A Reference Brief from the Energy Efficiency and Renewable Energy Clearinghouse (EREC)
This publication provides basic information on the
components and types of solar water heaters currently available and the
economic and environmental benefits of owning a system. Although the
publication does not provide information on building and installing your own
system, it should help you discuss solar water heating systems intelligently
with a solar equipment dealer.
Solar water heaters, sometimes called solar domestic hot water systems, may
be a good investment for you and your family. Solar water heaters are cost
competitive in many applications when you account for the total energy costs
over the life of the system. Although the initial cost of solar water heaters
is higher than that of conventional water heaters, the fuel (sunshine) is
free. Plus, they are environmentally friendly. To take advantage of these
heaters, you must have an unshaded, south-facing location (a roof, for
example) on your property.
These systems use the sun to heat either water or a heat-transfer fluid,
such as a water-glycol antifreeze mixture, in collectors generally mounted on
a roof. The heated water is then stored in a tank similar to a conventional
gas or electric water tank. Some systems use an electric pump to circulate the
fluid through the collectors.
Solar water heaters can operate in any climate. Performance varies
depending, in part, on how much solar energy is available at the site, but
also on how cold the water coming into the system is. The colder the water,
the more efficiently the system operates. In almost all climates, you will
need a conventional backup system. In fact, many building codes require you to
have a conventional water heater as the backup.
First Things First
Before investing in any solar energy system, it is more cost effective to
invest in making your home more energy efficient. Taking steps to use less hot
water and to lower the temperature of the hot water you use reduces the size
and cost of your solar water heater.
Good first steps are installing low-flow showerheads or flow restrictors in
shower heads and faucets, insulating your current water heater, and insulating
any hot water pipes that pass through unheated areas. If you have no
dishwasher, or your dishwasher is equipped with its own automatic water
heater, lower the thermostat on your water heater to 120°F (49°C). For more
information on ways to use less energy for water heating, contact The Energy
Efficiency and Renewable Energy Clearinghouse (EREC—see Source List at the end
of this publication).
You'll also want to make sure your site has enough available sunshine to
meet your needs efficiently and economically. Your local solar equipment
dealer can perform a solar site analysis for you or show you how to do your
own. You can also contact EREC for more information.
Remember: Local zoning laws or covenants may restrict where you can place
your collectors. Check with your city, county, and homeowners association to
find out about any restrictions.
Solar Water Heater Basics
Solar water heaters are made up of collectors, storage tanks, and,
depending on the system, electric pumps.
There are basically three types of collectors: flatplate, evacuated-tube,
and concentrating. A flatplate collector, the most common type,
is an insulated, weather-proofed box containing a dark absorber plate under
one or more transparent or translucent covers.
Evacuated-tube collectors are made up of rows of parallel,
transparent glass tubes. Each tube consists of a glass outer tube and an inner
tube, or absorber, covered with a selective coating that absorbs solar energy
well but inhibits radiative heat loss. The air is withdrawn ("evacuated") from
the space between the tubes to form a vacuum, which eliminates conductive and
convective heat loss.
Concentrating collectors for residential applications are usually
parabolic troughs that use mirrored surfaces to concentrate the sun's energy
on an absorber tube (called a receiver) containing a heat-transfer fluid. For
more information on solar collectors, contact EREC.
Most commercially available solar water heaters require a well-insulated
storage tank. Many systems use converted electric water heater tanks or plumb
the solar storage tank in series with the conventional water heater. In this
arrangement, the solar water heater preheats water before it enters the
conventional water heater.
Some solar water heaters use pumps to recirculate warm water from storage
tanks through collectors and exposed piping. This is generally to protect the
pipes from freezing when outside temperatures drop to freezing or below.
Types of Solar Water Heaters
Solar water heaters can be either active or passive. An active system uses
an electric pump to circulate the heat-transfer fluid; a passive system has no
pump. The amount of hot water a solar water heater produces depends on the
type and size of the system, the amount of sun available at the site, proper
installation, and the tilt angle and orientation of the collectors.
Solar water heaters are also characterized as open loop (also called
"direct") or closed loop (also called "indirect"). An open-loop system
circulates household (potable) water through the collector. A closed-loop
system uses a heat-transfer fluid (water or diluted antifreeze, for example)
to collect heat and a heat exchanger to transfer the heat to household water.
