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Estimating what I owe my kid when we charge at her place

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Add 10% to cover efficiency losses.
The efficiency losses are already counted in the app, I have noticed. The only question is whether or not they're actually measured (I think they likely are, since the car has all the information to do this, not including cabling losses outside the vehicle (and the car could account for that too, as it can measure it pretty accurately, though I doubt it does!)).
 
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Reactions: Big Earl
Yeah, in short it seems like the OP should just enter (or select if available) their child’s rate schedule into the Tesla app and just use that. It seems to account fairly accurately for energy usage from the wall (caveats for pre-heating, etc. - have not tested to see if that is tracked).

And then come back here with a screen shot!

My last charge session added ~16kWh (which as we know was actually ~15kWh, since it is 95.5% of what is displayed) and ~18kWh was logged in the app. So all makes sense (rounding error makes it impossible to accurately calculate efficiency here but I have looked closely at it in the past to eliminate rounding error, and it is ~90%). I should try a session at 120V just for science to see if I get ~75%. Hopefully Tesla will not let me down.
 
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An important factor is how touchy this issue is. If power costs are a real bone of contention between the roommates then it might be safer to work off of the house’s power meter and pay for everything. Otherwise the roommate who is paying the power bill might find some pretext for disputing the payment made by mapman after mapman mysteriously consults with his tesla app. The house’s power meter is something the paying roommate can access and trust, and he can only be grateful to get more than adequate compensation according to his own measuring device.
 
An important factor is how touchy this issue is. If power costs are a real bone of contention between the roommates then it might be safer to work off of the house’s power meter and pay for everything. Otherwise the roommate who is paying the power bill might find some pretext for disputing the payment made by mapman after mapman mysteriously consults with his tesla app. The house’s power meter is something the paying roommate can access and trust, and he can only be grateful to get more than adequate compensation according to his own measuring device.
Shouldn't be a problem - just trying to help everybody continue to play nice together.