Solar Uniform Energy Factor

Overview of SUEF Metric

The Solar Uniform Energy Factor (SUEF) Specification for domestic solar water heaters was created in late 2020 by a diverse group of stakeholders. It provides a method to determine a Uniform Energy Factor (UEF) value to compare the performance of solar water heaters with other water heater types on a consistent basis.

The specification defines the conditions, assumptions, and methodologies to be used to determine UEF for domestic water heaters directly utilizing solar energy. It is published as an optional appendix of the ICC 900/SRCC 300-2020, Solar Water Heating Standard and is designed to work in conjunction with the existing DOE Test Method (10 CFR 430, Subpart B, Appendix E).

The U.S. Department of Energy (DOE) created the UEF metric to quantify the energy efficiency of water heaters in 2014. The DOE Test Method for UEF (10 CFR 430, Subpart B, Appendix E) did not include water heaters utilizing solar energy. Therefore, this specification serves to fill the gap and provides the additional procedures needed to produce a UEF value for domestic solar water heaters. SUEF can then be used to compare performance directly with UEF values for water heaters using only electricity, natural gas or fuel oil.

The SUEF of a solar water heater is determined using one of four hot water draw patterns, depending on the size of the collector array, as shown below:

Total Gross Area of
Solar Collector Array*
DOE Draw Pattern**
0 – 24 ft2Very-Small Usage (10 gpd)
24 – 38 ft2Low-Usage (38 gpd)
38 – 64 ft2Medium-Usage (55 gpd)
> 64 ft2High-Usage (84 gpd)

*Total array area is the gross area of the collector from the OG-100 certificate, multiplied by the number of collectors in the array.
**DOE Draw Patterns are defined in the DOE Test Procedure, Tables III.1-4.

SUEF is used in the ENERGY STAR® Program Requirements Product Specification for Residential Water Heaters, Version 4.0 For certification it requires that systems have SUEF > 3.0 for systems with electric backup or SUEF > 1.8 for those using gas (in addition to other requirements). The SUEF ratings for ENERGY STAR certified solar water heaters are provided on the SRCC Listing document for the manufacturer.

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