Active systems use electric pumps, valves, and controllers to circulate
water or other heat-transfer fluids through the collectors. They are usually
more expensive than passive systems but are also more efficient. Active
systems are usually easier to retrofit than passive systems because their
storage tanks do not need to be installed above or close to the collectors.
But because they use electricity, they will not function in a power outage.
Active systems range in price from about $2,000 to $4,000 installed.
Open-Loop Active Systems
Open-loop active systems use pumps to circulate household water through the
collectors. This design is efficient and lowers operating costs but is not
appropriate if your water is hard or acidic because scale and corrosion
quickly disable the system.
These open-loop systems are popular in nonfreezing climates such as Hawaii.
They should never be installed in climates that experience freezing
temperatures for sustained periods. You can install them in mild but
occasionally freezing climates, but you must consider freeze protection.
Recirculation systems are a specific type of open-loop system that provide
freeze protection. They use the system pump to circulate warm water from
storage tanks through collectors and exposed piping when temperatures approach
freezing. Consider recirculation systems only where mild freezes occur once or
twice a year at most. Activating the freeze protection more frequently wastes
electricity and stored heat.
Of course, when the power is out, the pump will not work and the system
will freeze. To guard against this, a freeze valve can be installed to provide
additional protection in the event the pump doesn't operate. In freezing
weather, the valve dribbles warmer water through the collector to prevent
freezing. Consider recirculation systems only where mild freezes occur
once or twice a year at most. Activating the freeze protection more frequently
wastes electricity and stored heat.
Closed-Loop Active Systems
These systems pump heat-transfer fluids (usually a glycol-water antifreeze
mixture) through collectors. Heat exchangers transfer the heat from the fluid
to the household water stored in the tanks.
Double-walled heat exchangers prevent contamination of household water.
Some codes require double walls when the heat-transfer fluid is anything other
than household water.
Closed-loop glycol systems are popular in areas subject to extended
freezing temperatures because they offer good freeze protection. However,
glycol antifreeze systems are a bit more expensive to buy and install, and the
glycol must be checked each year and changed every 3 to 10 years, depending on
glycol quality and system temperatures.
Drainback systems use water as the heat-transfer fluid in the collector
loop. A pump circulates the water through the collectors. The water drains by
gravity to the storage tank and heat exchanger; there are no valves to fail.
When the pumps are off,the collectors are empty, which assures freeze
protection and also allows the system to turn off if the water in the storage
tank becomes too hot.
Pumps in Active Systems
The pumps in solar water heaters have low power requirements, and some
companies now include direct current (DC) pumps powered by small
solar-electric (photovoltaic, or PV) panels. PV panels convert sunlight into
DC electricity. Such systems cost nothing to operate and continue to function
during power outages.
Passive systems move household water or a heat-transfer fluid through the
system without pumps. Passive systems have no electric components to break.
This makes them generally more reliable, easier to maintain, and possibly
longer lasting than active systems.
Passive systems can be less expensive than active systems, but they can
also be less efficient. Installed costs for passive systems range from about
$1,000 to $3,000, depending on whether it is a simple batch heater or a
sophisticated thermosiphon system.
Batch heaters (also known as "bread box" or integral collector storage
systems) are simple passive systems consisting of one or more storage tanks
placed in an insulated box that has a glazed side facing the sun. Batch
heaters are inexpensive and have few components—in other words, less
maintenance and fewer failures. A batch heater is mounted on the ground or on
the roof (make sure your roof structure is strong enough to support it). Some
batch heaters use "selective" surfaces on the tank(s). These surfaces absorb
sun well but inhibit radiative loss.
In climates where freezing occurs, batch heaters must either be protected
from freezing or drained for the winter. In well-designed systems, the most
vulnerable components for freezing are the pipes, if located in uninsulated
areas, that lead to the solar water heater. If these pipes are well insulated,
the warmth from the tank will prevent freezing. Certified systems clearly
state the temperature level that can cause damage. In addition, you can
install heat tape (electrical plug-in tape to wrap around the pipes to keep
them from freezing), insulate exposed pipes, or both. Remember, heat tape
requires electricity, so the combination of freezing weather and a power
outage can lead to burst pipes. If you live in an area where freezing is
infrequent, you can use plastic pipe that does not crack or burst when it
freezes. Keep in mind, though, that some of these pipes can't withstand
unlimited freeze/thaw cycles before they crack.
