Solar water heaters: What you need to know

You use hot water at home every day when you shower, run a load of laundry, or turn on your faucet to wash dishes. Solar water heating systems use the sun's energy to heat the water in your home and can help you save on energy costs. 

Solar water heaters (also known as solar hot water) are an alternative to conventional water heating systems, including tankless coil water heaters, gas water heaters, electric water heaters, or heat pump water heaters (all of which use either gas, oil, or electricity to power them.) We have used solar energy to heat water for centuries, but not until the 1980s did solar hot water companies begin to take off in the U.S., making solar hot water a viable option for property owners and their domestic hot water needs.

With a solar hot water system, you can use the sun's power to save money and reduce your reliance on conventional energy sources such as oil, electricity, and gas. Solar hot water cuts down on greenhouse gas emissions in the atmosphere and also helps you save money long-term by reducing gas and electricity bills.

Solar hot water systems capture thermal energy from the sun and use it to heat water for your home. These systems have a few major components: solar collectors, a storage tank, a heat exchanger, a controller system, and a backup heater.

Collectors

The panels in a solar thermal system are known as "collectors," and are typically installed on a rooftop. 

These collectors are different from the photovoltaic solar panels you're probably familiar with because instead of generating electricity, they generate heat. Sunshine (or "solar power") passes through a collector's glass covering and strikes a component called an absorber plate, which has a coating designed to capture solar energy and convert it to heat. This generated heat is transferred to a “transfer fluid” (either antifreeze or potable water) contained in small pipes in the plate.

Collectors come in different sizes. The size and number of collectors you'll install depends on how much sunshine your roof gets and how much hot water you use in your home. They are also made in two styles: flat plate and evacuated tube. Flat plate collectors are generally less expensive, but can capture less sunlight and are less efficient in colder conditions. Evacuated tube collectors take up less space on your roof, but are heavier and more fragile.

Heat exchanger and storage tank 

Once the transfer fluid in your collectors heats up, it moves into a series of pipes known as a "heat exchanger," which is located inside the storage tank for your hot water. When these pipes are filled with heated transfer fluid, the heat is transferred from the pipes to your water, making hot water ready for your home for applications ranging from making food to pool heating and beyond.

Controller system

Most solar hot water systems have a controller system that ensures the water in the storage tank doesn’t get too hot. Controller systems can also prevent cold water from being cycled through the system when it's extremely cold outside and the transfer fluid isn't sufficiently warmed.

Backup heater

Lastly, every solar hot water system comes with a backup system. On cloudy days when there isn't enough sun to generate enough heated water from solar energy, your backup heater will kick in and generate hot water for your home with gas or electricity. Backup heaters will account for roughly 20 percent of your hot water use yearly.

Every solar hot water system has the same essential parts, but there are some differences in system design. You have to choose between a direct or indirect system, which impacts how fluid is heated in the collectors. You also have to decide whether you want an active or passive solar hot water system, which impacts how fluid moves through your system.

Direct vs. indirect solar hot water

The main difference between direct and indirect solar hot water is the type of fluid used to collect heat in the system. In an indirect system, solar energy is collected and held in a special antifreeze fluid. The antifreeze is circulated into your hot water storage tank, which heats water for use in your home. By comparison, in a direct setup, your water gets heat directly from the sun, rather than being collected in a transfer fluid first. 

Most solar hot water installations in the United States use indirect solar hot water systems. Indirect systems are more resistant to cold temperatures and are better at retaining heat energy during colder winter months. However, as they are part of a closed-loop system, indirect systems are also more expensive than direct systems. 

Direct solar hot water systems may work for some homeowners in the most southern parts of the country, but most U.S. residents will want to install an indirect system to avoid efficiency and heat loss during colder parts of the year.

Active vs. passive solar hot water

In your solar hot water installation, antifreeze fluid or water is moved throughout the system with a controller pump or with gravity. Controller pumps are only used in active solar hot water installations. Passive systems rely on gravity to move fluid and water around.

While passive systems are less complicated to install, they are also much less efficient than active systems. Additionally, some passive systems must have their storage tank installed higher than the collectors to work, which can put high stress on your roof. Most solar hot water systems in the U.S. are active, meaning they have a controller that pumps water or antifreeze fluid through the system.

Integral collector-storage passive system

Integral collector-storage systems are made of storage tanks that allow the sun to heat water. This heated water then goes into plumbing systems for various uses. They work well in warm temperatures with late-night hot water needs.

Thermosyphon system

These systems work similarly by using a collector on the roof which then allows the water to flow through a building’s plumbing system for their hot water systems. 

Compared to conventional hot water heaters, solar hot water heaters may be a cost-effective alternative. Cost estimates vary, but according to the Department of Energy savings from using a solar hot water heater could be around $274.46/year or potentially more depending on fluctuations in the price of natural gas. The estimate for the total cost of a solar hot water heater is around $4,000.