A supercharger tops out at 250 kW throttles down when the battery pack gets to ~115 F. Hot water heaters run from 120-140 F. Domestic circuits at at most ~7kW, so you'll realistically never be able to do anything useful with the waste heat. At low power like that battery charging is 98-99% efficient.

Another problem is that the coolant loop needs to be kept quite clean. The fins are only about a few millimeters wide and in some cars (eg Model 3) they're electrically hot. Any grit or buildup making its way into the system is a huge problem and just having a valve that can open can cause problems.