If the vacation home is small enough, and there is good solar access, you could use a roof mounted ICS solar water heater such as a “Sunbank SB-40G.” The collector includes 40 gal. storage and can include an internal electric backup heating element. There would be no additional water heater. The problem is that the latest version of the Performance software will not model this OG-300 type system! I am using this system now for a small ADU. I had to model it using EnergyPro 8.0 however, which WILL model OG-300 solar systems.