Most block heaters are 400-1500 watts. I think some (but not all) are thermostat controlled so they have a duty cycle. I would be surprised if every plug in a parking lot could supply 1400 watts continuous. My guess is they are designed for something under 1000 watts per receptacle.Wouldn't the block heater circuits be the equivalent of a class 1 charger, 8 KMs per hour of charge.
Has winter driving been factored in with the batteries supplying the heat? Heat exchangers are better but a friend got stuck on the 400 and the two and a half hour crawl didn't suck out the watts as much as the heating system.
Air conditioning in summer has a similar effect.
