Water heating is a major energy use. Hot water for showers, laundry, and washing dishes consumes a quarter of residential energy use worldwide; in commercial buildings, that number is roughly 12 percent. Solar water heating—exposing water to the sun to warm it—can reduce that fuel consumption by 50 to 70 percent.
The world’s first commercial solar water heater came to market in the 1890s. In the early 20th century, the technology took a leap forward by adding a separate storage tank. Solar collectors popped up on rooftops across California and Florida, but cheap energy in the post–World War II years stymied the industry in the United States.
Today, the technology is in use in many countries and almost every climate. In Cyprus and Israel, where the use of SWH has been mandated since the 1980s, 90 percent of homes have systems. Throughout its history, SWH has risen and fallen based on the price of energy, as well as government intervention to support it.
All told, SWH is among the most effective ways to convert solar energy into thermal energy. Payback periods are as short as two to four years, depending on specifics of system and location.
Clarence Kemp…The Climax: Perlin, John. Let It Shine: The 6,000 Year Story of Solar Energy. Novato, California: New World Library, 2013.
William Bailey’s Day and Night: Perlin, Shine.
China…70 percent of…capacity: REN21. Renewables 2016 Global Status Report. Paris: REN21 Secretariat, 2016.
Cyprus and Israel…90 percent of homes: IEA-ETSAP and IRENA. Solar Heating and Cooling for Residential Applications. Paris and Abu Dhabi: International Energy Agency and International Renewable Energy Agency, 2015.
large-scale installations [growing]: REN21, Renewables 2016.
“one of the most effective technologies”: Shukla, Ruchi, et al. “Recent Advances in the Solar Water Heating Systems: A Review.” Renewable and Sustainable Energy Reviews 19 (2013): 173-190.
Hot water…energy use: Ürge-Vorsatz, Diana, et al. “Heating and Cooling Energy Trends and Drivers in Buildings.” Renewable and Sustainable Energy Reviews 41 (2015): 85-98.
SWH can reduce…by 50 to 70 percent: IEA-ETSAP and IRENA, Solar.
United States…potential [impact of] SWH: Dutzik, Tony, Rob Kerth, and Rob Sargent. Smart, Clean, and Ready to Go: How Solar Hot Water Can Reduce Pollution and Dependence on Fossil Fuels. Boston: Environment America Research and Policy Center, 2011.
national ambitions for growth: Mauthner, Franz, Werner Weiss, and Monika Spörk-Dür. Solar Heat Worldwide: Markets and Contribution to the Energy Supply 2014. Gleisdorf, Austria: IEA Solar Heating and Cooling Program, 2016.
Project Drawdown defines solar water as: the use of solar radiation to pre-heat or heat water for residential and commercial use within buildings. This solution replaces conventional fossil fuel-based water heating.
Solar water systems can be differentiated by: the type of solar thermal collector used (unglazed and glazed flat plate collectors and evacuated tube collectors), and the type of system operation (active, which have circulating pumps and controls, and passive, which do not). This analysis estimates the emissions and financial benefits of a high adoption of solar water.
Total Addressable Market 
The total addressable market for solar water has been estimated using data from the International Energy Agency (IEA, 2016a) and Global Buildings Performance Network (GBPN, n.d.). It amounted to 6,100 terawatt-hour (therms) in 2014.
Adoption Scenarios 
Nine input scenarios were created in alignment with the IEA (2012 and 2016b) in order to capture the many possibilities for adoption. These represented low growth, aggressive growth, and early aggressive growth. Impacts of increased adoption of solar water from 2020-2050 were generated based on three growth scenarios, which were developed by statistically combining these nine input scenarios. All three scenarios were assessed in comparison to a Reference Scenario, where the solution’s market share was fixed at the current levels.
- Plausible Scenario: This scenario is modeled around the results of the nine input scenarios, taking values that were one standard deviation below the mean values for each year.
- Drawdown Scenario: This scenario is modeled around the results of the nine input scenarios, taking values that equal to the mean values for each year.
- Optimum Scenario: This scenario is modeled around the results of the nine input scenarios, taking values that were one standard deviation above the mean values for each year.
Solar water systems were assumed to supplement the existing electric and gas heaters in houses, so no first costs for the conventional technology were included. Financial and climate calculations were made assuming that solar water would replace electric water heaters 10 percent of the time and gas heaters 90 percent of the times. The solution’s first cost, however, was an average of 41 data points from IEA (2012), REN21 (2015), and other sources. The operating costs are represented by the heating cost for using either type of system, and come from the average of 5 data points. Emissions included electric and fuel energy sources, and used emissions factors from the guidelines of the Intergovernmental Panel on Climate Change (IPCC).
The integration process for this solution was mainly concerned with the water saving—home solution, which reduces the flow of hot water in residential buildings and can greatly reduce the need for water heating. Since the need for water heating is captured as the total addressable market for solar water, this total addressable market was reduced by the savings provided by the water saving—home solution.
The environmental impacts of the Plausible Scenario show a total greenhouse gas reduction of 6 gigatons of carbon dioxide-equivalent emissions. This is a result of adding 2 terawatts of additional heating capacity by 2050. These environmental benefits come at a cost of US$2.9 billion in marginal first costs (including installation and replacement costs), and US$774 billion in operations savings. 
The Drawdown and Optimum Scenarios show 11.9 and 17.7 gigatons avoided, respectively.
Despite high upfront costs, solar water heaters are an important solution due to their attractive financial and emissions savings. Country-specific evidence indicates that some other factors may keep adoption under 6 percent of the market, such as limited roof space, high upfront costs, and perhaps lack of knowledge and access to the technology in some regions. The countries with highest adoptions are those with firm national policies of encouraging solar water investment. Once purchased, residents can enjoy the clean, low-cost technology and keep greenhouse gas emissions low.
 For more on the Total Addressable Market for the Buildings and Cities Sector, click the Sector Summary: Buildings and Cities link below.
 To learn more about Project Drawdown’s three growth scenarios, click the Scenarios link below. For information on Buildings and Cities Sector-specific scenarios, click the Sector Summary: Buildings and Cities link.
 For more on Project Drawdown’s Buildings and Cities Sector integration model, click the Sector Summary: Buildings and Cities link below.
 All monetary values are presented in US2014$.
Full models and technical reports coming in late 2017.