Adding a water heating system to an existing house is seldom a good investment.

Payback of 10-20 years means the house will be sold before the money is recouped.

The big enetgy waster is space heating.

A house should be planned carefully, including the building site.

Long term problems such as shading should be considered, as well as the obvious need for proper orientation to the Sun.

Here is a good place to learn more...

                                                                           https://www.thenaturalhome.com/passivesolar.html