Energy Factor (EF) was the primary efficiency metric for water heaters before 2015, measuring the ratio of usable hot water output to total energy input. This metric provided a simple 0.5-0.95 scale for comparing water heater efficiency, though it had limitations in accounting for different usage patterns and fuel types.
Historical Context
The EF rating system served as the standard efficiency measure from the 1980s through 2014. It combined recovery efficiency, standby loss, and cycling loss into a single figure, making basic comparisons straightforward for consumers shopping for water heaters.
Limitations and Replacement
EF ratings did not account for different household sizes or usage patterns, sometimes overestimating efficiency for typical American households. The U.S. Department of Energy replaced EF with the Uniform Energy Factor (UEF) starting April 2015 to provide more accurate, standardized comparisons across tank, tankless, and heat pump technologies.
Current Status
While new water heaters use UEF ratings exclusively, understanding EF remains useful when evaluating older units or comparing historical data. Most legacy documentation and older professional certifications reference EF standards.