Earlier quoted context omitted.
Heating a big, isolated water tank was pretty common when I was working in the industry 25 years ago. Power was much cheaper during night so you used that to heat the water tank and daytime you used that water to heat the building. Lately this has been available to private customers also, price by the hour. (Sweden)
Check out https://mixergy.co.uk which recently came out with a nice solution.
An interesting note from the video is how they said they could detect the grid frequency from the plug to detect times of over supply and (in theory) that would be the right time to absorb some of that excess.
Some other interesting benefits (mentioned in the comments) about how it can periodically cycle the heat better than traditional water tanks to kill off bacteria.