Block heater is normally not more than 800 watts, and often 400W. If you have an electric in car heater that could be another 1000W, but might be 1250W. It's really only a benefit if you are plugging it in when it's below -10C and like andy said, if it is on for about 2 hours before you start it that's all you need.
You can get an outside plug that cycles between 0 and -15 and then is on continuous below that, but is off above freezing. You can also get temp sensor plugs that go under the hood to plug into so it self regulates the block heater.
(Message edited by rallyart on February 24, 2009)