Solar Water Heaters: How Do They Work? Should You Get One?

Types, components, and pros and cons of solar water heaters.

Using a solar water heater can save a household significant money and help reduce local and regional emissions that cause air pollution. Although they do require a higher initial cost for both the system and installation (compared to replacing an existing electric or gas-powered water heater), they will save 50% to 80% of the costs for heating water over time, according to the Department of Energy.

Depending on what system you buy, and how much hot water your household uses, this means they could pay for themselves in a couple of years. And you may also get a tax deduction for installing one.

Solar water heaters work by using the sun's energy to either directly heat water that can then be used in the house for hot-water needs, or by using solar energy to heat another fluid that's then used to heat the water. They can be active or passive, and all systems require a storage tank.

Solar water heaters can work in cold climates, less sunny places, and in a variety of conditions, though less effectively than in sunnier locales. But even if you live in a place where your solar water heater only preheats the water a little bit in winter, somewhat in fall and spring, and plenty on summer days, you will still save money and reduce emissions.

At night and on cloudy days, if you don't have enough warm water stored, you will need an additional heating element to raise water temperatures. Most people with solar water heaters in mixed or seasonal climates use them in conjunction with an on-demand water heater to raise the water temps a little further. Since these devices are warming already warmed water, they work even faster and more efficiently than if they were heating cold water.

Usually, solar water heaters are placed on the roof, facing south, so they get the best quality direct sunlight. However, they can also be placed in a garden, meadow, or other areas where they receive direct sunlight.

An active solar water heater can be direct or indirect. In the direct system, water is circulated via pumps through the solar collectors (usually on a roof) where it's warmed by the sun and then sent to a well-insulated tank for storage. These are useful in climates where it rarely freezes.

An indirect active solar water heater uses a special non-freezing heat-transfer fluid that's heated by the sun, which then warms stored water. These are useful in places where it freezes seasonally.

Passive systems are simpler and cheaper than active systems, but less efficient. There are a couple of different kinds, with some using hot-cold water differentials to move water around rather than pumps. The other kind simply uses whatever heat energy is available from the sun to preheat water and then uses a traditional water heater to raise the temperature to what's needed.

Every solar water heater must include at least two elements: a collector to gather the sun's energy and a storage tank. After that, other parts of the system depend on the type of solar water heater being used.

The primary components of any solar water heating system are one or more collectors to trap the sun's energy and a well-insulated storage tank. There are, of course, several types of solar water heating panels.

Flat plate collector panels have a glass or polymer cover with a dark plate underneath. As the sun shines on the panel, its heat is absorbed by the plate (and the dark piping that the water flows through) and transferred to the water.

Integral storage systems are black tanks filled with water that are kept inside a clear box that's well-insulated. This system is often used to preheat water that's then fully heated to desired temperatures for bathing or household chores by an additional system like a tankless water heater.

A third type, evacuated tube collectors, contain clear tubes with metal inside and are mostly used for commercial applications.

Solar water heater storage tanks can vary depending on the size of the home, the number of solar collectors, and the amount of hot water needed in the home. Typically, most systems have a large-capacity tank—80-gallons (or more)—which allows for warm water storage on overcast days. Some systems include two tanks, so there's one for immediate use and another one just for storage.

Pros

Cons

Solar water heaters cost about $3,500 to install, and that includes both labor and parts.

Solar water heaters do work in the winter, but they're least efficient when the sun's radiation is weakest. Short days reduce solar thermal input, and cold outdoor temperatures make it hard for water to remain hot. Most solar water heaters include freeze protection, which helps keep snow and ice from accumulating around them.

No electricity or electrical components are needed to run a solar water heater. Instead, these systems use solar collectors that draw energy directly from the sun.

Solar water heaters heat water during the day, when the sun is shining, and keep it hot in insulated tanks overnight. This could be difficult in frigid temperatures, but how well your solar water heater keeps water heated in the winter depends on the quality of the system.

"Estimating the Cost and Energy Efficiency of a Solar Water Heater." U.S. Department of Energy.