Low-cost energy at the tap
Solar water-heating systems are different from the solar devices that make electricity. Collectors on your roof still grab energy from the sun, but in this case the energy is used to heat water. Of course not everyone has the right roof or enough sun to do the job. But the average U.S. household spends 11% of its energy budget heating water. So why not investigate your home’s potential to provide you with a hot shower from this abundant, free source?
Besides saving you money, solar water heaters also provide big benefits to the planet by reducing greenhouse gas emissions, air pollution, and demand for fossil fuels. According to the U.S. Department of Energy, replacing an electric water heater with a solar water heater offsets the equivalent of 40% to 100% of a typical passenger car’s CO2 output.
By the way, if you had a solar water heater in the 1970s that didn’t last, don’t hold it against today’s products. There’s been a dramatic increase in reliability and efficiency since then.
This article covers solar water heating for your kitchen, bathrooms, and laundry–and even for heating your home. See our “Solar Heating for Pools and Spas” article for information about heating swimming pools and hot tubs.
Before You Go Solar
How low can you go? Using less energy is always less expensive than producing energy–even if that energy comes from the sun. So before you make a move to solar heating, look for ways to reduce your hot water use. Take shorter showers, and showers instead of baths. Install low-flow faucets and showerheads. If you’re buying a new dishwasher or clothes washer, choose an efficient model. Set your water heater temperature to 120°F, and turn it to “low” or “vacation” mode if you’re going to be out of town for more than a few days.
Blanket your tank. Cut energy use by wrapping the heater with an insulating blanket. They’re inexpensive and readily available at home improvement stores.
How They Work
Most systems have two basic parts: a collector in which water is heated by the sun, and an insulated tank where the hot water is stored. The collector sits on the roof (or in another sunny location), and the storage tank usually sits next to the home’s conventional water heater. Although most residential systems have a separate storage tank just for the solar heated water, some systems have tanks where the solar-heated and conventionally heated water mixes.
Solar water heaters provide 110°F to 180°F water when the sun is shining. To ensure plenty first thing in the morning and during cloudy stretches, however, most solar homes have a back-up fossil-fuel-fired water heater. The back-up heater can either be a conventional water heater or a tankless heater.
If your home is heated with hot water circulating in tubes under the floors, that’s another good reason to go solar. In such radiant-floor heating systems, the solar heater is backed up by a conventional one so you will always have heat when you need it.
- “Flat plate collectors” are the most common. A pump circulates liquid through the collector, which typically looks like a shallow glass-covered box. When the sun is shining, it heats up the liquid in the collector. When the liquid is hot enough, the pump comes on and moves the hot water to a storage tank. In some systems, the liquid is potable water that’s stored in a tank that feeds into the home’s hot water pipes. In other systems, the liquid is an antifreeze solution that flows through a heat exchanger to transfer heat (but not liquid) to the home’s potable water.
- An “evacuated tube collector” also uses a pump to circulate water. It’s more efficient than a flat plate collector, but a lot more expensive. Water is heated inside a vacuum, so there’s less heat loss than with flat plate collectors. Rather than looking like a glass box, the collector has rows of metal fins. As with a flat plate collector, the heated water is stored in a tank, usually next to the conventional water heater.
- “Batch collectors” are passive water heaters. They don’t use pumps, which reduces electricity use and maintenance. But they require a storage tank placed higher than the collector, so weight can be an issue if the collector and tank are on the roof. When water in the collector gets hot enough, it naturally rises and is replaced by cooler water from the storage tank. Batch collectors are best in mild climates where freezes are rare (in cold climates, they need to be drained for the winter months).
System Size and Location
- If you are adding solar water heating to an existing home, you’ll need to consider whether the roof can take the added weight, whether you have adequate unshaded south-facing space, and whether there’s room near your existing water heater for an additional storage tank and pipes. A solar designer or installer can evaluate these considerations for you.
