Originally Posted By: G-MAN
In my experience, if you know it's going to be cold enough overnight to warrant the block heater, it's best to just plug it in as soon as you've parked the car/truck for good. That way the block heater is keeping the coolant warm, not trying to warm up ice cold coolant.
That's exactly what I do when the real cold weather hits here. Only need to do it for 5 or 10 nights a year.