I'd go with a generator. Go out a few hours before you want to start the car, fire it up, and come back later. Propane generator is best and most efficient.

By the way, a stock-in-the-engine-block heater only uses about 275 watts, not 1000. The lower hose style heaters use about 380. (I've tested both types) Engine block style is more effective though, hence why the hose heater makes up for it with more juice.

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