Mythbusted: All Cars Need to Be Plugged in at Least 4 Hours on Cold Days

January 22, 2016

Winter Street Featured


On cold days, plugging in my block heater for four hours will make sure my car starts AND I won't waste power.


In most cases, you can probably get away with plugging in for less than four hours. Four hours is a good rule of thumb, but it's also a very conservative estimate.

Here's a more accurate breakdown of how long you need to plug-in the average gasoline engine car, 5W-30 oil, good/cold battery, automatic transmission, parked outside all night:

  • 0 to -15C-- no plug-in needed
  • -20C-- 0.5 hours
  • -25C-- 1.5 hours
  • -30C-- 2.5 hours
  • -35C-- 4.0 hours
  • -40C-- 8.0 hours

As you can see, only when the temperature reaches -40C do you need to plug in your car for more than four hours, and it rarely gets that cold, even in Saskatchewan.

So how did we come up with the four-hour rule? The original data we used all comes from a 2008 study at the University of Saskatchewan. This study tested engines by starting them in cold temperatures. Dozens of different variables were used including different types of engine oil, batteries, fuel types, and of course, temperatures. Out of that study, we came up with four hour rule as a conservative approach.

What is the actual magic number? If it's -25C out or warmer, we suggest plugging in your car for 1.5 hours to save the maximum amount of electricity.

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