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Model 3 home charging efficiency %?

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Lots of testing has been done. There's a main thread too, but can't find it. The EPA also has some relevant data.

There's three things:
  • 300W fixed overhead for coolant pumps, computers, etc. (biggest loss, usually)
  • AC-DC conversion losses
  • Delivery losses (from meter, through plug, to your car - usually negligible).
A 120V 12A setup (North American standard outlet) is the worst. You'll get about 75% of the energy delivered into the battery. Reducing the current would deliver much less. Cutting it to 6A would result in about 55% efficiency.

Level 2 charging at higher powers is best. Basically, you're trying to make the 300W overhead be a small portion of the power reaching the car. I think the max 240V setup with 48A would be close to 95%, but 32A isn't that much worse, maybe 91% or so to my recollection.

There was a noticeable jump for us switching from 120V/12A to 240V/24A in efficiency via the electricity bill.

Edit: to actually answer your question, for example:
120V, 12A: 53.3kWh to charge 40kWh (roughly)
240V, 24A: 44.0kWh to charge 40kWh (roughly)
 
I think the max 240V setup with 48A would be close to 95%, but 32A isn't that much worse, maybe 91% or so to my recollection.
5% strikes me as too much of a difference between 32 and 48 Amps

If overhead is 0.3 kW and 32 Amps is 8 kW then overhead is a 300/8000 = 3.75% inefficiency
At 48 Amps 300/12,000 = 2.5% inefficiency

This suggests that the higher Amps saves ~ 1.25%, presuming the other losses are similar. And at least in terms of resistive losses, the higher Amp charge will have 2.25x higher losses.

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No matter, your post was spot on. I summarize it as "get 240 volt charging."
 
After 1 year with the Model 3 Standard Plus (55KWh), I spent 3285 KWh in charge, while only 2688 KWh was delivered (and then available) by the battery. It represents a 18% loss.
Charging most of time at 240V, 3x5A. Rarely 3x16A or superchargers.
Up to now, I was thinking that a lower power would result in better efficiency. What does represent this constant overhead at 0.3 kW? Is it linked to AC-DC conversion ? Battery heating?
 
After 1 year with the Model 3 Standard Plus (55KWh), I spent 3285 KWh in charge, while only 2688 KWh was delivered (and then available) by the battery. It represents a 18% loss.
Charging most of time at 240V, 3x5A. Rarely 3x16A or superchargers.
Up to now, I was thinking that a lower power would result in better efficiency. What does represent this constant overhead at 0.3 kW? Is it linked to AC-DC conversion ? Battery heating?

The 300W accounts for overheads like the computers, coolant pumps, etc. that are all going during charging. The AC/DC conversion is closer to 94% (it varies). The overheads are one of the worst for available EVs. Any battery heating is subject to conditions of course, and would be significantly more energy than 300W. I can go into more detail of you'd like later - I'm on my phone right now so I'll spare myself the typing.

The formula I now recommend is something like this:

Net Energy = (Volts x Amps x 0.94) - 300​

This formula holds fairly accurate for me over a large range of values, but is slightly off at very low powers (it over-reports, i.e. reality is worse than the calculation).

If my understanding is correct, I can take your 3ph 5A and count it as 15A. Working the numbers, one would expect somewhere around 85.7% charging efficiency. Your kWh numbers work out to 81.8%, but I'm not sure how you are measuring energy delivered by the battery (you could be missing standby usage for example, if using the trip meters).

I have some data but not enough to conclude anything regarding heat losses. I don't expect they're as significant.
 
I just did the math for a 2015 P85D charging from 70% to 80% (8.5kwh).

The app said it was charging at 12A, 116V. In theory, it should have taken 6.11 hours to add 8.5kWh, but it actually took 7.83 hours.

12A * 116V * 7.83h * 1kWh/1000Wh = 10.899kWh.

So that's 10.9kwh to charge the battery 8.5kwh. In other words, only 77.9% of the power went to charging the battery. The rest (22.1%) was probably lost as heat and consumption. I imagine the efficiency drops as you get closer to a full battery.

I also plugged in far away from my panel, which could explain 116V instead of 120V, so there are more losses upstream.
 
After 1 year with the Model 3 Standard Plus (55KWh), I spent 3285 KWh in charge, while only 2688 KWh was delivered (and then available) by the battery. It represents a 18% loss.
Charging most of time at 240V, 3x5A. Rarely 3x16A or superchargers.
Up to now, I was thinking that a lower power would result in better efficiency. What does represent this constant overhead at 0.3 kW? Is it linked to AC-DC conversion ? Battery heating?

Faster charging means the coolant pumps and fans and computers run for less time, that might explain the difference?