I have 2 on my car: the OEM unit, which is a bolt-on external element type, is about 300 watts, if I recall. It warmed the engine (or at least the coolant temp sensor) about 40-50F above ambient at 30F.
I also bought & installed a 800w inline tank convective heater.
Guess which one I use most?
In praise of the lowly block heater - MetroMPG.com
Experiment: how long should a block heater be plugged in? - MetroMPG.com