What a solar water heater is
A solar water heater uses collectors on the roof to heat water directly, instead of making electricity the way solar panels do. A pump (or, in simple systems, natural circulation) moves water or antifreeze between the collectors and an insulated storage tank, and a conventional electric or gas heater tops up the temperature when the sun isn’t enough.
There are two common collector types. Flat-plate collectors are insulated boxes with a dark absorber plate under glass; evacuated-tube collectors are rows of glass tubes that lose less heat in cold weather. In warm climates without freezes, simpler open-loop systems heat the household water itself; elsewhere, closed-loop systems keep antifreeze in the collectors.
On the mainland, most homeowners who want to cut energy costs with the sun now install solar panels, and some pair them with a heat pump water heater instead. Solar water heaters make the most sense where water heating is a big share of the bill and electricity or propane is expensive.
Where solar water heaters are still common: Hawaii
Solar water heating has largely given way to solar panels on the mainland, but not in Hawaii, where electricity is the most expensive in the country and new single-family homes are generally required to include a solar water heater. Honolulu’s public permit records (City and County of Honolulu) show about 1,150 permits for solar water heaters in 2024 and 1,390 in 2023, roughly four in ten of them on new homes. The median declared value of work on retrofit permits since 2023 was about $4,100; permit valuations often understate the full installed price, so treat it as a floor.
How the installation works
- Sizing: the installer estimates daily hot water use from the number of people in the home and sizes the collectors and tank.
- Collectors: they are mounted on a south-facing roof plane (or a ground rack), tilted for the sun.
- Piping: insulated pipes run from the collectors to the tank, usually through the attic or an exterior chase.
- Tank: a solar storage tank, often 80 gallons or more, replaces or sits next to the existing water heater.
- Pump and controller: a controller compares collector and tank temperatures and runs the pump when the roof is hotter.
- Filling and testing: closed-loop systems are pressure-tested and filled with antifreeze, and the backup heater is connected.
Cost factors
| Factor | Impact on cost |
|---|---|
| System type | Closed-loop systems for freezing climates cost more than open-loop or passive systems used in warm areas. |
| Collector type | Evacuated tubes work better in cold weather but usually cost more than flat-plate collectors. |
| Tank size | Larger households need 80- to 120-gallon tanks, which cost more than a standard 50-gallon tank. |
| Pipe run | A long run from the roof to a basement tank in a finished home takes more labor. |
Who it suits
- Households that use a lot of hot water, since savings grow with use.
- Homes heating water with electric resistance or propane, the most expensive fuels for the job.
- Sunny, warm climates, where simpler systems work and produce more.
- Homes in Hawaii, where electricity costs the most in the country and new single-family homes generally must include one.
If you already plan solar panels, compare the cost of a heat pump water heater running on that solar power before choosing a separate thermal system.
Questions to ask your installer
- Do you recommend flat-plate collectors or evacuated tubes for our specific climate?
- Is this an active closed-loop system with antifreeze to protect against winter freezes?
- What size storage tank do we need based on a family of our size?
- What maintenance is required for the circulating pump and the antifreeze fluid?
- Will the backup heating element run on gas or electricity?
- Are there any state or utility rebates for solar water heating in my area? (The 30% federal credit that covered solar water heaters ended on December 31, 2025.)
What to watch out for
- Installers without solar thermal experience. These systems handle high temperatures and pressures, and a general plumber may not size the expansion tank, valves or insulation correctly.
- Overheating. When a household is away and the tank is hot, collectors can reach very high temperatures. Ask how the design handles it.
- Freeze protection. If temperatures ever drop below freezing where you live, make sure the system is closed-loop or drainback, not open-loop.
- Antifreeze and pump upkeep. Glycol breaks down over time and has to be tested and replaced every few years, and pumps eventually fail.