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There is another factor sometimes to consider in EBH cost calculation. The heat that leaves the engine compartment is sometimes considered totally wasted. But that's not the case if your car is in a garage. If you leave a 500W EBH on for 10 hours it will use 5kWh. Even if you suppose only 2kWh aids the engine warmup, that's 3kWh that heats the garage and indirectly the rest of the car. The EBH-powered engine block is no less efficient that a space heater once it gets close to equilibrium temp.
I had tried carefully for many years to time my EBH to come on 2 or 3 hours before startup, but now I realize that my effort is better spent on insulating and weatherstripping the garage itself. For that reason, sometimes I think I might as well switch on my EBH at night as soon as I hit off-peak rate and consider EBH to be just another home heating appliance. For those who have natural gas, of course, my conclusion does not make sense.
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