I have used pretty much all of the various types of block heaters. The ones that replace a frost plug is probably your best bet for efficiency and cost. The one in my car has been in for about 20 years but the cord has been replaced at least once. They use about 400 watts as someone mentioned and reach maximum benefit if left on for about 2 hours. I have a magnetic type which is OK but a bit troublesome since you don't want to leave it attached when the car especially a rocking rolling B21A is running. My 83 240 with a 7 year old battery and a frost plug heater started with no problem yesterday at -25F after being plugged in for 2 1/2 hours. This morning at -25 and not being plugged in it barely turned over and failed to start. I use an outdoor type timer and set it to go on two hours before I plan to start the car to save on electricity. Another good idea is an electric battery blanket which helps as well. The old light bulb trick does work OK and I have used it with some success in a pinch. Changing to a synthetic 0W30 or 5W30 helps as well. My old 89 VW Diesel ( a poor cold weather starter) started last year at -30F when the block heater timer failed but had 5W30 Mobil 1 in the crankcase.