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Charging efficiency

I apologize if there is already a thread on this. I am interested in how much electricity is lost in the charging process. If you pump 85 KWH into your battery, I am sure that your electric meter on your house will show that you used more than 85 KWH. My question is how much more. All of Tesla's analysis assumes that there is no loss - i.e. perfect transfer - from the AC current in your house into the DC battery in the Model S. Does anyone know how much that loss is and are there any factors that impact that loss - e.g. temperature, current level... And is there anyway to measure the loss. It seems to me that knowing that is critical to being able to calculate cost per mile.

charging efficiency is very very low - and I'm wondering if I have a problem.

I've been suspecting this for a while - but I finally got all the pieces together in one place yesterday and now I know:

I used 12.5 kWh yesterday in my daily driving - and then proceeded to dump 22.51 kwh into the car to charge it - that approaching only 50% charging efficiency and dramatically alters the cost of driving this car.

Does anyone know if this is expected?

50% is low. I've been using an Aeon Labs ZWave Smart Energy Monitor on my 14-50 feed for the past 50 days or 2,300 miles. My charging efficiency (kWh driven / kWh charged) is 71.3%.

I estimated that about 14% of my total kWh charged is attributable to the energy lost while the car was parked. If I subtract this 14% vampire loss from the total kWh charged, I get a 83% charging efficiency (kWh driven / (kWh charged - kWh lost while idle))

We have had our car for 18 days. We have logged 1070 miles. The car has always been charged at home on a dedicated meter.
Total kWh used per meter is 462
Total kWh used by car trip B (never been reset): 340
Charged up every night.

74% charging efficiency (including vampire loss at night)

I just updated my Electricity Cost calculations to account for charger efficiency AND "vampire load" (draw while parked), and once you look at all that, the overall efficiency (driving energy / wall energy) is like 70% (even if the average charger efficiency is 84%). That means that if your dashboard is telling you that you are using 345 Wh/mile, the real from-the-wall usage is more like 490 Wh/mile. If vampire load was reduced to about 10% of what it is now, these numbers would be more like 82% and 420 Wh/mi. Anyway, you can play with the spreadsheet by downloading from (or just see PDF snapshot at

See more on actual efficiency in this thread -

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