A lot of the world is already plugged into the sun for heating its water. I first saw entire communities with solar panels on the roof when I toured Israel. Of course, you can't beat the Middle East or the Mediterranean for long sunny days. I thought by now a lot of the American Southwest, Florida, and California would have roof-to-roof paneling, but I guess we're slow to change.
Freezing cold temperatures at night or in winter can burst pipes in a solar heater if you don't install a drain-back tank or pipe monitor system with anti-freeze valves. Then there's capacity and needs. A typical four-person family needs about 72 gallons of hot water a day for showering, cleaning, laundry, and cooking.
Solar Water Heat: What, Where, and Why
There are passive and active solar water heating systems. Passive systems circulate warm water when the sunlight is consistent. They're a good choice in warm or hot climates, but they freeze on you if you have cold winters. Active solar systems are better in the cold. These systems circulate their water supply with pumps and valves.
The U.S. Department of Energy says you need a well-insulated storage tank to generate the most savings from your solar water heater. An active, closed loop system relies upon a flat plate collector connected to your storage tank. The Energy Savers website has a calculator for determining the proper system for your home and environment.
You need to estimate the annual operating cost for the solar water heater based on the solar energy factor for the system you choose and the rates for heating the auxiliary tank power (by electric or gas). Also build several budgets around the cost and efficiency of passive vs. active solar water heating for your home. You can realize as much as a $500 savings in energy costs a year with a solar water heating system. Water heating accounts for as much as 15 percent of your total energy bill.
Solar water heaters are far less expensive to install and operate than photovoltaic arrays. You can build a passive, batch-style system for several hundred dollars. But you need to know about local building codes before lifting a hammer.
You might also consider installing an on-demand water heater for your kitchen or bath.