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Solar Power to recharge Tesla Model S with 60 Hwh battery.

Has anyone figured out what size battery system a person would need to store solar energy so it could be transfered when needed to recharge this EV assuming a 90 % depletion of the EV battery pack? I am having a local solar installer put in 88 of the 240 W 12 VDC solar panels to create 21 KW of solar energy but they are not sure how to size the battery storage system. We live in the mountains of NC. This will be their first system for the Tesla EV and I would sure appreciate any advice or experience that anyone is willing to share. Thanks, Larry

I have a quantity of 20 240W modules to power my two 40Kwh Teslas, that sounds to large, unless you are also going to power the house.

You need something like between 300W to 425W per mile, depending on how fast / hard you drive.

Simplistic answer: 111 % of the amount of juice you want to store for your car. Or to keep it simple, if you want to recharge an 85 kWh battery, you need close to a 100 kWh battery to do it, assuming one is near empty when you start, and the other is near empty when you finish.

Much simpler to have a grid tie system, and let the grid be your battery.

Or 67 kWh battery for the 60 kWh Model S, if it is to be fully recharged.


No need to store if NC buys power. You feed the grid in the day, and your meter runs backward. At night the tesla feeds.

is it possible to empty one battery into another? I'd always assumed you could only "average" them, to a midpoint. What's doing the pumpimg?

For ac stepup, its easy with a transformer. For dc, putting batteries in series increases voltage, or as you said, pumping. Dont take this to the bank though. Jat would be a better authority

He still has to charge the car with 240V 40A AC (80A with dual chargers and HPWC).

Yeah, best to feed the grid.


Please do keep us posted on your experience as planning to do same when I get mine in April.


There are a bunch of other factors too. How many cloudy days do you want to accommodate? What's your daily driving range and corresponding consumption level? Do you need to store multiple days worth of consumption? How much energy will you need for other purposes like powering your home or business?

As others have said most utility companies act as a really big battery, almost for free. So that's usually the most cost effective approach unless you want to really operate off-grid.

@gtc. If voltage of battery A is greater than voltage of battery B then charge flow from A to B till A is drained. The analogy is two water tanks connected with a pipe and if one tank is on higher ground then all the water will drain into the tank that is at a lower level.

Bad news: unless you can figure out how to do a DC charge from your storage batteries at home, you will need an inverter to provide AC to the car. This means that you will need to go through TWO conversions, each at 80-85% efficiency. This means that as little as 64% of the energy in the storage battery will make it to the car. So, to charge 90% of an 85 kWh battery would take .9*85/(.825*.825) = 112 kWh of storage. Unless you're off the grid, this is unlikely to be cost-effective. See my electricity cost calculations at

That does not sound like a description of this situation. At some point voltage and charge will match, and flow will stop. Without "draining" A.

I plan on using solar and batteries as well. I don't like the idea that the grid buys it from me at wholesale and I buy it from them at retail.. retail keeps going up around here..

I don't have it completely figured out but my current estimate is 16 panels charging 18 deep cycle batteries. I am probably way to large on this system but I plan on building it up from 8 panels and 9 deep cycle 150 ah batteries.

Very inefficient process. Our 111 panels are grid connected and we sell the excess. Batteries are a whole other level of conversion and wasted energy, plus huge upfront and maintenance issues. Best method is grid connection.

ziggy, you may not "like" it but it is cutting off your nose. If you don't go grid connect, you also forgo SREC credit sales, which for us average over $800 per month in CASH payments that we receive on top of selling the excess back. By the way, you aren't selling back at wholesale and buying retail. They only pay wholesale for the excess over your net usage. You get retail credit for your your actual use. When you have solar panels, you become an electrical producer. Why should the utility pay you any more than wholesale? It would be immoral, at the expense of rate payers.


Xantrex makes an inverter that can balance the grid and batteries. It's the XW6048. Unless your battery bank is ridiculously over-sized there will be times you'll have to conserve energy and times your panels sit idle unless the grid is available. SMA also has several great products for increasing the self-consumption of solar homes but most of these are currently only available in Europe.

I do agree that at present batteries don't make a lot of sense... Grid-Tie is the way to go; You'd have to be getting screwed by your utility pretty bad to make going "off-grid" worth it.

I am told Nissan fast chargers are incompatible with Tesla MS because they are DC. However, super chargers are DC too. Any one know what the issue is beyond plugs and connectors. We have Nissan fast chargers at work and a tesla and Volt folks cannot use them.

Communication between battery and charger are different, voltages are different, connectors are different. That's about it in a nutshell.

I have an issue here with not being able to tie into the grid, because they are installing an 800 mega watt solar panel array ( thousands of panels) about 1000 yards from my house, so there is no capacity left in the grid for my little system. Therefor the only way for me to use solar for my car is with a battery (of grid) system. So I would like to know how big a system I would need to power my 85. Buy the way I am in Canada and here they pay you .56cents/kwh for solar systems, and we pay about .22cents/kwh after all the delivery charges, taxes and other charges are factored in. I wished I had ignored my wife and installed a system 5 years ago when they were paying .88 cents/kwh.

Behold the reason Ontario is headed for the deepest energy cost pit this side of the UK.

I figure the 310 Kw (420 HP) would be enough to power a larger motor home. To simply, let's say the battery system was removed. Does anyone know the approximate size (No. of 225 W panels etc. run in a compatable wattage etc.) it would take to provide all energy requirements to run continuously throughout the day (provided a good day of sun etc.) (all variables assumed to be optimal/standardized to simplify). In other words, is there enough room on the roof of a 42 foot RV essentially to contain roughly 10,15,20 225 W solar panels linked in a series run to the engine to provide sufficient power. (Battery supplementation can be considered seperately).

At noon, south of 40°N and north of 40°S, on a cloudless day, between March and September. ;)

Well, a lot depends on your roof, location etc. I am in the NE, I have a 4.05 KW system on my roof (18 Sunpower 225). That system on average generates 15KWH/day year that math, you would need a system that is 5.1 times larger than mine, or a 22KW system to replenish an 85KWH battery pack to 90%. But again, much will depend on your location, etc.....

and btw, that would be one big system...

Nano solar ink

Search - Sun Plus Nanotechnology: Can Solar Energy Get Bigger by Thinking Small?

Not a chance there you could put enough solar panels on top of an RV to get it to operate in a cloudless day at noon on the equator.

...even if the solar panels operated at 100% efficiency.

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