We have the Refrigerator, which sort of "create their own cold" using electricity (these obviously have to be plugged in).
What do you call an electrical device that keeps things warm?
I would think a "Warmer", but the antonym of "warmer" is "cooler", and a "Cooler" is already a thing (it's an insulating device that keeps a cold thing cold, but it doesn't "generate" cold from being plugged in).
Please ignore the scientific atrocities of saying "generate" cold. I'm simply being brief.