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(background: I'm just playing with numbers, trying to estimate how many solar panels would be needed to fully supply the energy for a home and 2 Tesla cars)

I've been reading up on solar panels and on Tesla car charging. That page mentions that the conversion efficiency from DC (solar panel) to AC (what home appliances need) is 75-80%.

On some Tesla forum somewhere I read that the charging efficiency from the wall plug to the car is around 85% (can anyone confirm this?). Can I conclude from this info that a Tesla car is actually a DC device?

So, if I understand all of this correctly, I lose ~25% to convert solar panel energy from DC to AC (selling to the utility company) and then again ~15% to charge the car battery from the wall plug (buying from the utility company).

Is it possible to avoid both of these conversion steps by charging a Tesla Powerwall from the solar panels during the day, and then charge the car from the Powerwall during the night? (avoiding the energy grid completely, no net metering considerations)

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    \$\begingroup\$ An AC car wouldn't have batteries because losses. All (consumer-grade) cars have batteries. \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 19 '15 at 17:37
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    \$\begingroup\$ You won't have 0% loss as you still will need some form of power conversion - for peak efficiency, the output voltage and current of a solar panel should track the maximum power point (so a one part of the day, you might get low voltage high current from the cell, and another you might get the opposite). The question is then how efficiently can you convert the output of the solar panel to a stable DC voltage. \$\endgroup\$ – Tom Carpenter Jul 19 '15 at 17:50
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    \$\begingroup\$ I would make the following (somewhat optimistic) assumptions. 10% loss on DC-DC conversion between Solar panels and Powerwall. 10% loss on DC-DC conversion between Powerwall and car. This is based on typical-optimistic losses in DC-DC converters. This may or may not stand up to further scrutiny; we shall see. \$\endgroup\$ – Nick Alexeev Jul 19 '15 at 17:59
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    \$\begingroup\$ Here's an AC car ... en.wikipedia.org/wiki/AC_Cobra \$\endgroup\$ – Brian Drummond Jul 19 '15 at 18:18
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    \$\begingroup\$ For getting energy in: Powerwall is just a battery and AC charger. Bring your own DC-DC convertor for charging from solar, or DC-DC for providing mains power. Without a MPPT convertor you're throwing away some 20% of the solar cells anyway, so you might as well have one. \$\endgroup\$ – tomnexus Jul 19 '15 at 18:42
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Inverter efficiencies are typically horrible. If you run the inverter at maximum power output, the efficiency is typically poor (95%) but of course you almost never run at full power, so the efficiency is horrible (75%), and sometimes you only take a trickle of power, so the efficiency is 0 to 50%.

If the system is correctly sized and designed for efficiency, then 95% transfer efficiency is achievable.

Any inversion has similar losses. DC/AC then AC/DC loses efficiency, but it's only twice as bad as a single DC/DC converter, so in a well designed matched system you're only losing an extra 5%.

The efficiency you get will depend on the design of the systems you use. Typical solar inverter systems are designed to maximize the utility to the home owner, that is how useful the system is. If you are willing to accept a system that is designed with only one thought in mind -- using the minimum number of solar panels -- then you can certainly buy a system that is more efficient. (The systems I work on are better than 1%)

Now that panel prices have dropped, it is almost always cheaper to buy more panels.

Having said that, if your purpose is to charge a car, not sell power, then it is reasonable to look around and see what kind of systems you can buy. A correctly matched DC/DC system will be more efficient than a standard (one size fits all) solar inverter system.

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