A thermosiphon system relies on warm water rising, a phenomenon known as
natural convection, to circulate water through the collectors and to the tank.
In this type of installation, the tank must be above the collector. As water
in the collector heats, it becomes lighter and rises naturally into the tank
above. Meanwhile, cooler water in the tank flows down pipes to the bottom of
the collector, causing circulation throughout the system. The storage tank is
attached to the top of the collector so that thermosiphoning can occur. These
systems are reliable and relatively inexpensive but require careful planning
in new construction because the water tanks are heavy. They can be
freeze-proofed by circulating an antifreeze solution through a heat exchanger
in a closed loop to heat the household water.
Sizing Your System
Just as you have to choose a 30-, 40-, or 50-gallon (114-, 151-, or
189-liter) conventional water heater, you need to determine the right size
solar water heater to install. Sizing a solar water heater involves
determining the total collector area and the storage volume required to
provide 100% of your household's hot water during the summer. Solar-equipment
experts use worksheets or special computer programs to assist you in
determining how large a system you need.
Solar storage tanks are usually 50-, 60-, 80-, or 120-gallon (189-, 227-,
303-, or 454-liter) capacity. A small (50 to 60 gallon) system is sufficient
for 1 to 3 people, a medium (80-gallon) system is adequate for a 3- or
4-person household, and a large (120-gallon) system is appropriate for 4 to 6
A rule of thumb for sizing collectors: allow about 20 square feet (about 2
square meters) of collector area for each of the first two family members and
8 square feet (0.7 square meter) for each additional family member if you live
in the Sun Belt. Allow 12 to 14 additional square feet (1.1 to 1.3 square
meters) per person if you live in the northern United States.
A ratio of at least 1.5 gallons (5.7 liters) of storage capacity to 1
square foot (0.1 square meter) of collector area prevents the system from
overheating when the demand for hot water is low. In very warm, sunny
climates, experts suggest that the ratio should be at least 2 gallons (7.6
liters) of storage to 1 square foot (0.1 square meter) of collector area. For
example, a family of four in a northern climate would need between 64 and 68
square feet (5.9 and 6.3 square meters) of collector area and a 96- to
102-gallon (363- to 386-liter) storage tank. (This assumes 20 square feet of
collector area for the first person, 20 for the second person, 12 to 14 for
the third person, and 12 to 14 for the fourth person. This equals 64 to 68
square feet, multiplied by 1.5 gallons of storage capacity, which equals 96 to
102 gallons of storage.) Because you might not be able to find a 96-gallon
tank, you may want to get a 120-gallon tank to be sure to meet your hot water
Benefits of Solar Water Heaters
There are many benefits to owning a solar water heater, and number one is
economics. Solar water heater economics compare quite favorably with those of
electric water heaters, while the economics aren't quite so attractive when
compared with those of gas water heaters. Heating water with the sun also
means long-term benefits, such as being cushioned from future fuel shortages
and price increases, and environmental benefits.
Many home builders choose electric water heaters because they are easy to
install and relatively inexpensive to purchase. However, research shows that
an average household with an electric water heater spends about 25% of its
home energy costs on heating water.
It makes economic sense to think beyond the initial purchase price and
consider lifetime energy costs, or how much you will spend on energy to use
the appliance over its lifetime. The Florida Solar Energy Center (FSEC—see
Source List) studied the potential savings to Florida homeowners of common
water-heating systems compared with electric water heaters. It found that
solar water heaters offered the largest potential savings, with solar
water-heater owners saving as much as 50% to 85% annually on their utility
bills over the cost of electric water heating.
The FSEC analysis illustrates that the initial installed cost of the solar
water heater ($1,500 to $3,000) is higher than that of a gas water heater
($350 to $450) or an electric water heater ($150 to $350). The costs vary from
region to region, so check locally for costs in your area. Depending on the
price of fuel sources, the solar water heater can be more economical over the
lifetime of the system than heating water with electricity, fuel oil, propane,
or even natural gas because the fuel (sunshine) is free.
However, at the current low prices of natural gas, solar water heaters
cannot compete with natural gas water heaters in most parts of the country
except in new home construction. Although you will still save energy costs
with a solar water heater because you won't be buying natural gas, it won't be
economical on a dollar-for-dollar basis.
