First-hour rating
How many gallons of hot water a tank can deliver in its busiest hour, starting full.
First-hour rating (FHR) is the most useful number on a tank water heater's label — it's the gallons of hot water the unit can supply in one hour starting from a full tank, combining the stored water plus what the burner or element can reheat in that hour. It matters far more than raw tank size, because a 40-gallon gas heater with a strong burner can out-deliver a 50-gallon electric one over a busy morning. To size a tank, estimate your [peak-hour demand](/guides/what-size-water-heater-do-i-need-calculator) (about 20 gallons per shower, plus kitchen and laundry use) and buy a heater whose FHR meets or beats it. The federal EnergyGuide label lists FHR as "Capacity (first hour rating)" in the top-left corner.