How do solar water heaters work?

About 20% of a home’s energy use goes toward water heating, according to the U.S. Department of Energy. If you’re looking to save on energy for financial or environmental reasons, you might consider powering your hot water system with solar energy.

Solar water heaters work by absorbing sunlight in solar collectors and saving the heated water in a storage tank. Both active and passive solar water heaters are available; active systems have manual controls and pumps that circulate the water through the system to the home (these tend to be more expensive).

Key insights

• Solar thermal systems heat water for household use with clean energy from the sun.
• Solar water heating can reduce fuel consumption by 50% to 70%.
• For homes with long winters or rainy seasons, solar water heaters may not be as efficient.

Can you run a water heater on solar power?

Solar water heaters can actually be more efficient than traditional electricity (and can reduce fuel consumption by up to 70%). For this reason, they’re fairly popular in other parts of the world — Cyprus and Israel, for instance, have solar hot water mandates, with 90% of homes using solar heating systems — and they’re slowly taking hold in the U.S.

These systems do have a few unique requirements, though. Most notably, your roof needs an abundance of sunlight throughout the day. This means you’ll need a large, unshaded area that faces either south, southeast, southwest or west. For optimal performance and to ensure your investment is as safe as possible, your roof should also be relatively new and built with durable materials, such as asphalt.

“If you're considering a solar water heater, be sure to have your roof inspected by a qualified professional to make sure it can handle the weight of the system,” said Alan Duncan, CEO of Solar Panels Network USA, a company that helps educate individuals on renewable energy installations.

To help determine whether your home can handle a solar water heater, review the U.S. Department of Energy’s guide for siting a solar water heating system.

How solar hot water systems work

Active solar water heaters use pumps to help circulate water. Passive systems, including integral collector storage (ICS) systems and thermosyphon water heaters, don’t use any sort of water-moving technology — they rely on gravity and convection to move water through the system instead.

What is an integral collector storage (ICS) system?

An ICS system is a simple type of passive solar water heater that collects and stores cold city water in a tank. Three sides of the tank are insulated, and one side is glazed. The glazed side faces the sun and collects the heat; the sun’s rays pass through this side to heat the water inside. These are best for warm or hot climates because heat can easily be lost through the large plate.

What is a thermosyphon water heater?

A thermosyphon water heater is a type of passive solar water heater that separates the collector and the storage tank. With this system, the storage tank is located higher up than the collector, with gravity pushing colder water into the collector and warm water rising by convection to the storage tank.

“[A thermosyphon system] relies on the fact that hot water rises and cold water falls, so no external pump is needed to move the heated water around. This makes thermosyphon systems a very efficient way to heat your home's water supply using renewable energy sources,” said Ryan Collier, director of Heat Pump Source, a company that works with heating installation and repair in London.

Do solar water heaters work in winter?

In winter, sunlight is typically less potent and more elusive, leading to a drop in solar energy production — which means it’s important to install a backup water heater that can pick up the slack during the colder months.

If you live in an area with long winters, your home may not be the best candidate for a solar water heater, especially if you already have budget concerns. This is also true for homes in foggy or rainy climates (like in the Pacific Northwest). Without a significant amount of direct sunlight, solar water heaters aren't as effective.

What are the disadvantages of a solar water heater?

While solar water heaters can save some homeowners big money in the long run, there are a few downsides to purchasing and installing these machines. For starters, they’re expensive, usually costing somewhere between $2,000 and $6,000 to install (compared with $1,000 to $3,000 for a traditional water heater). Solar installation may be eligible for certain tax incentives, however.

You can expect to spend from $2,000 to $6,000 upfront on a solar hot water system.

Solar water heaters also require more maintenance, including annual pump and antifreeze checks and regular scaling prevention efforts. For some, like a reviewer from Arizona on our site, the added cost and headache of maintenance can be too much to handle.

“I had solar years ago in California,” they said. “I had one for my water heater and it never worked. … I said I'd never do solar again.”

Depending on the size of your household and your habits, a solar water heater also might not be suited to your needs. If you have a house full of people who regularly shower at night, for example, it might not make sense — these heaters create energy during the day, when the sun is out, and store whatever is left, which means nighttime hot water usage should be kept to a minimum.

Solar water heaters vs. home solar systems

You can also heat your water with a traditional photovoltaic home solar system. Rooftop solar panels convert sunlight into energy, which you could then use to run a water heater. Home solar systems are relatively inefficient when it comes to powering water heaters, though, because traditional heaters require so much electricity. A total home solar system is also significantly more costly.

Solar water heaters, on the other hand, only have one job, which makes them more efficient and cost-effective for heating a home’s water supply. Plus, they require much less rooftop room than a full home solar system.

That said, if you already have plans to install a home solar system, it could be worth testing powering your electric water heater with the solar energy your system creates. If you’re only hoping to cut down your hot water costs, however, a solar water heater is a much smarter investment.

Bottom line: Is a solar water heater worth it?

Solar water heaters protect homeowners against fluctuating utility prices and shortages and can save them an average of 50% to 80% on heating bills per month. With various tax incentives available, these systems are often a good investment. For existing or older homes, however, the setup may be more difficult — you need to consider available sunlight and roof structure — which may add to the cost of installation.

Most solar water heater installers offer a free consultation to see if your home is a good candidate; we recommend starting there if you’re interested in solar water heating.