Domestic Hot Water for Zero Energy Homes

Zero energy homes (ZEH's) in the near future will be simple, all electric, and supplemented with PV.

Hybrid PV/thermal solar collectors may be the best solution as costs come down. Sun Drum Solar is now offering a product. Dawn Solar also offers a building integrated hybrid PV roofing system. PVTSolar uses an air-based approach. When I worked at Solaron in the 80's, we abandoned air-based systems, and we were the world leader in air-based until that point. These are the only entries so far in the hybrid market sector. I expect to see more manufacturers enter the fray soon.

Whether or not a solar thermal DHW system is used, a grid-sourced backup water heater is required.
An affordable heat pump hot water heater (HPWH) is now on the market:
https://airgenerate.3dcartstores.com/index.html

This type of system uses only 25%-35% as much electricity as a standard resistance water heater, making it the best choice for a ZEH without solar thermal.
Another bonus of these heat pumps is that they can supply "free" summer cooling for the house.

In the near future as costs stabilize, we'll learn if this $700 is worth it for a home with solar thermal or hybrid collectors. In Denver, a home with 100-150 ft2 of solar thermal or hybrid collectors probably won't need it. The $700 marginal cost would take too long to recoup because the solar is providing 90%+ of the domestic hot water load.