You could make one and market it, I'm sure they would sell. Low amp heater that runs on rechargeable batteries, ideal for power failures.Your thread makes me wonder why is there no battery-operated heater in the event of power outages? I understand they'd draw far more power than an air pump, but with the advances in lithium batteries in some heavy duty power tools, I would think this would be possible, at least for the short term.
Name it the Enricosonic portable aquarium heater (patent pending, everyone else bugger off, this is our idea)