How do you connect it to your existing hot water system though? Already got a heat pump too, when you have the most sun the heat pump barely uses any power for hot water and don't need much.

I have wondered about solar/fire heated pool or hot tub, but that is adding energy demands in the first place. That said with the heat lately a cold pool could be nice. Can get 5000L for just under £100, 3-4m wide. Heating it could make it usable into autumn though.