Generally speaking a solar hot water heater will cost $3000 - $5000 to fit, and will save $300-$500 per year in electricity. If you have a mortgage, then the opportunity cost of spending $4500 is about $400 a year. Ie. if you paid off $4500 of your mortgage you would save that much in interest. So it never pays itself off. If you were able to get an interest-free loan, it would take about 10 years to pay off, and they have a 20-year life-cycle, so the second decade is the one where you get the free hot water.

Having said that, a properly installed SHWH will reduce your household electricity usage by about 30% on average. So while it won't save you any money, it will definitely reduce your carbon footprint.

There's a whole bunch of information on solar water heating systems at www.solarsmarter.org.nz

.....but I think that you could invest $4500 in lots of places and have all sorts of different returns - thats not the argument. It's probably the best thing you could do to reduce your household energy emissions...there are also interest free loans from the government (which is how I financed my solar hot water heater).....plus people are more likely to be making the decision to spend the money on a new car, ipod or other desirable object not the mortgage.....unless you are super organised and disciplined with your finances!