How Solar Water Heaters Work

This house in Golden, Colo., features a solar water heating system. See more green science pictures.
This house in Golden, Colo., features a solar water heating system. See more green science pictures.
Photo courtesy of NREL

Heating up water accounts for a good chunk of a home's energy use. It's a near-constant need -- we use hot water for showers, laundry, washing dishes -- and it adds up. In one year, an 80-gallon (302-liter) electric water heater averages about $500 to run, uses 4,800 kWh of electricity and contributes about 6,600 pounds (2,993 kilograms) of CO2 to the atmosphere [sources: Olsen, Reuters, SB].

A natural-gas-powered heater does better, but it still accounts for about $400 and 3,900 pounds (1,769 kilograms) of CO2 per year [sources: Energy Star, GNG, PGE].


There are greener options out there. Both high-efficiency and tankless water heaters can cut back on energy use. But solar is on another level. It's about as green as hot water can get. A solar water heater is typically used in conjunction with a traditional heater, since weather affects solar hot-water production. The traditional heater supplements the solar heater. Adding a solar water heater to a water-heating system can reduce energy bills and corresponding CO2 emissions by 50 percent -- sometimes even more, depending on where you live [source: PE].

Solar is an excellent clean energy source: Its fuel, sunlight, is limitless, free and emits nothing when converted into energy. The problem with solar, as most of us know, is its efficiency. Solar photovoltaic technology, or PV, is less efficient at converting its fuel into electricity than, say, a wind turbine. But when you're talking about heating water (as opposed to powering light bulbs or stereos), the sunlight doesn't need to become electricity. It needs to become heat. And turning sunlight into heat is no problem.

Which is not to say solar water heaters have no drawbacks. In this article, we'll find out how a solar water heating system works, which factors determine its efficiency, how you could make your own, and why you might or might not want to.

At its core, a solar water heater does one thing: It uses sunlight to warm water. The same thing is happening when you leave a glass of iced tea in the sun: After a while, it's not iced anymore. Of course, a home water heater has to work faster and bigger than that, so the system has to be more complicated.

But sometimes, only slightly.

Solar Water Heating Systems

A basic solar collector system
A basic solar collector system

The core of a solar water heater is a solar collector and a storage tank. A solar collector is basically a glazed, insulated box with a dark-colored interior and, usually, a bunch of tubes or passageways for water flow. (Glazing is a coat of material, typically glass, that aids in heat retention.) The solar collector turns the sun's radiation into heat. A storage tank is exactly what it sounds like. It holds the water.

That's the basic setup, and some systems aren't much more complicated than that. The first distinction among solar water heaters is cut and dry: passive or active? An active heater uses electrical pumps and controls to move water around the system. A passive heater uses nothing but forces of nature. Passive is the simpler of the two.


There are two primary types of passive systems:

  • Batch: This is as uncomplicated as a water heater gets. It's just one or more water tanks inside a solar collector (no tubes in this one). The water warms up right inside the tank, and either gravity or natural convection (the tendency of hot water to rise) moves water from the tank to a home's pipes.
  • Thermosiphon: The water tank is separate from the solar collector. Cold water moves through the tubes of a solar collector, and natural convection pumps the resulting hot water into a storage tank. From the storage tank, the water travels into the home's water pipes.

Active systems typically fall into one of three categories:

  • Direct: Water moves through the solar collectors and into a storage tank with the help of electrical pumps and controls.
  • Indirect: Instead of heating water, the solar collectors heat a "heat transfer" fluid, such as antifreeze. The antifreeze then flows into the sealed piping of a heat exchanger, where it is surrounded by water. The water picks up the heat from the antifreeze (but never mixes with it), and is then pumped into a storage tank.
  • Drainback: A drainback system is like the indirect system except that it uses distilled water as the heat-transfer liquid, and it has a separate "drainback" tank for the distilled water. Pumping all of the heat-transfer liquid out of the system and into an interior tank makes it ideal for colder climates, since the liquid isn't exposed to extremely cold weather.

Whether the solar system is passive or active, it costs a lot more than you'd pay for a gas or electric model (more on price later). But some people are cutting back on the cost by making their own. As it turns out, the most basic type, the batch heater, is a relatively simple build-at-home project.

Homemade Solar Water Heaters

Building a solar water heater isn't exactly for beginners. It requires installing pipes, glass and, preferably, insulation. But for a do-it-yourself type, it's an ideal money- and planet-saving project. You can build a batch water heater for less than $100.

A batch heater is also called an integral passive solar water heater -- "integral" because the solar collector and storage tank are combined. It's the simplest system to build at home, and it really only requires a few basic parts. (This is just a brief overview; for complete instructions, see DIY sidebar.)


