I've had my Model S for a month now and been doing just fine charging on a standard 120V outlet in the garage, given that I drive 30 - 40 miles a day. Still, I've been curious how the efficiency compares to 240V charging. I hooked up a Kill-a-Watt meter the other day and watched it like a hawk (worried about smoking it out), but also got some good data out of it. Quick caution: I don't recommend charging with any sort of device inline due to risk of overheating and fire. While the Kill-a-Watt is rated for 1875w (125v @ 15A), I don't know that it can safely handle that rate for a sustained period of time. Also, it's cheap and made in China. Please don't do this overnight and risk catching your house on fire. Ok, with that out of the way, here's the data: Battery was down 5.9kwh from driving 15 miles before coming home to charge. Garage temperature ranged between 50 - 55F. Charged for 5hr 45min to full standard charge. Car showed 2kw/2miles per hour rate of charge. Car showed 6kwh recovered at the end of charge (more like 6.25kwh based on when it flipped to 6). Car showed 15 miles recovered at end of charge. At the wall 8.54kwh were used. Current draw reported by the car (116v-117v @ 12A) Current draw at the wall was 1490w (116v-117v @ 12.82A) Current draw at the wall after charging complete was 1w Some conclusions: Charging efficiency at 120V: 73% [edit: shokunin points out I forgot to factor in idle load draw. With that, charging efficiency is 85%] Real rate of rated miles recovered: 2.6 miles per hour of charging Real rate of watts recovered: 1.09kw/hr Some open questions: I heard a circulation pump running the whole time during charging. It stopped when charging completed, so I assume that was for thermal management of the batteries. It could be that some of the charging losses were from heating the batteries, in which case the charging efficiency number would be higher. I'd need to repeat this experiment on a colder and warmer day to say for sure. One interesting thing I noticed: With the car fully charged and still plugged in, opening the door to start the cabin air heater drew no current from the wall (meter still read 1w). This means opening up the door in the morning to preheat the cabin will draw from batteries, not the wall. But, turning the heater on with the phone app without opening the door does appear to draw from the wall, at 250w. So, lesson learned - preheat from the app, not at the car, to draw from wall power. Cranking the heat up to max only increased this number by a few watts, presumably extra current from running the fan blower faster.