A homeowner in a sunny climate has the opportunity to install a solar water heater in his home for a cost of $2400. After installation the solar water heater will produce hot water for 15 years. The machine will require maintenance in year 5 and year 10. Each maintenance will cost $700. At least how much must the homeowner save on water heating costs every year if this is to be a sound investment?

Assume that the useful life of the machine is 15 years, and the interest rate is 11% per year.