Solar energy to heat water, to warm buildings and to provide low temperature heat for industry and agriculture is a well understood technology1. Of these applications, solar hot water is the most established with a proven track record of performance. Yet in the U.S., solar water heaters are an uncommon sight on residential or commercial buildings. Various estimates put the number of installed systems at about one million (1), less than 1% of U.S. households. Many of these systems were installed decades ago during the 1970s and 1980s when federal tax incentives were in place. Today’s U.S. market for solar water heaters is far below its potential. According to one estimate, only 6000 systems were installed last year, (2), a very small fraction of the approximately 10 million gas and electric water heaters shipped each year (3).

This paltry deployment of solar hot water in the U.S. exists despite the potential benefits for displacing consumption of fossil fuels. Consider the potential energy savings and environmental benefit of replacing gas and electric water heaters with solar hot water. Energy consumption in buildings represents 39% of primary U.S. energy consumption, nearly equally split between residential (21%) and commercial (18%) (4). Water heating accounts for approximately 12% of the residential total, equivalent to 733 billion kWh (2.5 quadrillion BTU) and for 6% of the commercial total, equivalent to 322 billion kWh (1.1 quadrillion BTU) (5).

The number of buildings appropriate for solar depends in part on population density and climate. The actual market penetration depends on many factors, including among others first and life-cycle cost, availability, customer preference, and local building and plumbing codes and standards. For the sake of argument, assume a market penetration of 50% of current electric water heaters and 20% of gas water heaters (optimistic but achievable goals). A reasonable estimate of the average solar fraction of these systems (the fraction of the energy provided by solar with the remainder supplied by the current energy source) is 0.5. With this assumed scenario, solar water heaters would displace about 88 billion kWh (0.3 quadrillion BTU) of the primary electricity and natural gas in residential buildings. The consequent reduction in greenhouse emissions would be 6.7 million metric tons of carbon (6). If commercial water heating is treated in the same manner, an additional 50 billion kWh (0.17 quadrillion BTU) of primary energy consumption could be avoided with a carbon reduction of 2.7 million metric tons. The total projected carbon savings is equivalent to removing 7 million passenger cars from the road (7). Note that this analysis does not consider the additional benefit of reduced peak demand (8).

Based on these projections, it is reasonable to question why domestic solar hot waters are not more widespread and to urge measures to redress the problem. There are reasons for optimism. Unlike some of the “clean” energy options that receive substantially more federal support, the technical risk is low. There are numerous well-designed and manufactured systems and components; the independent U.S. Solar Rating and Certification Corporation (SRCC) currently lists 12 companies that offer a wide range of certified solar water heating systems for use with either electric or gas auxiliary heat (9). In addition, some communities have recognized the benefits of increasing the number of solar water heaters. There are financial incentives offered by many states, cities, and utility companies (10), and federal tax credits for the purchase of residential photovoltaic panels and solar hot water systems have been proposed.

There is a general consensus that the major obstacle to deployment is the relatively high initial purchase price, including installation, relative to electric and gas water heaters. In many areas of the country, availability, particularly in-time response to customers whose conventional water heater must be replaced, and customer, trade, and institutional acceptance of new products may hinder purchase. The funding for research and development (R&D), marketing, and public education needed to hurdle these barriers is insignificant. The market in the U.S. is too weak to permit significant research and development by the existing solar manufacturers. The weak market is probably also to blame for the lack of investment by the major suppliers of conventional water heating and space heating equipment. The dearth of industry support for R&D may be a harbinger of a declining U.S. manufacturing base for flat plate collectors in the face of competition from overseas where the market is stronger. Public funding is also inadequate. For example, the solar water heating program of the U.S. Department of Energy (USDOE) is two percent of the budget for solar energy technologies, which itself is a meager $80.3 million U.S. out of the $1.25 billion U.S. allocated to the Office of Energy Efficiency and Renewable Energy. The bottom line is that a mere $1.75 million U.S. is available to support the rating and certification efforts of SRCC and the scientific and engineering endeavors aimed at advancing the technology by national laboratories, universities and industry.

What can and should be done to increase use of solar hot water in the U.S.? First and foremost, research and development must be better supported at the federal and state levels, at least until there is sufficient market and profit for the solar manufacturers to adequately support in-house R&D. Current funding levels are inadequate to maintain competency and world-class research and testing facilities, much less support major innovation. Moreover, congressional earmarks reduce the funds available for competitive solicitations for innovative concepts.

  • Efforts to identify more cost-effective materials and manufacturing processes should be expanded. One recent focus has been on development of polymer-based systems, which will be most cost effective when production capacity is scaled up. This effort requires substantial investment to develop materials that are durable and compatible with potable water and to develop designs and manufacturing processes that take advantage of the cost savings potential of replacing glass and metal with plastic. An assessment of the “cradle-to-grave” efficiency of energy conversion, which considers each step from raw to scrap materials, and ecological impact of material options would provide a useful comparison of this route with competing concepts.

  • The U.S. market demands aesthetically pleasing designs. Initiatives to develop systems that are part of the building envelope should be supported for both existing and newly constructed buildings. The solutions will be different. The projected benefits described here are for existing buildings, which in the short term are a larger potential market than new buildings; all water heating equipment in current buildings will be replaced eventually with new equipment. However, new buildings represent an opportunity for major innovation by incorporating the solar systems into roof and wall systems constructed off site in large manufacturing facilities. In 2003, nearly 1.4 million new homes were built.

