For most of us, performance matters, whether we’re shopping for a new car or choosing a solar water heating system. For cars, their estimated fuel economy—miles per gallon—can influence which model offers the best value. Although these EPA testing numbers aren’t necessarily “real-world,” they can give us a guideline to go by. Solar hot water (SHW) system performance is not much different. In this case, systems are evaluated by an independent testing agency—then certified by the Solar Rating and Certification Corp. (SRCC). And their ratings are the next best thing to real-world performance.
Our list of solar water heating systems is condensed from the SRCC’s Operating Guidelines 300 (OG-300) catalog, since we didn’t have room to list the more than 500 SRCC certified systems. Instead, we tried to include every SHW system manufacturer, but pared the list to 130 individual systems, which were selected by two criteria: typical residential tank size (40 to 120 gallons) and typical collector sizing (1 square foot of collector area to every 1 to 2 gallons of water stored).
Only seven or eight efficient systems per manufacturer were included in each climate category. To compare apples to apples, all the systems listed in our table are assumed to have electric backup heating. The included data is current as of March 1, 2008—for updates, visit the SRCC Web site. If you want a more detailed look at all the systems, but don’t want to wade through the SRCC’s 309-page catalog, check out our complete spreadsheet of OG-300 systems at www.homepower.com.
In HP123, we featured a guide to selecting a solar hot water collector—the “engine” of a SHW system that gathers the energy. While the collector is the most important component in a solar water heating system, it is only one component of several that work together. Once the energy is gathered, it needs to be stored for on-demand use. The other components of an SHW system facilitate the storage and distribution of the solar-heated water, and greatly influence how much hot water is available.
While choosing a collector is important, knowing how the entire system will perform is crucial. And getting an idea of how one system stacks up against another will help you maximize your investment. The SRCC OG-300 standards provide a relative performance comparison of various solar water heating systems. Certification requires testing the collectors under the OG-100 standard and testing the entire system. (Note that some collectors are integrated with the storage tank, such as integral collector and storage and thermosyphon systems, and are listed only in the OG-300 catalog.) Before a system can be certified, a design and installation review, and a performance computation must be completed.
Collectors and systems are tested under standard laboratory conditions that are certain to be different from those at your home. Testing is a combination of durability and performance, with the test procedures for performance specified by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE Standard 93, “Methods of Testing to Determine the Thermal Performance of Solar Collectors”).