ASK THE EXPERTS: Tesla Charging

Tesla Roadster
EV charging with renewable energy can be most easily accomplished with a grid-tied system.
Tesla Roadster

After numerous Internet searches, I have yet to get an answer to my most burning current question: Which 12-volt DC to 110 AC inverter do I need to buy so I can charge an electric car? I want to use my own stand-alone solar-electric array—five 33-watt, 12-volt Siemens PV modules and two pairs of 6-volt deep-cycle golf cart batteries with extended amp-hour capacity—installed 12 years ago. I am willing to take an extra day or two to charge the car or, as income allows, add more PV modules. 

Jon Dieges • via email

Without factoring in typical system losses, the most you can get out of your array in “perfect” conditions is 165 W (5 x 33 W), which is not enough to charge any EV I know of. Even the slowest Tesla Roadster EV charging regimen (30 hours) requires more than 10 times that amount—1,800 W (15 A at 120 VAC). The charge rate probably tapers off over time, but you can check with Tesla owners to find out how many kilowatt-hours it takes to accomplish a full charge.

The bottom line is that you need a much larger array and battery bank to accomplish car charging directly from a stand-alone PV system. However, a grid-tied solar-electric system negates the need for an expensive battery bank and related balance-of-system components required for stand-alone systems and thus can make sense for car-charging. Your array can simply offset your energy consumption when the sun is shining and you can draw on the grid for any supplemental electricity required or tap into your PV-production credits if you need to charge at night. Your EV will get charged, then, no matter when you plug in.

With the grid as backup to the PV array, you are not relegated to the slowest, low-wattage charging means, but can take advantage of the fastest possible charging—regardless of the size array you can afford. 

Michael WelchHome Power senior editor

Comments (6)

Jim and Elaine Stack's picture

I feel GRID Tied solar is best. I give my clean solar back to the utility during the Peak Hours to help them and the planet and get paid for it too. I then charge Off Peak because there is excess in the GRID that can't be stored or ramped down so they dump it to ground! Why do you think they give you low Off Peak rates.
I call this F.I.T. Feed in Transportation. $1 of electric replaces at least 1 gallon of gas at $3-4 a gallon. Reduces pollution by 20 pounds a gallon too! That's about a 4 times pay back!

An electric vehicle is about 80% or more efficient, a gas car 15% and diesel 30%. An electric can do REGENERTIVE braking gas and diesel can't. An electric doesn't need a transmission gas and Diesel do. An electric doesn't need OIL changes, G & D do.
It all adds up.

Joshua Bills's picture

That's great for you! In our state (KY) net metering combined with time-of-use (TOU) rate class option is being challenged by one of the larger investor-owned utilities here. The statute clearly states that credits are accounted for at the rate generated under TOU. However, this utility has responded unfavorably to one of their customers, who wants to install PV, and is currently provided service under an optional TOU rate class (it's even called Low Emission Vehicle - LEV - rate class). The utility's response is to allow excess credits at the premium value, but they will "bin" those credits so that the customer can only utilize their bank of peak-power generated credits during peak power periods. Since this family has shifted their loads to under 1-kWh per day during peak rate periods, net metering is not an option! This case is before our public service commission now.

Joshua Bills's picture

Whatever happened to the idea of using the EV battery as a main off-grid battery for a home? Realizing there are challenges--inverter/charger for high voltage battery system in vehicle--second stationary battery system to store energy while vehicle is away from home--but are those insurmountable obstacles? Seems a possible solution for grid NIMBYs out there.

Jim and Elaine Stack's picture

JB, Nissan has V2G that can be used for Solar battery back up but so far only sells it in Japan. So does Mitsubishi. All you need is a Quick Change port and a solar system that can then sync on the AC from an inverter running off it. Also a disconnect for the main power like a generator uses. I think this is another big step that the Solar installation companies will see and embrace. Now that electric vehicles and Renewables are using the same DC power it can be a whole new world, an EVworld. .

Fred Golden's picture

Start with about 500 - 800 watts of modern solar panels, and you will have a good start at recharging a car. A prosine or Outback 2,000 - 3,000 watt inverter will get the job done. 6 Golf cart batteries can act as the storage until you plug in the car, L16 would be better though, if you can lift them!

Home Power has several advertizers of solar panels, some with panels in the $1 o $1.50 per rated watt. It is not like the $200 cost that one might have paid for 33 watt panels 10 years ago.

Michael Welch's picture

Hi Fred. You would very quickly empty those batteries, then be stuck with no charging source. Better would be to put those PV modules on-grid, and use the grid for charging.

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