First-hour rating (FHR)
The first-hour rating is how many gallons of hot water a storage water heater can deliver in one hour starting from a full tank — the single most important sizing number for tank-type heaters, more informative than tank size.
FHR combines stored hot water with how fast the unit reheats incoming cold water, so a 50-gallon tank with a strong burner can out-deliver a 60-gallon tank with a weak one. DOE sizing guidance matches FHR to your busiest hour: roughly 45+ gallons for 1–2 people, 55–62 for 3–4, and 75+ for five or more.
Size from your real peak hour — back-to-back showers, laundry, and dishes — not from the number printed on the tank. Two households with the same tank can have completely different experiences if their peak hours differ.