- For ideal performance, the solar collector should be located in an unshaded area that faces south. It should be tilted at an angle close to the latitude (a 37° angle for 37° latitude). But any orientation within 45° of south and any tilt from 15° to 60° will work well enough. If you have a pitched roof, you can mount the collector flush with the roof rather than having it jut out. Most solar water heaters today are streamlined and look like skylights on the roof from the exterior.
- Solar water heaters for indoor water use save the most money if your household uses a lot of hot water-and if you use it from late morning through early afternoon when the solar collectors have maximum exposure to the sun.Smaller households that use hot water mostly early in the morning and in the evening can benefit, too, but the savings will be smaller.
- Solar water heaters are most cost effective in sunny climates. In cloudy climates, you may need a larger, more expensive system.
- Homeowners who install a solar water heater are eligible for a federal income tax credit of up to $2,000 (the credit doesn’t apply to pool or hot tub heaters). Check the Database of State Incentives for Renewables and Efficiency (DSIRE) for information about this and other incentives and rebates for solar water heating.
- Solar water heating is much less expensive than solar electric and usually pays for itself in four to nine years. Most systems cost from $2,000 to $4,500. A typical household with an existing electric water-heating system could save up to $500 a year by going solar. If you have gas water heating as opposed to electric, the savings are lower and the payback longer. But as natural gas supplies decline and prices rise, solar water heating will become a hedge against rising energy costs.
Maintenance and Warranties
Solar water heaters can last for 20 to 30 years, although they do require periodic inspection and routine maintenance. You can do this yourself, or hire a solar technician to do it for you. Maintenance tasks include making sure the collector isn’t shaded by nearby trees; hosing it down periodically to clean off dust and bird droppings; checking the condition of the collector, plumbing, wiring, roof penetrations, and support structures; replacing any antifreeze; and making sure the pump is operating properly. Check your owner’s manual for specific maintenance requirements.
Warranties on solar water heaters vary from 3 to 10 years, depending on the manufacturer and the installation contractor. Favor products with longer warranties. Make sure the bids you receive contain details about what the warranty covers; there may be different warranty periods offered for the installation and the collector itself.
Some handy people buy solar water heating components and do the installation themselves, but it’s not a project for beginning do-it-yourselfers. Most homeowners have the systems professionally installed.
Whether you install it yourself or hire a pro, make sure the system you choose is:
- Certified. Choose a solar water heater system that’s certified by Solar Rating and Certification Corporation (SRCC), an independent organization that verifies the performance of solar equipment.
- An Energy Star. The government’s labeling program for the most energy-efficient appliances and equipment, Energy Star, will begin covering solar water heaters in 2009. To earn the Energy Star label, a solar water heater must have certification from the SRCC and a “solar fraction” of at least 0.5. A solar fraction is the portion of the water-heating needs served by the solar system rather than the back-up water heater. A solar fraction of 0.5 means that 50% of the hot water is supplied by the solar heater and 50% by the back-up equipment.
Before hiring a contractor, ask the following questions:
- Does the solar professional have experience designing and installing the type of system you want? If you are in the market for a solar water heater, don’t hire a contractor who has only installed photovoltaic systems.
- How many years has the contractor has been in the solar business and how many installations have they done? Solar is booming in many parts of the country; be cautious about hiring a newbie contractor.
- Is the contractor licensed? Some states require solar contractors to have special licenses; check with the contractors’ license board in your state about requirements.
- What specific services will the solar contractor provide? Most offer a “turnkey” service: they’ll analyze your site and water heating needs, design an appropriate system, procure the equipment and materials, obtain any necessary permits, and install the system.
- Are the bids you received based on comparable information? When evaluating bids, make sure they are for the same type and size system. The bids should include all costs associated with buying and installing the system, including hardware, installation, and permits. The bids should include an estimate of how much energy will be saved in kilowatt-hours or “therms” (which each contain 100,000 Btu).
- For general advice on what questions to ask contractors and other tradespeople, see our “What to Ask Your Contractor” article.
- Check out our Green Directory of pre-vetted GreenCheck® Solar Providers.