Date registered: Sep 2008
Vehicle: '09 ML550, honking great motorhome, M3, PT Cruiser
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If you Google around a little, you'll find the general consensus is that Vernl is correct, and most block heaters are around 450 watts. I must be showing my age, as every block heater I have ever used was in the 100 to 150 watt range -- but I haven't used one for over 20 years, so obviously they have grown in power.
The University of Saskatchewan claims that there are few benefits to using a block heater for more than about four hours, regardless of ambient temperature. But they also point out that you won't do any harm leaving it on for longer than that. I pay 8-1/2 cents per kilowatt hour for electricity, so leaving a 450 watt heater plugged in for 4 hours would cost 15 cents, and leaving it plugged in for 10 hours would cost 38 cents. That's $11 a month. You'll probably save that in reduced engine wear alone.