Well, your guess is a nice assumption but not accurate. I grew up in northern minnesota and we used them all the time, all my cars have them (except the HCH of course because it came from AZ). They really do not heat up the car or engine (significantly), they simply stop it from freezing and keep the oil from gelling so that it starts easier. True, once running your car will warm up faster than it would without the block heater, but there's still a significant warm up time just like in fair weather. It's a disaster-avoidance tool, not a comfort accessory.

I wonder, using all that electricity to heat your car to save gas mileage? Have you considered whether that's really saving you anything in the big picture when you add the electricity for your heater? Just curious.

As for the difference, I don't know the wattage of a block heater, but they're basically a little resistive heating element put in place of a frost plug that provides residual heat. Maybe 500 watts, but that's a total guess.