[frabert]

This! Or, if you don't have geothermal heating but have an electric water heater, maybe temporarily increase the temperature it goes to: maybe it's normally set to go to 65C, then when you detect that you have negative prices and your batteries are full and your water already hot, maybe heat the water to 70C and store that little bit of extra energy as heat! If you have thermostatic valves in your bathrooms, you won't even notice the difference except by the fact that your water heater now can apparently hold a little bit more water than usual :)

[john61]

I have a heat pump for hot water and calculated this with an offered floating energy tariff. It is not economical because the high net tariffs are not floating but fixed per kwH and negative / very low prices are seldom here and only for a short period of time available.

[cogman10]

Assuming regular negatives (more than once a day) you could also tie the heating to the grid prices with maybe an hour buffer around your high water usage times to make sure you are up to temp.

Modern water heaters will keep temp for a shockingly long period of time.