  • In support of the stated DOE goal of zero energy buildings (homes that produce all of the energy consumed based on an annual net energy balance), solar thermal systems that can supply both hot water and space heating (space heating represents the largest fraction of energy consumption in residential buildings) should be developed for the U.S. market. Such systems are not uncommon in other countries but systems must be developed to integrate with U.S. residential space heating systems and home construction. This endeavor should address cost-effective and efficient long-term storage, considering seasonal or annual, rather than daily, energy requirements, and interface with a variety of heating systems, including forced air systems.

  • Combined solar thermal/photovoltaic (PV) systems, both concentrating and nonconcentrating configurations, which produce electricity and heat, should be investigated, in particular for integration into the building envelope. However, the direct use of electricity generated with photovoltaics to heat water should not be pursued. Using solar energy to produce electricity and then converting the electricity to heat is very inefficient.

  • In addition to expanded research and development, lawmakers and public policy decision makers should consider more extensive incentive programs for solar thermal water heating and space conditioning, for a variety of entities, including companies that currently offer solar systems, HVAC manufacturers and distributors, water heater manufacturers and distributors, public utilities, builders, and the consumer.

  • Efforts to effect behavioral changes through public education should be increased. Support for curriculum development in renewable energy technologies at all levels of instruction should be supported. At the university level, the lack of research funding is one reason for the lack of advanced technical courses offered in the field and the few graduates who are trained to move the technology forward.

In summary, the potential for reducing consumption of electricity and gas, and the consequent emission of greenhouse gases justifies a far greater investment in low-temperature solar thermal systems. These technologies are suitable for a number of applications, among which hot water is the most likely candidate for near-term deployment on a substantial scale. We should not simply wait until the cost of fossil fuels is so high that the current technology is less expensive than gas or electric hot water heaters. Policies should be put in place to recognize the costs of carbon released into the atmosphere. Solar should not have to justify itself on the basis of the current cost of electricity and gas. That said, strong and sustained investments in R&D can be expected to lead to cost-competitive systems, which will be attractive to U.S. households and businesses.


I have been very fortunate to follow in the footsteps of the great pioneers in solar energy, including Professor John I. Yellott, and the past recipients of the award given in his honor. I owe a great debt of gratitude to the students and colleagues who have contributed to my research in solar hot water and who have made working in the solar energy field such a personal pleasure.

The paper is a summary of remarks presented at the John I. Yellott Award Lecture at the ASME International Solar Energy Conference in Portland, Oregon in June 2004. The ASME Solar Energy Division John I. Yellott Award is a biennial award sponsored by the ASME Solar Energy Division. This award, in honor of the Division’s first Chair, recognizes ASME members who have demonstrated sustained leadership within the Solar Energy Division, have a reputation for performing high-quality solar energy research and have made significant contributions to solar engineering through education, state or federal government service, or in the private sector. John I. Yellott (1908–1986) had a long and remarkable professional career beginning with a fascination with supersaturated steam at Johns Hopkins University and culminating in his remarkable work in solar energy that began in 1955 when he moved to Phoenix to become the first executive secretary of the Association for Applied Solar Energy (AFASE). A solar engineer and consultant with a worldwide reputation, he taught mechanical systems and environmental controls in the Arizona State University’s School of Architecture from 1963 to 1985.

Based on a survey of the major U.S. solar hot water manufacturers conducted by Les Nelson, Western Renewables Group, 30012 Aventura, Ste A, Rancho, Margarita, CA 92688. The data agree well with the shipped collector area data reported by the U.S. DOE Energy Information Administration found at
U.S. Department of Energy, Energy Efficiency and Renewable Energy
Building Energy Data Book
, which may be downloaded at
The energy consumption data and carbon emission factors presented in this paper are abstracted from the
Energy Information Administration
, EIA,
2004 Annual Energy Outlook
, which may be downloaded at and the
2004 Building Energy Data Book
. There is some variation from year to year, but energy consumption for heating water is projected to remain relatively stable until 2025. Primary energy consumption accounts for the generation, transmission, and distribution losses.
This is an estimate based on average miles driven, gas mileage, and carbon content of gasoline. A calculator for such estimates can be found at various sites including the U.S. Climate Technology Gateway at
The U.S. Solar Rating and Certification Corporation (SRCC) estimates that, on average, for every solar water heater installed, 0.5 kW of peak demand is deferred. See the SRCC Web site: or write to: SRCC, c/o FSEC, 1679 Clearlake Road, Cocoa, FL, 32922-5703.
The list of SRCC certified and rated solar water heating systems as well as collectors is available at http://www.srcc.org
The Interstate Renewable Energy Council maintains a database of state incentives for renewable energy that as of January 2005 lists programs in 42 states and
. 2005,
Database of State Incentives for Renewable Energy
, available at,
, P.O. Box 1156, Latham, NY 12110-1156.

An often cited number for which no hard data were found.

The split between electricity/gas is 1.181.15 quadrillion BTUs in residential buildings and 0.450.59 in commercial buildings.

The carbon emission is based on 16.02kgCmillion BTU for electricity and 14.40kgCmillion BTU for natural gas. The data can be converted to a carbon dioxide equivalent by multiplying by 226.