Heat pump water heaters already exist. These are hybrid things where a traditional electric water heater is fitted with a heat pump. The heat pump can increase the water temp but cannot deliver enough, so heating elements are still needed to reach a usable temp.
I’m wondering if that design can be improved on this way: instead of powering the heat pump from the wall, the heat pump can be connected directly to a PV. I think that would be more efficient and cheaper because PV output is not normally directly usable. IIUC, it’s variable D/C which must be regulated and/or inverted to A/C involving more hardware, conversion, and waste. But exceptionally, I’ve heard that a PV can directly power a compressor with no middleware. Any reasons this would be infeasible or uninteresting?
Of course the tank still needs wall power for the heating elements, but would use less wall power and entail less conversion loss.
Shit.. sounds about right.
Although the /water is hot enough/ scenario could be addressed mechanically: bigger water tank, lower heating element raised and the heat pump heating the bottom exclusively where it could /always/ add heat because it would never be hot enough at the bottom.
(edit) after some thought, it would superficially make sense to get a factory water heater (tank) and not tamper with it at all. Just have a PV-powered HP heat water before it enters the stock water heater (in a tank or coil). Thus there could be 2 heat pumps (but for economy the stock tank would just be a simple non-heat pump type thus 1 HP). I guess this is still a dead idea anyway if it’s true that a PV cannot simply directly connect to a compressor.