The cost of solar water heaters, like solar PV systems, is reasonably high up-front, but is likely to be less than conventional water heating in the long run. According to the Department of Energy, your water heating bills should drop by 50%-80%. Plus, there are plenty of non-financial benefits of using solar power.
A rooftop solar thermal collector.
The cost of solar thermal energy depends on:
price of fuel from local utility, used to heat water
federal, state and local government incentives
the amount of hot water you use
your system’s performance
the type of back-up heating you use
the solar radiation resource in your area
Reinforcing your roof to hold the weight of a solar collector can add to the solar water heating cost. Another factor to keep in mind is that some systems require more frequent and costly maintenance than others.
You can estimate the annual operating cost of a solar water heating system once you know your system’s solar energy factor (the measurement of efficiency for solar thermal systems) and the cost of your backup heating. Check out this great U.S. Department of Energy guide to calculating the cost of the system you have in mind.