Solar water heaters use the sun’s thermal energy to warm water for your home. They are not the same as the photovoltaic (PV) panels that produce electricity. Instead, a solar water heater is a solar thermal system: it captures heat from sunlight and transfers it to water in a storage tank, with a conventional backup heater ready when the sun cannot do the whole job. Understanding how these four basic jobs work together will help you decide whether a solar water heater fits your site and climate.
Solar thermal energy, not solar electricity
According to ENERGY STAR’s guide on how solar water heaters work, all solar water heaters include a collector and a storage tank, and they use the sun’s thermal energy to heat water. This is fundamentally different from photovoltaic panels, which convert sunlight into electricity. A solar thermal collector absorbs the sun’s heat and passes it to water or a heat-transfer fluid. The U.S. Department of Energy’s 2003 consumer guide explains that solar heaters, or solar thermal systems, collect the sun’s energy to heat air or a fluid, which then transfers solar heat directly or indirectly to your water. Keeping this distinction clear is important when you research equipment and plan a project.
The four jobs every solar water heater must do
Every solar water heating system, no matter the design, performs four essential jobs: collect sunlight, move heat, store hot water, and provide backup heating. ENERGY STAR and the DOE guide both describe these functions, and the Florida Solar Energy Center’s 2006 manual (which focuses on Florida conditions) breaks them down in detail.
Collect sunlight
The collector is the part that faces the sun. ENERGY STAR lists three common collector types: batch collectors (also called integrated collector-storage or ICS), flat-plate collectors, and evacuated tube collectors. Batch collectors heat water in dark tanks or tubes inside an insulated, glazed box. Flat-plate collectors, as ENERGY STAR describes them, use copper tubes attached to a dark absorber plate inside an insulated, weatherproofed box with a glass cover. Evacuated tube collectors work like a thermos: each tube contains a glass or metal inner tube surrounded by a larger glass tube with a vacuum between them, so very little heat is lost. ENERGY STAR notes that evacuated tube collectors can even work in overcast conditions and operate in temperatures as low as -40°F.
Move heat
Once the collector has absorbed heat, something must move that heat to the storage tank. This job is done by the circulation system. In active systems, an electric pump moves water or a heat-transfer fluid. In passive systems, natural convection does the work without any pump. The DOE guide explains that active systems use pumps and controls, while passive systems work without this added equipment. The FSEC manual adds that both active and passive systems can be either direct (the potable water itself circulates through the collector) or indirect (a separate fluid collects heat and passes it to the potable water through a heat exchanger).
Store hot water
A well-insulated storage tank holds the heated water until you need it. The DOE guide states that most solar water heaters require a well-insulated storage tank, and that solar storage tanks have an additional outlet and inlet connected to and from the collector. In a two-tank system, the solar tank preheats water before it enters the conventional water heater. In a one-tank system, the backup heater is combined with the solar storage in one tank. The FSEC manual emphasizes that water itself is a good thermal mass for storing sensible heat, and that tanks are highly insulated to reduce heat losses.
Provide backup heating
Solar water heaters usually work with a backup. A backup heater, usually a conventional electric or gas water heater, steps in when the sun is not enough. The DOE guide describes how the solar system preheats water before it reaches the backup heater, reducing the amount of conventional energy needed. The FSEC manual notes that the backup can be integrated into the same tank or kept as a separate unit downstream.
How water or heat-transfer fluid moves through the system
The way heat moves from the collector to the storage tank defines two important choices: active versus passive circulation, and direct versus indirect loops. These choices affect freeze protection, efficiency, and cost.
Active and passive circulation
Active systems, according to ENERGY STAR, use electric pumps, valves, and controllers to move water from the collectors to the storage tank. These are common in the U.S. The FSEC manual describes several active control methods: differential controllers that turn the pump on when the collector is 5 to 20°F warmer than the tank bottom, photovoltaic-controlled systems that use a small PV module to power a DC pump only when the sun is strong, and timer-controlled systems used in tropical climates. Passive systems, by contrast, require no pumps. ENERGY STAR says natural convection moves water from the collectors to the storage tank as it heats up. The DOE guide notes that passive systems work during power outages but are not usually as efficient as active systems.
Direct and indirect loops
A direct system circulates household water straight through the collector. ENERGY STAR says these are preferable in climates where it rarely freezes. An indirect system uses a non-freezing heat-transfer fluid that never mixes with the potable water. The fluid circulates through the collector and a heat exchanger, where it transfers heat to the water in the tank. ENERGY STAR and the DOE guide both state that indirect systems make sense in freezing climates. The FSEC manual adds that indirect systems are also used where water has high mineral content, because the combination of high dissolved minerals and high temperatures can accelerate scale buildup in piping.
Thermosiphon: moving water without a pump
A common passive design is the thermosiphon system. The FSEC manual explains that in a thermosiphon system, the storage tank must be located above the collector. Cold water from the bottom of the tank flows down to the collector inlet. As the sun heats the water in the collector, it expands slightly and becomes lighter. Heavier, denser cold water from the tank pushes the lighter heated water up through the collector outlet and into the top of the storage tank. This natural displacement continues as long as the sun shines, filling the tank with hot water by the end of the day. The DOE guide describes thermosyphon systems similarly, noting that warm water rises as cooler water sinks, and that the collector must be installed below the storage tank. Both sources caution that the weight of the water-filled tank on the roof requires careful attention to roof design.
Freeze protection and overheating: two critical safeguards
Solar water heaters face two opposite threats: freezing temperatures that can burst pipes and collectors, and stagnation temperatures that can damage components or create scalding hot water.
Freeze protection
The FSEC manual lists five basic methods to prevent freeze damage: indirect antifreeze fluid circulation, draining (manual or automatic), water flow, thermal mass, and auxiliary heaters. Indirect systems inherently protect the collector loop with antifreeze. Drainback systems, described in the FSEC manual, empty the collector and exposed piping by gravity whenever the pump is off, so no water remains to freeze. Direct systems in mild climates may use freeze valves or rely on the thermal mass of the collector, but ENERGY STAR warns that batch collectors are generally not recommended for cold climates because outdoor pipes could freeze. The DOE guide emphasizes that passive direct systems should not be used where temperatures often go below freezing.
Overheating and scalding protection
On sunny days when little hot water is drawn, a collector can reach very high temperatures. The FSEC manual notes that stagnation temperatures can be as high as 400°F for some collectors. To protect people from scalding, ENERGY STAR recommends a tempering valve that mixes cold water with the hot water before it reaches the tap. The DOE guide and FSEC manual also describe pressure and temperature relief valves as essential safety components that release high pressure and temperature. Any system connected to a home’s pressurized drinking-water supply must follow local plumbing and building codes and may require a permit and a licensed plumber. The materials that touch potable water must be safe for that use, and the roof structure must support the weight of collectors and a full storage tank.
What the sun provides changes by site and season
The amount of solar energy available for heating water is not the same everywhere or all year. The FSEC manual’s solar basics section explains that insolation, the sun’s electromagnetic energy falling on a surface, varies with latitude, season, time of day, and weather. At the latitude of Orlando, Florida (about 28°N), a horizontal surface can receive a maximum of around 300 Btu per square foot per hour at noon on a clear June day. In winter, the sun’s path is lower and shorter, and the angle of incidence is less favorable, so a fixed collector receives less energy. The FSEC manual shows that for year-round use, a collector tilt roughly equal to the local latitude suits year-round use, but a tilt of latitude plus 15° favors winter collection and latitude minus 15° favors summer. Shading from trees or buildings also reduces available energy, so a site assessment is essential before you choose a location. For more on evaluating your site, see our site assessment guide.
At a glance: jobs, parts and what to look for
The table below summarizes the four jobs, the key parts that do them, and the main points to check when you evaluate or plan a system. Use it alongside our operating principles overview and the DIY solar water heater basics article.
| Job | Key part(s) | What to look for |
|---|---|---|
| Collect sunlight | Collector (flat-plate, evacuated tube, batch) | Orientation (south-facing in northern hemisphere), tilt, glazing, absorber coating, durability |
| Move heat | Circulation system (pipes, pump or natural convection, heat exchanger if indirect) | Freeze protection method, check valves, insulation, proper pipe slope for drainback |
| Store hot water | Storage tank | Insulation, tempering valve for scalding protection, pressure and temperature relief valve, potable-water-safe materials |
| Provide backup | Conventional water heater or backup element | Integration with solar tank, compliance with local codes, proper sizing |
For a deeper look at collectors, visit our collectors page. Storage tank details are covered in the storage section. Safety devices like tempering valves and relief valves are explained in our guides on scalding prevention and pressure and temperature protection. Because any system tied to a home’s water supply must meet plumbing and building codes, always review the codes and professional review page before you start.
Frequently asked questions
Is a solar water heater the same as solar electric panels?
No. Solar water heaters are solar thermal systems that capture heat from the sun to warm water. Photovoltaic (PV) panels convert sunlight into electricity. The two technologies use different equipment and serve different purposes. ENERGY STAR’s guide makes this distinction clear: solar water heaters use the sun’s thermal energy to heat water.
Can a solar water heater work in a cold climate?
Yes, if the system is designed for freezing conditions. Indirect systems circulate a non-freezing heat-transfer fluid through the collector and a heat exchanger, protecting the outdoor loop. Drainback systems empty the collector when the pump is off. The FSEC manual and ENERGY STAR both describe these freeze-protection strategies. However, passive direct systems and batch collectors are generally not recommended where temperatures often drop below freezing, according to the DOE guide and ENERGY STAR.
Do I need a backup water heater with a solar system?
Yes. Solar water heaters preheat water, and the sources describe a backup heater for days when solar heat is not enough. A conventional backup heater, either integrated into the solar tank or installed as a separate unit, ensures you have hot water during cloudy periods or high demand. The DOE guide and FSEC manual both describe this arrangement.
Ready to explore the principles in more detail? Head to our operating principles category page for guides on how solar thermal systems work, what to consider before you build, and how to match a design to your climate.