Paybacks vary widely, but you can expect a simple payback of 4 to 8 years
on a well-designed and properly installed solar water heater. (Simple payback
is the length of time required to recover your investment through reduced or
avoided energy costs.) You can expect shorter paybacks in areas with higher
energy costs. After the payback period, you accrue the savings over the life
of the system, which ranges from 15 to 40 years, depending on the system and
how well it is maintained.
You can determine the simple payback of a solar water heater by first
determining the net cost of the system. Net costs include the total installed
cost less any tax incentives or utility rebates. (See the box for more
information.) After you calculate the net cost of the system, calculate the
annual fuel savings and divide the net investment by this number to determine
the simple payback.
An example: Your total utility bill averages $160 per month and your water
heating costs are average (25% of your total utility costs) at $40 per month.
If you purchase a solar water heater for $2,000 that provides an average of
60% of your hot water each year, that system will save you $24 per month ($40
x 0.60 = $24) or $288 per year (12 x $24 = $288). This system has a simple
payback of less than 7 years ($2,000 ÷ $288 = 6.9). For the remainder of the
life of the solar water heater, 60% of your hot water will be free, saving you
$288 each year. You will need to account for some operation and maintenance
costs, which are estimated at $25 to $30 a year. This is primarily to have the
system checked every 3 years.
If you are building a new home or refinancing your present home to do a
major renovation, the economics are even more attractive. The cost of
including the price of a solar water heater in a new 30-year mortgage is
usually between $13 and $20 per month. The portion of the federal income tax
deduction for mortgage interest attributable to the solar system reduces that
amount by about $3 to $5 per month. If your fuel savings are more than $15 per
month, the investment in the solar water heater is profitable immediately.
Tax Incentives and Rebates
Some local or state governments offer tax incentives to encourage residents
to invest in solar energy technologies. Check with your state or local energy
office or Department of Revenue for information. Some electric utilities offer
rebates to customers who install solar energy equipment because these
installations help utilities reduce peak loads. Peak loads are periods when
the utility must generate extra power to meet a high demand. Heating water in
the evening is one example.
Solar water heaters offer long-term benefits that go beyond simple
economics. In addition to having free hot water after the system has paid for
itself in reduced utility bills, you and your family will be cushioned from
future fuel shortages and price increases. You will also be doing your part to
reduce this country's dependence on foreign oil. The National Remodelers
Association reports that adding a solar water heater to an existing home
raises the resale value of the home by the entire cost of the system. You may
be able to recoup your entire investment when you sell your home.
Solar water heaters do not pollute. By investing in one, you will be
avoiding carbon dioxide, nitrogen oxides, sulfur dioxide, and the other air
pollution and wastes created when your utility generates power or you burn
fuel to heat your household water. When a solar water heater replaces an
electric water heater, the electricity displaced over 20 years represents more
than 50 tons of avoided carbon dioxide emissions alone. Carbon dioxide traps
heat in the upper atmosphere, thus contributing to the "greenhouse effect."
Be a Smart Consumer
Take the same care in choosing a solar water heater that you would in the
purchase of any major appliance. Your best protection is to consider only
certified and labeled systems. One such label is put on by the Solar Rating
& Certification Corporation (SRCC), a nonprofit, independent third-party
organization formed by the state energy officials, and consumer advocates to
certify and rate solar water heaters.
A national standard (OG-300) addresses a variety of concerns, including
safety and health, durability and reliability, installation, performance, and
operation and maintenance. To meet this standard, a system is rigorously
tested. A certified solar water heater carries the SRCC OG-300 label, and the
system performance is listed in a published directory. A similar program has
been established for Florida by FSEC. Both SRCC and FSEC provide collector
testing and rating programs.Obstetrics and Gynecology
Find out if the manufacturer offers a warranty, and, if so, what the
warranty covers and for how long. If the dealer you are buying the equipment
from goes out of business, can you get support and parts from the
manufacturer, or from a local plumbing contractor?
Make sure that the workers who are actually installing the system are
qualified to do the work. Ask the installation contractor for references and
check them. When the job is finished, have the contractor walk you through the
system so you are familiar with the installation. And be sure that an owner's
manual with maintenance instructions is included as part of the package.
