dm - which state/climate are you in? What do you use to heat water now, NG or electric?
If you don't have to worry about freezing, a solar water system will be cheaper and payback quicker.
I've been pricing out replacement conventional NG water heaters, and it looks like we would only pay ~ $250 ~ $300 annual gas costs at our ~ $1/therm. I'm having a hard time imagining that a system sized to cover, say half that usage would be cheap enough to install and maintain for a decent payback. A thing mounted on the roof, plumbing from roof to basement, controls, pumps running? Could I get all that for $1500? - and that wouldn't even be a 10 year payback when you consider the lost income on the $1500. At even 3%, that takes a $45 annual bite out of a $150 annual savings...
With my luck, it would leak and do $6,000 of damage
In a warm climate, and with electric WH and high KWh rates, I could see where the math could work out.