I worded that awkwardly, Mergatroyd - we plug in when the Environment Canada website is predicting such lows - and when our arthritic joints talk to us, since E.C. isn't clairvoyant. If I'm suspecting a drop, I'll plug in, but not turn the breaker supplying power to the outdoor plug-in on until 1-2 hours before I have to drive the car.
Don't you love the days when you drive to work in balmy 2C, but drive home in -20 and snow with howling winds? Or those mornings after a freezing rain + heavy snowfall when you have to brush & scrape the car windows?