply to be a power producer, and sell to the grid. This gets around net zero regulations. However, if one goes out and "sells" x amount of power, then one has to put a minimum of the same amount of power on the grid. One can be a producer and not have to sell.

Ellipsys: Anything to do with "Casino Royale?"