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A solar water heater sits on a rooftop in Brazil.

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Rising prices and what often looks like an energy crisis may mean you are paying more attention to how your home uses energy. If you want to save money on your bills, start with your water heater. That unassuming boiler in your basement can use 14% to 18% or your home’s monthly energy budget, according to the Department of Energy.

While simply turning down the temperature of your water heater is a good place to start, in some cases using a different fuel source can make an even bigger difference. Example: Solar water heaters use the heat of the sun to heat water. Although they are widely adopted in some parts of the world, they are not very popular in the United States at this point.

Given their cost and the drop (until recently) in the price of solar panels, solar water heaters may not make sense for many homeowners. But in some cases they can still reduce your carbon footprint and save you some money.

Below is everything you need to know before gluing one to your roof, including whether you should. You can also find out about if unplugging your devices really saves you moneyand 28 Ways to Cut Your Home Costs Right Now.

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What are solar water heaters?

Solar water heaters vary in design, efficiency, capacity and price, but they all replace much of the gas or electricity used to heat water with clean, free sunlight. The three basic designs all have a way to collect heat, a tank to store hot water, a backup heater when your system can’t keep up, and some sort of circulation system.

Manifold water heaters heat water in tubs or pipes, usually painted black to capture more heat from the sun. To prevent the water from getting too hot, cold water can be mixed in periodically. These heaters are better suited to warmer climates. Where freezing could be a problem, they will need to be drained during the cold months to prevent damage to the system.

Flat collectors sit on a metal plate, often painted black, to absorb heat from the sun. The heat moves from the plate to the tubes filled with water. Water circulates through heated tubes to and from the storage tank, keeping stored water warm.

Evacuated tube collectors are the most efficient models. The water is heated in a tube surrounded by a larger vacuum-sealed glass tube. Because there is no air between the heating liquid and the outside world, very little heat is lost.

Solar water heaters can heat water directly or indirectly. In indirect radiators, the sun heats a heat transfer liquid (often a mixture of water and propylene glycol), which then transfers its heat to water in a tank. Since the freezing point of the heat transfer liquid is lower than that of water, it can operate in colder climates. Evacuated tube collectors can operate in temperatures as low as -40 degrees Fahrenheit, according to the US Environmental Protection Agency.

Water must circulate through the system so that warmed water can be stored and fresh water can be heated. This happens in two ways. Passive systems rely on hot water rising and cold water falling. Natural water movement circulates water through the system. Or, systems can actively circulate water, using a pump. This is the most common type of system in the United States, according to the EPA.

Buy a solar water heater

Before buying a solar water heater, there is a bit of research to do.

First, your roof (the most common location for solar water heaters) must be in good condition and getting enough sunlight. If you need to replace your roof, do it first. Depending on the system, you can get by with more or less sun, but a recommended measure is at least six hours of sunshine on a clear day, all year round. A roof facing directly south (in the northern hemisphere) is best, but heaters can be moved 45 degrees F in either direction and still work reasonably well.

Second, as with any major purchase, you should get multiple quotes. Installers with local knowledge may be able to give you a better idea of ​​how well your water heater will perform in your local climate.

Accurately sizing your water heater requires some complicated calculations, but we can rely on a few rules of thumb. To provide hot water for two people, a solar water heater must have a 20 square foot collector plate. For each additional person, the collector should be 8 square feet larger if you live in sunnier areas, such as the southern United States, and 12 to 14 square feet if you live in the north. Water reservoirs should be 1.5 gallons per square foot of collector plate, or up to 2 gallons per square foot for sunnier climates.


Solar water heater on a roof in Spain.

Jose A. Bernat Bacete/Getty Images

Two other metrics you’ll want to check are solar energy factor and solar fraction. The solar energy factor of a system is the amount of energy supplied by the solar part of the system divided by the amount of energy supplied by gas or electricity. The scale goes from one to eleven; the Department of Energy says two or three are typical. The solar fraction is the energy delivered by the sun divided by the total energy used by the system (including what is wasted along the way). A typical solar fraction is between 0.5 and 0.75 and changes depending on the climate of a region. All EnergyStar certified solar water heaters provide these numbers and are helpfully compared by EnergyStar.

Hypothetically, a family of four living in the northern part of America would need a collector plate of at least 44 square feet and a storage tank of 66 gallons. One vendor, Duda Diesel, offers a system with an 80-gallon tank for $3,499. This one vendor offers systems ranging from 26 gallons ($2,175) all the way up to 264 gallons ($11,477).

In the United States, solar water heaters are eligible for the federal tax credit, which currently sits at 26%, but is expected to drop to 22% in 2023. For a system that costs $3,877, you would be eligible for receive $1,008 when you file your taxes.

There may also be maintenance costs, especially if you have hard water that leaves deposits. These can decrease the efficiency of your system and require temporary descaling. Maintenance costs are generally minimal.

Is a solar water heater a good deal?

Solar water heaters are significantly more expensive than conventional water heaters. You will likely spend thousands or more to install one and it will take at least a few years to save more money than you spent.

How quickly a solar water heater will pay back its cost in energy savings depends on the local cost of energy, whether you heat water with gas or electricity and even when using hot water . (Moving showers, dishes, or laundry in the evening, when there’s plenty of solar-heated water at the tap, will require less backup power than doing those things earlier in the day.) You can find detailed instructions for calculating your payback period, but solar water heaters take longer to pay for themselves if they replace natural gas water heaters than if they replace water heaters electrical. If your hot water costs are high, you could recover your expenses in a few years, much faster than the 20 years or more that a solar water heater can operate.

A more cost effective way to switch to solar power could be to run an electric water heater with solar panels generating electricity on your roof. Solar panels are getting cheaper, and they can replace the energy needed not just to heat water, but for all of your other home electricity needs. A solar panel on the roof can be sized to include the needs of an electric water heater and, depending on the efficiency of the water heater, may require only a slightly larger solar panel. And your the utility may pay you for any excess electricity you generate from solar panels. Any excess energy created by a solar water heater will not be offset.

Although solar water heaters are less expensive than photovoltaic solar panels, the latter will likely save more money in the long run and meet more of your energy needs. While local prices, your energy usage, and the climate will always impact this calculation, solar water heaters will likely save you money and keep some carbon emissions out of the atmosphere. Solar panels can outperform solar water heaters on both counts.