Tesla power usage monitoring with DCM

I have a Tesla Model Y that I charge with the mobile connector via a 120V 20A (5-20) outlet. And I use Dedicated Circuit Monitoring to track the power consumed while charging. When I compare the power added to the battery (from the Tesla app) to the power measured by Sense DCM at the circuit breaker box I have found there is a ~20% (on average) difference in power. For example after completing a charge session the Tesla app will show 10 kWh were added to the battery. But Sense shows that 12 kWh were actually consumed.

I’m trying to figure out if these are normal losses associated with getting power from the wall to the battery (and everything in between)? Or if I have a problem with my home’s setup.
Have any other Sense DCM users compared their measured actuals to the Tesla app numbers?

10 kWh added to the battery
12 Kwh pulled from the wall

10/12 = 83.3% efficient charger in the car.
That sounds about right and for a 120 volt I am even impressed.
120 V AC → almost 400 Volt DC is step up transformation.
I have a lot of respect that the tesla engineers are able to do that so efficiently.
I would have imagined that 240 volt AC → 400 V DC would match those numbers.

Would be interesting to find how how efficient that is.

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I’m seeing some difference with a 240V HPWC, but not 20%. Here’s a charging session that lasted from 3:30AM until 4:25AM. DCM says I was applying 11.6kW pretty much continuously. The car says I added 10.16kWh. DCM put in 55/60h x 11.65kW = 10.7kWh vs 10.16kWh into the car. That’s about a 5% difference.

The numbers are from Sense and Tesla but integrated together in Home Assistant.


10.16/11.6=87.5% efficient

87.5% on 240 Volt
83.3% on 120 volt charging

83.33/87.5 = 4.77% (almost 5% like you said) more efficient to charger your Tesla on 240 volt vs 120 volt.

Thanks for the data!

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Thank you for sharing, this is exactly the data I was hoping to see. That 5% inefficiency number for 240V is what I had been hearing on other forums. But seeing the raw data really drives it home. Any idea if the Tesla HPWC is a more efficient product? Say, for instance, versus the Tesla mobile connector at 240V and a similar charge rate (if that is even possible)?

Anything but a tesla supercharger is just a pass-along of power to the internal charger in the car.
I call it a sort of intelligent extension cord.
So both a mobile connector & HPWC just pass along 240 volt but the HPWC tells the car charger to charge with more amps. The efficiency therefore is 100% the same. It solely depends on the internal car charger that converts 240 V AC → DC of the battery pack.
Would the efficiency go up with 30amps charging vs 48 amp charging with the internal car charger is something for a different episode :wink:


Ahh, ok I see. So the HPWC enables faster charge rate by allowing higher current (compared to the mobile connector at 240V). And my understanding is that higher charge rates (and, therefore, shorter charging sessions) improve efficiency as well.

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exactly the HPWC just tells the car charger to draw more amps from the same 240 Volt input.

I think charging with more power probably is more efficient. But you have to think of things like cooling (afaik the internal charger in Tesla uses the cooling liquid to keep the electronics cool) and aging of electronics under heath for longevity.
My chevy bolt is able to charge at 30 amps but I have limited it to 26 amps (86%) just to not drive everything to the top.

Maybe @kevin1 can try some tests with different charging current to see if the efficiency changes with his tesla’s :wink:

I just watched this video. In my mind, it sounds like a Goldilocks problem - you want to find a rate that is not too fast and not too slow, not too long and not too short, all at the right temperature.

Things like battery conditioning (based on ambient temperature), battery heating (and invocation of cooling) and even the car staying awake, can work against you and efficiency. Charge too quickly and the cooling system needs to get in the game. Charge too slowly and these parasitic losses are working against you for a longer period of time. I don’t know how much resistive line loss from the breaker box to the car plays or or how the inverter / charger efficiency varies with charge rate and input voltage. I do know that 240V delivers the same power as 120V using half the current, so series resistive power loss is 4x greater in comparable power delivery (P=I^2 * R). That’s why we have super high voltage transmission lines.


That video was great. Very interesting to see that level of comparison! Do you have any idea what app he was using to see all that real time data on his car?

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