I had a customer who had the solar system and a regular electric water heater for back-up. She tested the system by shutting down the electric water heater completely. She told me she has never turned the electric water heater back on again. The only cost she has for hot water is a pump that runs to circulate the glycol water mix and the maintenance costs involved which she does every few years. Almost free hot water is better than any gas or tankless system. The gas water heater is best suited for what you want to do in the future though but you will probably end up with two hot water tanks. The gas and the solar because I do not believe the two are compatible or can be retrofitted. Check with a solar water heater plumbing specialist on this. Good luck.