So I have an extra Hot water tank, live in Florida, and its summer. My next house project is a Guerrilla-Solar Hot Water heater because I live in the country and hate permits.
The Plan is to paint the tank Black & lay a concrete base for it outside, run the cold water line to the outdoor tank, from there run a line to the interior tank and boom 2 40 gallon tanks + Cut down my daytime electric use.
At night it is not a waste of energy because the sun is heating that area anyways.