That’s also an idea that’s been around for a while. Pre-heat your hot water system input, thus reducing the load on whatever you use in your HWS, gas, electric, or other. I’ve not seen it implemented though, presumably it’s quite a manufacturing problem, bonding water pipes to the back of PV panels, secure interconnects, pressure relief valves, etc. It would have a significant effect on the price of a PV panel, and the efficiency increase would need to justify it.