  • Electric water heater tank (used is fine, as long as it's in good shape)
  • Black paint
  • Plywood box (large enough to hold the tank)
  • Sheets of glass
  • Hinged lid for the box (to reduce nighttime cooling)
  • Insulation material
  • Pipes/fittings
  • Mountings (for roof, side of house, or ground level)

Construction is pretty straightforward:

  • Paint the water tank black.
  • Secure glass to the top of the box.
  • Insulate the box and the additional lid and cut holes in the box for inflow and outflow pipes.
  • Secure the water tank inside the box.
  • Route incoming cold water into the bottom of the tank, and outgoing hot water from the top of the tank to the home's water-heater tank
  • Mount unit in desired location (roof is usually best for sun-exposure).

While building the water heater may be pretty easy, there are other factors to consider. You have to determine the prime location for heater so it's exposed to the most sunlight per day, which can take some calculating. You also need to make sure the ideal location can support the weight of the setup. And as with any other water heater, you need to figure out what size tank you need so you don't end up running out of hot water in the middle of your shower -- and determine how much glazing surface area you need to heat that water volume.

If you're not comfortable making these determinations, you might want to shell out the cash for a professionally built setup.

Either way, there are some general pros and cons associated with solar water heaters. We'll start with the upside.

Benefits of Solar Water Heaters

The benefits of solar water heating are numerous and considerable. First, you're going to save money on your electric bill.

Most likely, your water-heating electricity use will be cut by at least half [source: PE]. How much you actually save depends on the climate where you live. If you're in the U.S. Northwest, where the sun doesn't shine much during a good portion of the year, your savings will be lower than if you live in an extremely sunny place like Arizona, because there's less fuel available for your system. But if you live in Arizona, you could decrease your water-heating bill by up to 90 percent [source: EWH].


And then there's the corresponding reduction in pollution. A 50 percent reduction in traditional (emitting) energy use means a 50 percent reduction in CO2 emissions. So installing a solar water heater would reduce your hot-water carbon footprint by half. At the same time, you're conserving nonrenewable fuels for applications for which there are currently no easily available renewable energy sources.

Because of the benefits of solar water heating, adding a unit to your home will also increase its value. So you could end up getting back whatever money you put into a solar heating system when you sell your house.

Which brings us to the primary negative: the money you put in. While sunlight is free, the system required to convert it into hot water for your home can cost a pretty penny if you go the professional route.

Concerns About Solar Water Heaters

This school in Nepal uses solar water heater collector panels.
This school in Nepal uses solar water heater collector panels.
Photo courtesy of NREL

The No. 1 drawback to solar water heating is the upfront price. You can buy a new, high-efficiency gas or electric water heater for $500, and probably less; a solar water heater will run you anywhere from $1,000 to $3,500 installed [source: ToolBase]. You can lower that price by installing it yourself or building your own, but most people get it installed by experts since there's usually roof work involved. There are, however, federal (and possibly state) financial incentives available for installing solar water heaters, so your end cost will probably be lower.

It's important to remember, too, that you'll make that money back. But how long it takes to recoup the upfront investment depends on where you live, how much hot water you use, and the cost of electricity. If you live in an extremely sunny locale, use tons of hot water, and the cost of gas or electricity goes up each year, you're going to be saving more and you could make back the initial investment in just a few years. But if you're in Chicago, the cost of traditional energy decreases and you take five-minute showers, wash clothes in cold and wash dishes manually, your annual saving won't be as significant. In that case, it could take a decade to see a financial return on your investment.


Some other concerns include:

  • Temperature: If you live in a very cold location, direct heater models (batch, thermosiphon and direct-active) might be unavailable to you due to the risk of freezing.
  • Home orientation: To have an efficient setup, you need to have a mounting location with considerable sun exposure; city dwellings may not qualify.
  • Water quality: If your home's water is particularly hard or acidic, you may not be a candidate for an active system. Hard or acidic water can corrode water-circulation systems.
  • Power requirements: Since active systems rely on electrically powered machinery, they won't work during a power outage.
  • Building regulations: Some areas, like those prone to earthquakes, have strict weight limits for roof-mounted equipment. A solar water heater might be too heavy.

As with solar-powered systems in general, a solar water heater may or may not be ideal for your particular circumstances. But it does offer some significant benefits that could make it worth the effort to make it work.

For more information on solar water heaters and related topics, look over the links on the next page.

Lots More Information

Related HowStuffWorks Articles

  • Bainbridge, David. "Build a Solar Water Heater: An Integral Passive Solar Water Heater." Mother Earth News. January/February 1984.
  • Dulley, James. 'Breadbox' Heater Saves Dough. Chicago Sun-Times. May 28, 1985.
  • Olsen, Ken. "Solar Hot Water Heater: A Primer." Arizona Solar.
  • Patterson, John. "Solar Hot Water Basics." Home Power Magazine.
  • Solar Hot Water. U.S. Department of Energy.
  • Solar Water Heaters. Energy Savers.
  • Solar Water Heaters. ToolBase Services.