block heater
A block heater is an electric heater that heats the engine of a car; they connect to the normal AC power lines (nominally 120 volts in North America). This allows easier starting in cold temperatures, by keeping the engine oil warm so that it does not gel and the viscosity is low as well as aiding the vaporization of the fuel. In addition to easier starting, block heaters reduce pollution as cold engines have much higher emissions.
I have installed one in Oct. 400W, heat block model (not Oil, not cooleant)
Engine area temp increased 13C in 1 hr on 15C outside temp enviroment.
So no need to heat more than 2 hr, I think!