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

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

Chuck Lusin | 6 marzo 2013

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.

Chuck Lusin | 6 marzo 2013

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

GoTeslaChicago | 6 marzo 2013

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.

GoTeslaChicago | 6 marzo 2013

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

jbunn | 6 marzo 2013


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.

Brian H | 6 marzo 2013

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?

jbunn | 6 marzo 2013

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

Chuck Lusin | 6 marzo 2013

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

jbunn | 6 marzo 2013

Yeah, best to feed the grid.

thomas2V | 6 marzo 2013


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


SUN 2 DRV | 6 marzo 2013

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.

crttnarayan | 6 marzo 2013

@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.

EVTripPlanner | 6 marzo 2013

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

Brian H | 7 marzo 2013

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.

ziggy | 7 marzo 2013

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.

Pungoteague_Dave | 7 marzo 2013

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.

Pungoteague_Dave | 7 marzo 2013

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.

Brian H | 7 marzo 2013


nwdiver93 | 7 marzo 2013

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.

crttnarayan | 8 marzo 2013

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.

Timo | 8 marzo 2013

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

BAMMM | 8 aprile 2013

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.

Brian H | 8 aprile 2013

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

willwebreathe | 8 luglio 2013

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).

Brian H | 8 luglio 2013

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

gasnomo | 8 luglio 2013

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.....

gasnomo | 8 luglio 2013

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

tsimonel | 23 giugno 2014

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tes-s | 23 giugno 2014

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.

tes-s | 23 giugno 2014

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

PV_Dave @US-PA | 23 giugno 2014

Grid-tie is absolutely best when available without tens of thousands of dollars in connection fees (i.e remote locations). It's certainly possible to power your house and Tesla off PV/battery, but I would consult a professional on sizing if you're not highly experienced in solar already. Lots of variables, and off-grid they need to be right. When grid connected a sizing error has no capacity implications, just cost. Off-grid, if you deplete your batteries you're stuck, and if you over-size them you're wasting thousands (even tens of thousands) of dollars. Your driving habits alone will have a huge impact on the costs, i.e. is the car typically home during the day, how many miles per year, peak miles per week, etc.

For off-grid car charging I'd look seriously at the SMA Sunny Island products, as they're very flexible per inputs and scale well. Fortunately the Tesla allows you to specify whatever max charge rate you need. But unfortunately this is still not a simple problem to solve well, unless you don't mind spending more on the solar/batteries than the car.

tes-s | 23 giugno 2014

It would be nice if Tesla and Solar City would get together and create an off-grid offering. I doubt they will since it is more academic that real - most people will be grid-attached.

I would like a system like that to have DC charging for the Tesla, and a complicated algorithm to determine where to prioritize the power, with some manual overrides - if you are going away for 2 days, you want to get the Tesla fully charged and don't care much about the house batteries which can charge while you are gone. If you are not going to drive for a couple of days, then you would want to prioritize charging the house batteries.

Big thing would be DC charging. Silly to waste energy converting DC to AC and then back to DC to charge the Tesla. Even for people with grid-attached PV systems, wouldn't it be more efficient to use the DC from panels to directly charge the Tesla? | 23 giugno 2014

The problem with Solar DC is the voltage can range from 600V to 0V, and the charging the car via DC requires an exact voltage (current can vary). The Supercharger and in-car chargers deliver the exact voltage required to safely charge the batteries, which will vary somewhat as the car charges up. If you put too high a voltage, you will destroy the batteries. Too low, and either you'll get no charge, or worse, you'll drain the batteries (and likely fry the solar panels).

To convert any DC voltage to another DC voltage efficiently requires conversion to AC. This can be done at higher frequencies than the 60 Hz house current for improved efficiency and lower costs. No matter how it's done, you'll be forced to go through a DC-AC-DC conversion. No shortcuts allowed!