I hope you have the time to focus on this, as I think a single heat pump which provides hot water along with space heating and cooling could make a meaningful difference in the economic return of going all-electric.

When I played with this idea, I wondered if the space heating and cooling heat pump might be coupled with a standard electric resistance water heater of the sort that is common in the US, 150-225 liter, with upper and lower electric elements of 4-5 kW each. Both elements have adjustable aquastats; when the upper stat calls for heat, the upper element is turned on until the water reaches setpoint, at which time power is directed to the lower stat, which sends power to the lower element until water temp reaches the setpoint.

My thought was to remove the lower element and in its place install a direct exchange (DX) coil which is is incorporated into the refrigerant line immediately downstream of the compressor (and upstream of the reversing valve), so that it acts as a de-superheater. Whenever the compressor operates, whether for space heating or cooling, the heat pump delivers heat to the lower half of the water tank. The upper electric element continues to heat the upper half of the tank whenever the upper stat calls for heat, providing both a supplemental and backup energy source. The advantages are that the water heater tank is readily available and inexpensive, the cost of the DX coil should also be modest, and a significant fraction of annual water heating load would be free in summer or low cost in fall or spring due to high heat pump efficiency. Heat pump capacity might have to be increased to accommodate winter peak load, though there may be an opportunity to heat water during the day when space heating loads are not at peak.

Another feature that might add value, as well as complexity, would be to make the heat pump dual source, i.e., it pulls heat from (or sends heat to) outdoor air or a ground source heat exchanger, whichever provides greater efficiency.