Heated pet beds come in all shapes and sizes, some with electric heating elements and some without, so don't be in a hurry to buy the first one you come across. (pawster.com)
In the first - ever study of its kind, they encased electric heating elements in hand - sized plastic boxes. (sciencemag.org)
I'd look into the possibility of installing heat pumps with electric heating elements for very cold days (one per unit, hopefully, in the units which currently have electric heat) these can be installed in the window much like typical air conditioners and they will function as air conditioners in the summer - some can be installed through the wall. (biggerpockets.com)