Today I decided to use my "Killawatt" meter to measure how much my Model S draws while parked and full charged. I unplugged it from my NEMA 14-50 connector and put on the standard 120V connector.
To my surprise, the car started charging, even though it had been connected to 240V overnight. It drew 12A at between 122 - 123 V and took 1h 11 minutes to reach a full charge again. During that time, it drew about 1.73 kWh, confirming that the meter is relatively accurate.
Then, when the charge was complete, the draw dropped to my meter's minimum reading of 0.01A. The kWh read-out, after over 2 1/2 hours, is still at 1.73 kWh. It's about 78 F in my garage (25.5 C), and so, now at 124 V, it looks like I'm drawing about 1.24 W. That's about 0.03 kWh in 24 hours - i.e a negligible amount.
I assume it helped that my test happened under optimal temperature conditions, where the battery pack neither gets cooled nor heated, but it gives me a good measure of comfort to know that counteracting the dreaded "vampire drain" will not suck me dry unbeknownst to me. In fact it looks like it will cost me less than a cent per day.
This, of course does not account for the range loss that we do experience, when we leave it unplugged, but I'm also guessing that at 75 F that will be less. Perhaps the difference really is only in the heating and cooling cycles, which of course *have* to draw more energy than I measured.
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