ASK THE EXPERTS: Tesla Charging

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 Welch • Home Power senior editor