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This is an excellent question. Understanding the answer provides some insight into why, 50 to 100 years from now, solar water heaters will still be the best way to heat water.  And why you should go ahead and install a solar water heater today.

The issue is conversion efficiency. A typical flat plate solar water heating collector transfers about 63 percent of the solar energy that strikes it directly into the water (or heat transfer fluid, in indirect circulation systems).

A typical photovoltaic (PV) solar electric cell converts only 15 percent or so of the energy striking it into electrical energy under ideal conditions: PV cells lose efficiency as operating temperatures rise. Additional inefficiency occurs when an inverter changes direct solar electric current (DC) into alternating current (AC). The end result is that a PV cell with a 15 percent rated peak efficiency only delivers about 10 percent of the energy striking its surface to a demand load.

So here is the problem: The solar electric PV panels would require more than six times the roof area of a flat plate solar thermal collector to meet the same (water heating) demand load.

Meeting the hot water needs of an average Florida family of four requires about 40 square feet of flat plate solar water heating collector surface area, so to do the same job with PV panels would take about 40 x 6 = 240 square feet of PV panel surface area.

Completely. All you do is set your pool pump’s time clock or automation system to run during the daylight hours, then set the solar pool heater’s automatic control temperature indicator to your preferred temperature. The control system takes care of the rest. Sensors compare the temperature at the solar collectors with you pool water temperature. Whenever the solar collector temperature is at least four degrees warmer than your pool water, the control system adjusts a motorized valve to divert pool water through the solar collector panels if your pool is not at the desired temperature.

And even if your pool cannot reach your preferred temperature setting during the coldest winter weather, your pool will always be warmer than a neighbor’s similarly situated unheated pool, without you spending a penny on expensive fuel.

A lot depends upon how you use your pool and your tolerance for colder temperatures. Residents of northern climates are completely comfortable swimming in 78°F water and this is usually the temperature required for competitive swimming events. On the other hand, Florida residents usually like their pool water a bit warmer; at least 80°F or more. And if you are heating your pool for therapeutic reasons, you will want at least 85°F water and possibly as warm as 90°F.

A Central Florida screened pool will typically be at least 80°F or warmer for about three months and may reach 83–85°F from mid August to early September. Open pools are a bit warmer because the pool surface receives more direct solar energy, and may stay comfortable for an extra month or more.