When most people hear the phrase "solar panels," they envision photovoltaic, roof-mounted units generating electricity for a home or business.

An older, more widely-used solar application is thermal water heating. These systems are comprised of a solar collector linked to a storage tank and may be active (with circulating pumps) or passive. Useful in all climates, solar water heating systems can, in some regions, supply as much as 85 percent of the domestic hot water energy need.

The two most popular types of solar water heaters are flat-plate and evacuated-tube collectors.

Glazed, flat-plate collectors are housed in weatherproof, insulated boxes. The units are outfitted with a dark plate to absorb heat, which sits under one or more covers made of glass or polymer plastic. Unglazed variants of the same configuration are typically used to heat pools and do not have a cover.

Evacuated-tube solar collectors are made of parallel rows of transparent glass tubes. Each tube is made of a glass outer layer and an inner metal absorber with a fin attachment.

The fin serves to absorb the sun’s rays while also inhibiting radiant heat loss. These systems are smaller and more efficient, but also cost more. In terms of installation expenses, however, flat-plate and evacuated-tube systems are equal.

In direct circulation systems, pumps move the household’s water through the collectors and back into the home. These work best in climates where the temperature does not fall below freezing. Indirect circulation systems move a heat transfer fluid with a high freeze temperature through the collector.

This fluid then moves through a heat exchanger, where the water is warmed before it flows into the home. These systems are best suited for colder, freeze-prone regions.

Various substances can be used as a heat transfer fluid based on thermal capacity, freezing and boiling points, viscosity, and flash point. Both air and water are options, but both have disadvantages. Air has a low heat capacity and tends to leak out of collectors and associated tubing.

Water has a high freezing point, can cause corrosion, and, if "hard," will leave mineral deposits in tubes and pipes. Other choices are glycol/water (antifreeze) mixtures, hydrocarbon oils, and refrigerants, although the latter are being phased out due to their damaging effect on the earth’s ozone layer.

Evacuated-tube systems need only the light they receive to perform their task, and continue to function under cloudy conditions regardless of ambient temperature

These are the most popular systems in use in Northern Europe. Typically, they are paired with a back-up hot water supply, either through a two-tank system (joining the solar supply to the home’s existing water heater) or in a single tank arrangement with an alternative heat source.

Unlike the photovoltaic cells that are the focus of so much discussion about the use of solar power for generating electricity thermal solar water heating systems, have a stable, proven track record. They are used in hundreds of thousands of homes around the world. At an opening price of approximately 4,900 euros ($7,100 USD), solar water heating offers an excellent return on the investment for homeowners who are looking for ways to lower their electric bill and use clean, renewable energy.