What you need to know about solar water heaters: The rooftop solar you might not have heard of

Solar water heater on a roof in Spain.

Jose A. Bernat Bacete / Getty Images

If, like me, you’ve been thirsty enough to drink from a bottle of water you left in the car on a sunny day, you know how well the sun can heat water. Let it sit for a couple of hours and you can enjoy boiling water on a scorching hot day.

That’s the concept behind a solar water heater, a type of solar technology that, while widespread in parts of the world, is battling for wider adoption in the United States Reduce your carbon footprint and save some money, a solar water heater might make sense for you.

Below is everything you need to know before putting one on your roof, including whether you should even do it.

What are solar water heaters?

Solar water heaters are actually a little more complex than an in-car water bottle. They differ in design, efficiency, capacity, and price, but they all replace a good deal of the gas or electricity used to heat water with clean, free sunlight.

Solar water heaters come in three basic designs, but they all have a way to collect heat, a tank to store hot water, a backup heater in case your system can’t keep up, and some sort of circulation system.

Batch collector water heaters heat water in tubs or pipes that are usually painted black to collect more heat from the sun. Cold water can be mixed in regularly so that the water does not get too hot. These heaters are best suited for warmer climates. Where freezing could be a problem, they will need to be drained during the cold months to avoid damaging the system.

Flat-plate collectors use a metal plate, often painted black, to absorb the heat of the sun. Heat migrates from the plate to water-filled tubes. The water circulates to and from the storage tank through heating pipes to maintain the hot water supply.

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Evacuated tube collectors are the most efficient models on the market. Water is heated in a tube that is surrounded by a larger, vacuum-sealed glass tube. Since there is no air between the heating fluid and the outside world, very little heat is lost.

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

Water must be circulated through the system so that heated water can be stored and new water can be heated. It does this in one of two ways. Passive systems are based on rising and cold water. The natural movement of water circulates water through the system. Or systems can actively circulate water with a pump. This is the most common type of system in the United States, according to the EPA.

Buying a solar water heater

Before you buy a solar water heater, you need to do a little research.

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

Second, as with any major purchase, you should seek multiple quotes. Local installers may be able to give you a better idea of ​​how your water heater will perform in your area’s climate.

Sizing your water heater exactly takes a little complicated math, but we can rely on a few rules of thumb. To provide hot water for two people, a solar water heater should have a collector plate of 20 square meters. For each additional person, the collector should be 8 square feet if you live in sunnier areas like the American South, and 12-14 square feet if you live in the north. Water tanks should be 1.5 gallons per square foot of collector plate, or up to 2 gallons per square foot for sunnier climates.

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Solar water heater on a roof in Brazil.

Lucas Ninno / Getty Images

Two other metrics to check are the solar energy factor and the solar fraction. The solar energy factor of a system is the amount of energy that is provided by the solar part of the system, divided by the amount of energy that is provided by gas or electricity. The scale ranges from one to eleven; the Department of Energy says two or three are typical. The solar fraction is the energy provided by the sun divided by the total energy used by the system (including the energy wasted along the way). A typical proportion of the sun is between 0.5 and 0.75 and changes depending on the climate of a region. All EnergyStar-certified solar water heaters provide these figures and EnergyStar compares them in a helpful way.

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 66 gallon storage tank. One supplier, Duda Diesel, is selling an 80-gallon tank system for $ 3,877.27. This one vendor offers systems from 26 gallons ($ 2,427) up to 264 gallons (12,784).

In the United States, solar water heaters qualify for federal tax credit, which is currently 26%, although it is expected to decrease to 22% in 2023. For a system that costs $ 3,877.27, you’re entitled to $ 1,008.09 back when you turn in your taxes.

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

Is A Solar Water Heater Good Business?

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

How quickly a solar water heater pays for itself on its energy savings depends on local energy costs, whether you use gas or electricity to heat water and even if you use hot water. (Moving showers, dishwashing or laundry to the evening when there is enough solar heated water from the tap requires less reserve energy than doing these things earlier in the day.) You will find detailed instructions on how to calculate your payback period, but solar water heaters will pay for themselves longer when they displace natural gas than when they displace electricity. If your hot water bills are high, you can amortize your expenses in a few years, much faster than in the 20 or more years a solar water heater can run.

A cheaper way to go for solar power might be to run an electric water heater with electricity-producing solar panels on your roof. Solar panels are getting cheaper and cheaper and can replace the electricity that is needed not only to heat water, but also for any other electricity demand in the home. A solar system on the roof can be dimensioned so that it meets the requirements of an electric water heater and, depending on the efficiency of the water heater, only requires a slightly larger solar system. And your utility could pay you for excess electricity you generate from solar panels. Excess energy generated by a solar water heater is not compensated.

Although solar water heaters are cheaper than photovoltaic solar panels, the latter are likely to save more money and meet more of your energy needs in the long run. While local prices, your energy usage, and the climate will always affect this calculation, solar water heaters will likely save you money and keep some CO2 emissions out of the atmosphere. Solar panels can outperform solar water heaters in either case.

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