400 square feet of photovoltaics generates less water heating power than 65 square feet of a solar water heater.
Hot water is expensive! Water heating consumes 30% of the average total domestic energy usage (electricity and natural gas together). Solar energy is a sustainable and relatively economical way to reduce that bill. The latest technologies are lowering the price point to an affordable range which can eventually pay for itself.
Some of these technologies work even in cloudy places like Seattle, and cold places like the Arctic, Antarctic, and even outer space!: heat pipes, a.k.a. evacuated tube solar collectors. They can produce a lot of heat even in Seattle.
For Seattle, they publish this: http://www.solar-rating.org/ratings/annuals/WASEATTLE20070911.pdf
The top performing commercially available system for the Seattle area, according to the independent Solar Rating and Certification Corporation, is the ThermoMax "Mazdon 90-R80" system (90 tubes, 80 gallon tank), which saves an estimated 3100kWhr/year of electricity, based on Seattle ground water temperatures, latitude, and weather.
According to my recent Seattle City Light bill, they charge $0.0793/kWh for electricity after the first 3600kWh per year (summer rates). Cost savings for this system would be 3100*0.0793 per year or about $245/year. If the cost were $3000, the payoff time would be about 12 years.
China is producing solar hot water heaters at a cost of US$190 for a basic model (see article at http://www.planetark.org/dailynewsstory.cfm?newsid=36636), and has more than 30M units installed in homes.
In a heat pipe, sunlight shines through a glass vacuum tube into an inner boiler/condenser tube, boiling off the water (or glycol) from a wet black ribbon. It condenses in a heat exchanger at the top end of the tube as your cold house water is pumped by, and dribbles down the ribbon all cool to be boiled again in a heat-absorbing cycle. Because you can change the boiling temperature of water by changing the atmospheric pressure in the tube, it works even in freezing weather, so long as the steam is hotter than the cold water in your house.
Since the absorption takes place in a tube that is itself inside a vacuum, the outside temperature, hot or cold, cannot influence what goes on inside the vacuum tube. That's why this works in outer space (in the application for which the technology was originally designed by NASA), in Antarctica, in the desert, and even in Seattle. (See Heat Pipe Factoids.) As long as a greenhouse in your neighborhood can get warmer than the city water main temperature (i.e., ground temperature) for at least part of the day, this system can extract enough solar energy to make a significant difference.