[hwillis]

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.

[inflatableDodo]

I'm talking about having a domestic supercharger. You can get up to 70kW at the company head fuse as a standard residential customer in the UK. You can also go higher, but you have to apply for a quote, so at the moment getting a true domestic supercharger is a bit pricy.

However, they are going to have to upgrade it all anyway if we get loads of electric cars.

As for the coolant loop, I wasn't suggesting to have a valve that can open, just a closed loop to the socket and a heat exchanger that then heats the water flowing through the handle of the plug.

You then store the water in an insulated tank and use it as a preheated supply feeding your main water heating system.