First Hour Rating

First Hour Rating is a standardized measure of how much hot water a heater will supply during peak demand — typically a morning shower, laundry, and dishwasher running at once. It's the number stamped on the EnergyGuide label and reflects both tank capacity and recovery rate, so a small tank with a screaming burner can match a larger tank with slow recovery.

FHR matters because tank volume alone tells you nothing about real-world performance. A 40-gallon electric with a 4500-watt element might deliver only 40 gallons of hot water in the first hour, while a 40-gallon gas unit with a power-burner recovery can hit 80+ gallons FHR. Contractors use FHR to right-size replacement units for a home's actual hot water demand.

When you're replacing a water heater, look at the old unit's FHR first if you have paperwork, or ask the homeowner about run-outs. If they're burning through hot water before the shower ends, you need higher FHR, not a bigger tank. Tank size and FHR don't always track — understand the recovery rate and you'll spec it correctly.