Sizing:
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The size of the system is determined by the necessary minimum and maximum hot water temperature. In winter, the system’s output and incoming water temperature are usually at their lowest. On hot days, it is important that the system doesn’t make the water too hot. The changes in temperature and solar radiation between summer and winter, and day and night need to be considered. To properly size a solar water heating system, the total collector area and storage volume need to be determined.The size of the collector can be estimated by allowing 15-20 sf of collector area per family member. The size of the storage tank depends on the size of the collector. Typically, the storage tank needs to hold 1.5 gallons per square foot of the collector. A 50 gallon storage tank is usually large enough for a two-member family. An 80 gallon tank is appropriate for three to four people. For larger families, a 120 gallon tank is used.
Sitting:
How the collectors sit on a roof greatly affect the amount of heat they absorb. An appropriate angle to tilt the collector needs to be determined. Typically, the ideal position is to place the collector facing south. The chart below shows the average solar radiation in summer and winter in Pennsylvania at various tilt angles.
It is critical to determine the optimal tilt angle for the collector in order to receive the best performance. The slope of the roof, shading factors, and the available hardware all play into the determination of the tilt angle.
Energy Efficiency
The energy effiency of a solar water heating system can be determined by its solar energy factor. The solar energy factor is defined as "the energy by the system divided by the electrical or gas energy put into the system." The higher the number, the more energy efficient the system. The SEF can range from 1 to 11, but systems commonly have a factor of 2 or 3.
Cost
Initially, a solar hot water system can be more expensive than other systems. However, since solar energy is free, there is a payback period. The payback period depends on climate and available solar heat. Due to the warm weather in Florida, the payback period is less than 7 years. In many cases, the monthly cost of solar heat is less than other conventional water heaters. The chart below compares relative costs and payback periods around the globe. The table assumes that solar hot water systems have energy savings of 200 kWh per month, or 6.6 kWh per day.
The table shows that the payback period is much shorter in southern hemisphere countries where there is good sunshine.Installing a solar water system decreases water heating bills by 50% to 80%.