Active solar systems have circulating pumps and controls, and typically use flat-plate solar collectors. These solar panels employ insulated metal boxes, copper pipes with black absorber plates, and glazing to produce hot water temperatures between 120o-140o F for domestic use. Because these systems circulate water through the collectors, they are better suited for producing large volumes of hot water. For that reason active solar systems are generally used for larger homes, or for heating pools and spas.
Passive Solar Water Heating Systems
Passive solar systems have no pumps, controls, or moving parts. Passive systems come in two basic designs. One is the Batch Type Integrated Collector- Storage (ICS), which operates on in-line water pressure to move the hot water from the ICS to your domestic storage tank. The other type is a thermo-siphon system which has a separate panel and a well-insulated storage tank. These systems use the natural flow of gravity to move the hot water into the solar storage tank located above the panel on the roof. They typically produce between 60-80 gallons of hot water per day. The simple nature of these systems provides the most reliable and cost-effective solution for solar water heating on a smaller scale. Multiple units can be connected together to make more hot water.
Payback on Solar Hot Water Heaters
Paybacks for solar water heating systems vary widely depending on the amount of hot water you use, and in the case of electric water heaters, how much electricity you use overall in your home. A solar water heating system has a typical payback period of 8-10 years. However, the new baseline limits for residential electrical consumption in California establish a tiered rate structure that imposes much higher rates for over-baseline consumption. A similar excess rate applies to over-baseline use of natural gas. This can easily double the cost of electric water heating (with rates as high as 33 cents per kWh) and reduce the payback time significantly.