# Charging an electric car with solar panels?

Biks: I’m not sure you understand. An off-grid PV system already has a substantial battery system which costs a very significant percentage of the total system cost. You need the battery system to handle electrical usage at night or when cloudy as well as peaks of usage.

Why don’t you give us some numbers as to how much you expect to drive and someone might give an estimate of how big a system you need.

And the consider the overall cost of that solar setup vs how much power you expect to get out of it over the lifetime of the system. Hint: It’s not “free”.

The OP said money was no object, and he has acres of land, so he’s not worried about whether it’s practical or not.

Using Tesla’s listed specs, the Model S miles/kWh ranges from 3.12 mi/kWh (for the 80kWh battery) to 3.47 (for the 60 kWh) battery. Let’s say it gets 3 mi/kWh both to account for less-than-optimal driving and to simplify calculations. So you’ll need 8.3 kWh a day to drive 25 miles. Let’s round it to 10 kWh to further simplify things.

So you need to recharge 10kWh every day. Assuming you’re still near Worchester, MA, NREL says you get about 2.87 to 5.60 kWh/m^2 per day of sun depending on the month.

NREL assumes the panels are 16% efficient. 10 kWh / .16 = 62.5

Then you need to account for losses converting DC to AC. NREL assumes 77%. 62.5 / 0.77 = 81.17

81.17 kWh per day / 2.87 kWh per m^2 per day = 28.29 m^2.

So you’ll only need about 30 square meters of panels, which is about a 3 kW array. Here’s what that might look like; it’s not insanely large. This crude calculation roughly corresponds with the NREL calculator linked to above (input a 3kW system for your location and you get about 3662 kWh per year, or about 10 a day), but is way off from Stranger’s math… I think maybe he didn’t take panel efficiency into account? 13 m^2 seems small for a 53kWh/day system.

Solar Voltaic is now about .40 to .45 per watt before subsidies.

I have a SV array on my house. 8.5kw. It makes more than all the power I use, I could charge a car with it if I had a plug in electric, but I’d probably need a somewhat bigger system to cover all my current electric us plus a plug-in car.

When we were talking to solar companies about a system, whether we planned to buy a plug-in hybrid or all electric car was a factor in sizing our system/choosing the kind of panels we installed.

My system uses the grid as battery and backup, so I could not charge the electric car w/o the grid, but assuming I was charging the car on a reasonably sunny day, the power to the car would be coming from my roof. At night or on a cloudy day, all or some of the power would be coming from the utility, but because of “net metering” my surplus generation at other times could offset and replace this power.

“wh” would be watt-hours. Watts of power, multiplied by hours of time.

So solar panels, like every other possible source of electrical energy, would produce 3 wh per watt if you ran them for three hours, or 8 wh per watt if you ran them for eight hours.

Maybe he means watt hours per day per watt of installed panels, i.e. a 1000 watt array might produce 3000 to 8000 watt-hours on a day of average sun?

In my last job, I drove an electric forklift, the battery in which—at about 3500 pounds—is heavier than most cars. I got it into my head, one day, to wonder about a comparison of that battery to gasoline.

The battery had a sticker on it that gave its capacity at 850 amp-hours. At 48 volts, that comes to 40.8 kilowatt-hours.

So, how much gasoline do you suppose it takes to match the capacity of this battery? Ten gallons? A hundred gallons? A thousand gallons? Keep in mind, like I said before, that this battery weighs about 3500 pounds, which is more than most modern cars.

It turns out that a gallon of gasoline has about 33.7 kilowatt-hours of energy that can be released by burning it. So that battery can hold the equivalent energy of about 1.2 gallons of gasoline.

The three different figures quoted above, of the alleged capacity of the Tesla S’ battery, amount to the equivalent of {53,60,85} 33.7 ÷ gives an answer of about {1.57, 1.78, 2.52} gallons of gasoline, respectively. This doesn’t, of course, take into account the tremendous advantage that electrical motors have over an internal combustion engine, but even so, at best, you can multiply those numbers by a factor of about three; and see one major reason why we are still using internal-combustion-engine-powered cars.

Another reason has to do with the rate at which batteries may be charged. I’m not going to try to recreate my math right now, but I figured out a while back that in order to charge an electrical car at a rate that is comparable to pumping gasoline into a conventional car, you’d need to be charging at a rate of a few million watts. I don’t know if it’s feasible to have usefully-placed charging stations that have access to that much electrical power, but I very much doubt if we are anywhere close to having batteries that can withstand being charged at such a rate.

Definitely do not try this at home. Taking a quick look at the circuit breakers for my apartment, it looks like they add up to 395 amps, which, at 120 volts, would be 395 120 × 47400 watts, or 47.4 kilowatts.

Yeah, all very real problems. That’s why folks are busy researching more energy-dense batteries (lithium sulfur might double energy density compared to li-ion), ways to exchange entire batteries at charge stations, etc. And why Tesla created their Supercharger network with 120kW chargers. (But even then, that’s still a 30 minute wait as opposed to 1-2 min for a full tank of gas).

And here’s a better video of the Tesla S battery swap process that Dewey Finn posted earlier, taking less than 2 minutes.

Actually, I thought the video I linked to showed what was actually going on below the car much better than the video produced by Tesla.

Forgot to address this part. Tesla’s high power home charger is 20kW (240V, 80A), so you’ll need a 20kW array to match that output, which is pretty big but not out of the question if you have a lot of acreage. Their commercial Supercharger stations, on the other hand, offer 120kW and if you installed that much solar capacity at home you might as well become a power plant and pay somebody to chauffeur you around on a sedan chair. You could feed them homegrown kale and soy to maintain the off-grid aspect. Or just get a horse.

In normal use, however, there’s no reason to really worry about charging time (especially for a 25-mile range) because you can just charge it slowly, overnight, from the solar energy you accrued in your batteries.

My bad. I didn’t take a close look and just thought it was amateur footage of the same event. Didn’t realize it offered a better angle.

BTW, if you do watch that video, mute your speakers. The oohing and aahing of the attendees is really annoying.

aNewleaf: remind me not to party with you. :-/

Yeah, if you don’t limit yourself to commercial vehicles, you can get one of these that can run directly off solar power. (And only sits one. And is tiny. And has no air conditioning)

Huh? That car seats four. And it yields surplus energy, (it makes more energy then it uses) so maybe airco is a future option.

For charging the daily commuting vehicle, you need the solar panels at work…

(unless you charge batteries at home , and then use the batteries to charge the car… )

Oops, I was talking about solar cars in the World Solar Challenge race.

And you are correct, those are cars designed to win the race, so, they are ultra light and indeed only seat one person, like this 2011 prototype. But in the meantime the lessons learned from those prototypes are applied to other prototypes that more approach a modern family car.