Some numbers on the cold

Last night I took some data down on how much electricity the car used overnight while charging (should have done this in the summer months as well!).

To start with I recorded my power usage for the day (at least what the car thought it was using, this value corresponded with 59% of the battery remaining):
Trip meter display

These numbers are somewhat typical for my daily commute. Ironically this commute was on a rather cold day with just under 1/4″ of snowfall overnight. Preconditioning the car combined with everyone’s cautious driving caused me to only use 7.2kWh of electricity! These past few cold days I’ve been using closer to 8kWh of electricity for my 31 mile commute.

Right when I arrived home I grabbed my meter reading (the car’s EVSE has its own meter):
Meter reading 1

At the time I plugged it in the outside temperature was 35F and thus the car stated “Waiting to charge” with my normal start time of 1:00am. As the evening went on the temperature dropped–when it hit freezing the car switched from waiting until 1:00am to waiting to start at the current time–it was starting its warming/trickle charge cycle.
This morning I recorded the meter right before driving away (with no preconditioning, outside temp: 20F):
Meter reading 2

So overnight the car used a total of 10kWh to keep itself warm, and charge up to 100%. Thus 7.2kWh of that went into the battery along with some burned up in the charger due to the charging efficiency (according to the EPA charging efficiency is roughly 80%) which would be about 2kWh. This leaves about 1 kWh used overnight to keep the car warm. (I’m being purposely vague here because I don’t know the exact efficiency of my EVSE + car combination. Doing the math directly yields 1.36 kWh which is close enough to my 1 kWh value.)

The numbers above align rather nicely with the daily average power consumption reported by my electricity provider. The daily averages per month also seem to be reflecting the additional power usage as the temperatures drop:

  • September: 9.9 kWh/day
  • October: 10.1 kWh/day
  • November: 12.8 kWh/day

Around here the temperatures took a dive in November which is reflected in the numbers. There are other factors in those numbers as well: In September and early October we were driving around a bit more each day due to various school/sporting activities (which may be the reason that there is only a 0.2 difference between September and October).

At this point, with our early blast of arctic air, I’ve been very pleased with the performance of the FFE–I was expecting a much greater drop in total range as an affect of cold weather.

Update: Power required for a precondition. Prior to driving the car across town for Thanksgiving I had set it up to precondition to 85F. This was the perfect opportunity to measure how much that takes. Before the car started the meter looked like this:
Power meter
Just before leaving I grabbed a picture of the meter (when the car was done preconditioning):
Power meter
Thus the car used about 5kWh of electricity to warm itself up from the ambient 25F.

 

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