ev charging efficiency

~ ~ ~ ~ ~

i'm just trying to save money

~ ~ ~ ~ ~

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.

the results






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 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.

the lease on this car is up in a couple months so i'll also run this
experiment on a different car when i get one.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
written by peter beard on 2026-03-01, last updated 2026-03-15
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -