Solar water heating, also called solar thermal, uses the sun’s energy to heat water for your home, swimming pool, or even your living space. Unlike photovoltaic (PV) panels that generate electricity, solar water heating systems capture sunlight and convert it directly into heat. A typical system includes solar collectors, a storage tank, and a backup heater to ensure hot water is available even when the sun isn’t shining. According to the U.S. Department of Energy’s 2003 consumer guide, these systems are reliable, adaptable, and pollution-free because they use renewable energy.
How solar water heating works
Solar water heaters come in a wide variety of designs, but all include a collector and a storage tank, as ENERGY STAR explains. The collector absorbs sunlight and converts it to heat, which is then transferred to water or a heat-transfer fluid. The heated water is stored in an insulated tank until it is needed. When hot water demand exceeds what the solar system can provide, a conventional electric or gas backup heater steps in. ENERGY STAR notes that a certified solar water heating system is generally designed for use with an electric or gas backup water heater.
Collector types
The collector is the heart of the system. ENERGY STAR describes three main types used in residential applications:
- Flat-plate collectors are the most common type, according to the 2003 DOE guide. They consist of copper tubes attached to dark absorber plates inside an insulated, weatherproofed box covered with tempered glass. ENERGY STAR says a typical flat-plate collector holds about 40 gallons of water, and two collectors can provide roughly half of the hot water needed for a family of four.
- Integral collector-storage (ICS) or batch collectors heat water in dark tanks or tubes within an insulated, glazed box. The water stays in the collector until it is drawn, so it can become very hot. A tempering valve mixes in cold water to prevent scalding. ENERGY STAR warns that batch collectors are incompatible with closed-loop circulation systems and are generally not recommended for cold climates.
- Evacuated tube collectors are the most efficient collectors available, according to ENERGY STAR. Each tube contains a metal absorber tube surrounded by a larger glass tube, with a vacuum between them to minimize heat loss. These collectors can work well in overcast conditions and operate in temperatures as low as -40°F. Individual tubes can be replaced as needed. ENERGY STAR notes that evacuated tube collectors can cost twice as much per square foot as flat-plate collectors.
The 2003 DOE guide adds that unglazed flat-plate collectors made from polymer materials are typically used for pool heating. These collectors lack a glass cover and are simply a dark absorber plate.
Circulation systems: active vs. passive
Solar water heating systems are also defined by how they move water or heat-transfer fluid. ENERGY STAR distinguishes between active and passive circulation:
- Active systems use electric pumps, valves, and controllers to circulate water or fluid. ENERGY STAR says they are common in the United States, and the 2003 DOE guide says they are usually more efficient than passive systems, although they rely on electricity to operate.
- Passive systems require no pumps. They rely on natural convection: as water heats up, it becomes lighter and rises, pushing cooler water down. Passive systems can work during power outages and are typically less expensive to purchase and maintain, according to the 2003 DOE guide. However, they are not usually as efficient as active systems.
Direct and indirect systems
Both active and passive systems can be either direct or indirect, as detailed in the FSEC manual. In a direct system, the potable water that you use in your home circulates through the collectors and back to the storage tank. ENERGY STAR says direct systems are preferable in climates where it rarely freezes. Freeze protection is necessary in cold climates.
An indirect system, also called a closed-loop system, uses a non-freezing heat-transfer fluid that circulates through the collectors and a heat exchanger. The fluid never mixes with the household water. Instead, it transfers heat to the potable water inside the storage tank. ENERGY STAR states that indirect systems make sense in freezing climates. The FSEC manual adds that indirect systems are also used in areas with very hard water to prevent mineral buildup in the collectors.
One common passive design is the thermosiphon system, where the storage tank is mounted above the collector. As water warms, it rises naturally into the tank. The 2003 DOE guide notes that contractors must pay careful attention to roof design because the water in the storage tank is heavy. Thermosiphon systems can be direct or indirect.
System types at a glance
The table below summarizes the most common system configurations, drawing on information from ENERGY STAR, the DOE guide, and the FSEC manual.
| System type | How it circulates | Climate fit |
|---|---|---|
| Active direct | Pump moves household water through collectors | Mild climates where freezing is rare |
| Active indirect | Pump moves antifreeze fluid through collectors and heat exchanger | Cold climates with freezing temperatures |
| Passive thermosiphon (direct or indirect) | Natural convection; tank above collector | Mild to moderate climates; indirect version for freeze protection |
| ICS / batch | No pump; water heated and stored in the collector unit | Mild-freeze climates only; not for areas with distinct winters |
For a more detailed breakdown, see our guide on solar water heating system types.
Applications for solar water heating
Domestic hot water
Heating water for showers, laundry, and dishes is the most common residential application. ENERGY STAR states that an ENERGY STAR certified solar water heating system can cut your annual hot water costs in half. The system preheats water before it enters your conventional water heater, reducing the amount of energy the backup heater needs to use. The DOE’s 2003 guide suggests that sizing a solar water heater involves determining the collector area and storage volume needed to meet 90% to 100% of your household’s hot water needs during the summer. It describes a contractor guideline of about 20 square feet of collector area for each of the first two family members, with additional square footage for extra occupants depending on the region. For active systems, the storage tank size typically increases with the collector area, at about 1.5 gallons per square foot of collector. These numbers are from a 2003 publication and should be discussed with a qualified contractor today.
Pool heating
In 2003, the DOE guide described pool heating as widely cost-effective. Unglazed polymer collectors are common, and the pool itself serves as the storage tank. The pool’s filtration pump circulates water through the collectors. The 2003 DOE guide notes that a solar pool heating collector is typically sized to equal 50% to 100% of the pool’s surface area, depending on location and how long the pool is used each year. For example, a 15-by-30 foot pool in Florida might require a collector equal to 100% of the pool’s square footage, while in northern California a system sized at 60% to 70% of the pool’s area might be sufficient for a 6- to 8-month season. A pool cover helps reduce heat loss. Learn more about solar pool heating.
Space heating
Solar space heating systems use the sun to warm the air or water that heats your home. The DOE’s 2003 guide explains that space heating usually requires a larger, more complicated system than water heating alone, because heat must be stored for use at night and during winter. These systems are often combined with water heating. The guide states that the area of solar collector is generally about 10% to 30% of the floor area of the house, but many factors influence the exact size, including insulation levels, airtightness, and local climate. Because this information is from 2003, current sizing methods may differ. Visit our applications page for more context.
Is solar water heating right for your home?
Before you buy, ENERGY STAR recommends checking whether your site is suitable. Most systems are installed on the roof. For a roof to work well, it should receive direct sunlight between 10 AM and 4 PM year round, face south (within 30° of due south, not magnetic south), and be sturdy with shingles in good condition. If your roof is not appropriate, a ground-mounted system on unshaded land may be an option. The DOE guide adds that unshaded areas facing southeast or southwest can also work, and a solar professional may recommend west-facing roofs in some cases.
Climate plays a big role in system selection. ENERGY STAR advises that if temperatures are likely to fall below 42°F, you should choose a closed-loop system with antifreeze or a drainback system that automatically empties the collectors when freezing conditions threaten. Batch systems should be avoided in climates with distinct winters. For more on matching a system to your local conditions, see our articles on climate and sunlight and roof and site suitability.
Maintenance and longevity
Periodic maintenance helps a solar water heating system operate efficiently. ENERGY STAR suggests that in addition to hiring a qualified contractor for annual inspections, you should periodically check several items. On the roof or ground, make sure the collector is clean and not shaded, the sealing and glazing are not cracked or yellowing, fasteners are sound, piping and wiring are well-connected and insulated, and roof penetrations are well-sealed. Inside the home, check that the pressure relief valve is not stuck, pumps (in active systems) activate when the sun is shining, and pipes are free of mineral buildup. If you live in an area with hard water, you may need to add de-scaling agents every few years, such as running a vinegar solution through the collector loop every three to five years.
The FSEC manual details several freeze protection methods. Indirect systems use antifreeze fluid that circulates in a closed loop. Drainback systems automatically empty the collectors when the pump shuts off, so no water remains to freeze. Manual drain-down systems require the owner to open valves and drain the collector loop before freezing weather. All systems except those in tropical climates need some form of freeze protection. For more on keeping your system in good shape, read our guides on routine maintenance and cold weather protection.
Frequently asked questions
Does solar water heating work in cold climates?
Yes, but the system must be designed for freezing conditions. ENERGY STAR recommends closed-loop indirect systems with antifreeze or drainback systems that remove water from the collectors when temperatures drop. The FSEC manual notes that indirect antifreeze fluid circulation is frequently used in cold climates. Proper installation and freeze protection are essential.
How much can I save with a solar water heater?
ENERGY STAR states that an ENERGY STAR certified solar water heating system can cut your annual hot water costs in half. Actual savings depend on your household’s hot water usage, local energy prices, climate, and system size. The DOE’s 2003 guide mentions that solar heating systems can insulate you from rising fossil fuel costs, but it does not provide specific dollar savings. To understand current incentives, check with official sources such as the ENERGY STAR Special Deals Finder or your state energy office.
What is the difference between solar water heating and solar electric panels?
Solar water heating (solar thermal) captures sunlight to heat water or a fluid directly. Solar electric panels (photovoltaic or PV) convert sunlight into electricity. A solar water heater does not generate electricity, and PV panels do not heat water. The two technologies serve different purposes and are not interchangeable.
For a deeper dive into system selection, components, costs, and maintenance, explore our solar water heating basics hub.
