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 (11)

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.

Jim Hollander's picture

Your statement of 1 gallon of gasoline = 20 pounds of pollution is counter-intuitive since 1 gallon of gasoline weighs under 8 pounds, how can combustion of 8 pounds of gasoline result in 20 pounds of pollution?

Michael Welch's picture
From fueleconomy dot gov:

When gasoline burns, the carbon and hydrogen separate. The hydrogen combines with oxygen to form water (H2O), and carbon combines with oxygen to form carbon dioxide (CO2).

A carbon atom has a weight of 12, and each oxygen atom has a weight of 16, giving each single molecule of CO2 an atomic weight of 44 (12 from carbon and 32 from oxygen).

Therefore, to calculate the amount of CO2 produced from a gallon of gasoline, the weight of the carbon in the gasoline is multiplied by 44/12 or 3.7.

Since gasoline is about 87% carbon and 13% hydrogen by weight, the carbon in a gallon of gasoline weighs 5.5 pounds (6.3 lbs. x .87).

We can then multiply the weight of the carbon (5.5 pounds) by 3.7, which equals 20 pounds of CO2!
Jim and Elaine Stack's picture

Jim Hollander , GERAT QUESTION AND GLAD YOU KNOW SOME SCIENCE. The bad CARBON Combines with good Oxygen and makes more pollution that it started as. CARBON Makes Oxygen into deadly Carbon Monoxide, carbon dioxide and Sodium dioxide along with other bad combinations. So you are playing a losing roulet with 4 to 1 odds against you. It's as dumb as Fracking and Lead acid batteries. They all create more problems than any small use they create.

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 Hollander's picture

Can we tie into our Tesla Model S P85D's power by tapping into the battery pack under the chassis? (Provided we did not have free SuperCharging from Tesla Motors.)

Jim in NH

Jim and Elaine Stack's picture

No if you don't have Super Charging you can do any DC Fast Charging since the car won't allow it. It can't tell the difference.

Tesla also doesn't support V2G. So far Only Nissan 2018 LEAF, BYD e6, ACPropulsion T-Zero ,MINI-E eBox support V2G/H

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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