Storage Water-Heater Sizing (First-Hour Rating)
Whether a storage water heater meets the household peak-hour demand: the first-hour rating (FHR) is the usable storage plus one hour of recovery, with recovery = input x efficiency / (8.33 x rise), checked against the peak-hour draw. The manufacturer's rated FHR on the EnergyGuide label governs the final selection; this is a sizing check, not the rating.
Formula and source
recovery_gph = input_btuh x efficiency/100 / (8.33 x rise_F); first-hour rating FHR_gph = tank_gal x usable_fraction + recovery_gph; the verdict compares FHR to the peak-hour demand.
First-principles recovery Q = 8.33 x gph x delta-T with the DOE/AHRI first-hour-rating definition (usable storage plus one hour of recovery), by name, not reproduced.
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