ev charging efficiency

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i'm just trying to save money

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if you don't care about the process you can just skip to the results

i've been going around measuring the energy consumption of everything
i can in my house to see if there are any obvious savings i could
make. i drive an electric car (Polestar 2) and charge it in my garage
so that's obviously going to be
one of the top energy consumers in my house.

the car will tell me how much energy it uses but i'm wondering how
much it actually draws from the wall when i charge it. i don't
drive a whole lot so i just use the 120V charging cable that came
with the car, which is probably the least efficient way to charge it,
and i'm curious just how much energy is wasted.

here's the procedure i followed:

i used a Kill A Watt P4400 to measure everything else
so i used it with my car too.
i charged the car to the manufacturer's recommended maximum
value (90%) and then reset the trip computer. i drove the car about a hundred miles, then noted the energy consumption listed by the trip computer.

then i charged the car back up to 90% while it was plugged into the
kill-a-watt and compared the two measurements.

trial 1



Photo of the
          Polestar 2 trip computer showing distance traveled and
          energy consumption.
Photo of
          my Kill-A-Watt with the EV charger plugged into it showing
          energy consumption.



95.4 miles at 34.6 kWh/100 mi = 33.0 kWh of energy consumed.
the kill-a-watt says the charger pulled 47.20 kWh from the wall.

that means we got an efficiency of 33.0 / 47.20 or 69.9%.

that seems very, very bad.

this makes me suspicious that perhaps unfavorable weather
(the temperature during this test was between 24F and 45F)
is to blame so i plan to re-run this experiment about a
month from now when the weather should be warmer.

my hypothesis is that the BMS heats the battery if it's too cold
while charging and that may be where the extra
energy consumption came from.

trial 2

i re-ran the experiment under warmer temperatures and got this result:



Photo of the
          Polestar 2 trip computer showing distance traveled and
          energy consumption (second trial).
Photo of
          my Kill-A-Watt with the EV charger plugged into it showing
          energy consumption (second trial).



the temperature during this run ranged from 47F and 83F,
much closer to the ideal temperature for charging the battery.

118.2 miles at 31.0 kWh/100 mi = 36.6 kWh of energy consumed.
the kill-a-watt reads 48.61 kWh drawn from the wall.

that's an efficiency of 36.6 / 48.61 or 75.3%.

that's better but still worse than i would have expected.
i have a few more weeks with this car so i'll run one more trial and
see if i get the same results now that the weather's more consistent.

results

trial | distance (mi) | kWh (car) | kWh (battery) | efficiency
------|---------------|-----------|---------------|-----------
  1   |     95.4      |   33.0    |     47.20     |    69.0%  
  2   |    118.2      |   36.6    |     48.61     |    75.3%  

i'll also run this experiment on a different car
when i replace this one.

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written by peter beard on 2026-03-01, last updated 2026-04-24
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