A Bright Future
A solar water heater is a long-term investment that will save you money and
energy for many years. Like other renewable energy systems, solar water
heaters minimize the environmental effects of enjoying a comfortable, modern
lifestyle. In addition, they provide insurance against energy price increases,
help reduce our dependence on foreign oil, and are investments in everyone's
You might also consider other solar energy systems for your home. Systems
similar to the solar water heater are used for space heating and swimming pool
heating. In fact, pool heating is a major market for solar energy systems. For
more information on these systems, contact EREC.
The following organizations can provide you with information to help you
find the solar water heater that is right for you.
American Solar Energy Society (ASES)
2400 Central Avenue, Unit
Boulder, CO 80301
Fax: (303) 443-3212
ASES is a nonprofit educational organization founded in 1954 to encourage
the use of solar energy technologies. ASES pub-lishes a bimonthly magazine,
Solar Today, and offers a variety of solar publications through its catalogue.
Florida Solar Energy Center (FSEC)
1679 Clearlake Road
Cocoa, FL 32922-5703
Fax: (407) 638-1010
FSEC is an alternative energy center. The FSEC staff conducts research on a
range of solar technologies, offers solar energy workshops, and distributes
many free publications to the public.
Solar Energy Industries Association (SEIA)
1616 H Street, N.W.
Washington, D.C. 20006-4999
SEIA provides lists of solar-equipment manufacturers and dealers and
Solar Rating and Certification Corporation (SRCC)
1679 Clearlake Road
Cocoa, FL 32922-5703
Web site: http://www.solar-rating.org/
SRCC publishes the thermal-performance ratings of solar energy equipment.
The SRCC offers a directory of certified solar systems and collectors as well
as a document (OG-300-91) that details the operating guidelines and minimum
standards for certifying solar hot-water systems.
For information about many kinds of energy efficiency and renewable
energy topics, contact:
The Energy Efficiency and Renewable Energy Clearinghouse (EREC)
P.O. Box 3048
Merrifield, VA 22116
(800) DOE-EREC (363-3732)
Fax: (703) 893-0400
EREC provides free general and technical information to the public on the
many topics and technologies pertaining to energy efficiency and renewable
You may also contact your state and local energy offices for
region-specific information on solar water heaters.
The following publications provide further information about solar water
heaters. The list is not exhaustive, nor does the mention of any publication
constitute a recommendation or endorsement.
"Let the Sun Provide Your Shower," S. Baldassari, Countryside & Small
Stock Journal, (78) p. 55, November/December 1994.
"Solar Hot Water for the 90s," M. Rosenbaum, Solar Today, (5:5), p. 20,
"Solar Water Heaters Now," Home Mechanix, (87:760) p. 67, November 1, 1991.
"Solar Water Heating: A Viable Technology Alternative," M. Sheffer, Energy
User News, (19:9), p. 44, September 1994.
"Solar Water Heating in Pennsylvania," M.B. Sheffer and A.S. Lau, Solar
Today, (8:1), p. 12, January/February 1994.
"Wisconsin Public Service Company's Orphan Solar Program," J. DeLaune,
Solar Today, (9:3), p. 32, May/June 1995.
Books, Pamphlets, and Reports
Consumer Guide to Solar Energy, S. Sklar and K. Sheinkopf, Bonus
Books, Inc., 160 East Illinois Street, Chicago, IL 60611, 1991.
The Homeowner's Handbook of Solar Water Heating Systems, B.
Keisling, Rodale Press, 1983.
Home Energy Magazine, 2124 Kittredge Street, No. 95, Berkeley, CA,
94704-9942. (510) 524-5405. Home Energy Magazine is a source of information on
reducing energy consumption.
Solar Today, 2400 Central Avenue, Unit G-1, Boulder, CO, 80301.
(303) 443-3130. Solar Today covers all the solar technologies, both mature and
emerging, in a general-interest format.
March 1996; revised 2/2000
This report was prepared as an account of
work sponsored by an agency of the United States government. Neither the
United States government nor any agency thereof, nor any of their employees,
makes any warranty, express or implied, or assumes any legal liability or
responsibility for the accuracy, completeness, or usefulness of any
information, apparatus, product, or process disclosed, or represents that its
use would not infringe privately owned rights. Reference herein to any
specific commercial product, process, or service by trade name, trademark,
manufacturer, or otherwise does not necessarily constitute or imply its
endorsement, recommendation, or favoring by the United States government or
any agency thereof. The views and opinions of authors expressed herein do not
necessarily state or reflect those of the United States government or any
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