First Hour Rating (FHR) measures the total volume of hot water a water heater can supply during the first hour of use, expressed in gallons. This practical metric combines tank capacity with recovery rate to indicate real-world hot water availability for typical household demand patterns.
Calculation Method
FHR is determined by starting with a fully heated tank and measuring how much hot water (at 105°F or higher) can be delivered in 60 minutes. The calculation includes the initial hot water volume plus water heated during that hour. For example, a 50-gallon electric heater might have an FHR of 50-60 gallons, while a 40-gallon gas model could achieve 80-100 gallons.
Consumer Applications
FHR helps households right-size their water heater based on actual usage patterns rather than tank capacity alone. A family needing multiple consecutive showers should prioritize units with higher FHR ratings.
Standards and Selection
The U.S. Department of Energy uses FHR for water heater labeling and efficiency comparisons. This metric provides more meaningful guidance than capacity alone for estimating whether a unit meets household needs.