What size water heater do you need?

How we work it out

Tank: first-hour rating. Count what uses hot water in your busiest hour, usually the morning. AHRI's worksheet puts a shower or a bath at 20 gallons, a clothes washer load at 32, a dishwasher load at 14, food prep at 5, hand or face washing, a shampoo or hand dishwashing at 4, and a shave at 2.

first-hour rating ≥ gallons used in your busiest hour

The first-hour rating is on the yellow EnergyGuide label as "Capacity (first hour rating)." It's the federal test of how much hot water a full tank delivers in an hour, so it beats tank size for comparing: a 40 gallon gas tank with a fast burner can out-deliver a 50 gallon electric.

Tankless: flow and temperature rise. Add up what runs at once: a shower is 2.5 gpm (the federal limit) or 2.0 (WaterSense), a sink 2.2, a clothes washer 3.3 and a dishwasher 1.3, the figures ENERGY STAR uses. Showers, sinks and washers mix to about 105°F; dishwashers want 120°F.

load = Σ gpm × (delivery temperature − incoming temperature)

A tankless unit's flow table holds gpm × rise nearly constant. Navien's NPE-240A2 does 11.2 gpm at a 35°F rise, 8.7 at 45°F and 5.6 at 70°F, all about 392. So a unit keeps up when its gpm × rise is at least your load.

Incoming water. Ground water tracks the air above it, lagging by about a month. We use the formula from NREL's Building America House Simulation Protocols with the nearest NOAA station's 1991 to 2020 normals:

T = (T_air avg + 6) − (0.4 + 0.01 × (T_air avg − 44)) × (warmest month − coldest month) ÷ 2

That's the coldest month, the one to size a tankless unit for.