Water Heater Size Calculator

First Hour Rating is the gallons of hot water a storage tank can deliver in its first hour of use, starting with a full tank. It combines the tank's stored capacity with how fast the burner or element can reheat incoming cold water. The DOE EnergyGuide label on every storage heater publishes the FHR, and it is the number you actually want to match to your peak hour demand. A 50 gallon tank with a high-recovery gas burner can post an FHR of 80 to 90 gallons, while a low-recovery electric model of the same tank size might list 60 to 65 gallons. Match the FHR to your peak hour, not the raw tank gallons.

Tank heaters cost $400 to $1,200 installed and use $250 to $700 per year in energy. Tankless heaters cost $1,500 to $4,500 installed and use $200 to $400 per year, since they only fire when you draw hot water. The break-even on the higher upfront cost is typically 10 to 15 years, right at the edge of a tank's lifespan. Pick tankless if you want endless hot water, have space constraints, or plan to stay long enough for the savings to compound. Pick tank if upfront budget matters more, your hot-water use is bursty (multiple showers back to back), or you live somewhere with very cold inlet water that limits tankless GPM.

Inlet water in northern states averages about 50 F in winter, while in the Sun Belt it sits closer to 70 F. Heating water from 50 F to a 120 F setpoint takes 70 F of rise, versus 50 F of rise from a warmer source. That is 40 percent more energy per gallon and a slower recovery rate on a storage tank. For tankless heaters, the GPM rating drops sharply as the required temperature rise grows: a unit rated at 7 GPM for 45 F of rise might only deliver 5 GPM at 70 F of rise. The calculator adds 15 percent to peak hour demand for cold climates and bumps tankless GPM up one tier to compensate.

A typical family of four with two bathrooms, a dishwasher, and a clothes washer has a peak hour demand around 75 to 80 gallons. A modern 50 gallon gas tank with a 40,000 BTU burner posts an FHR near 80 to 90 gallons, which covers that peak comfortably. Electric 50 gallon tanks recover more slowly, so they post FHRs around 60 to 65 gallons and tend to need a 65 or 80 gallon model for the same household. If your peak hour demand exceeds the FHR of a 50 gallon tank, step up to 65 or 80 gallons rather than running out of hot water mid-shower.

Heat pump water heaters (also called hybrid electric) move heat from surrounding air into the tank instead of generating it, so they use 60 to 70 percent less electricity than a standard electric tank. Annual operating cost drops from about $500 to $700 down to $150 to $250 in most US climates. The upfront cost is $1,500 to $3,000 installed, roughly $1,000 more than a standard electric tank, but federal tax credits and many utility rebates cover $500 to $2,000 of that gap. Payback typically lands in 3 to 5 years for households replacing an electric tank, and the unit needs a roughly 100 cubic foot space (a basement or utility room) for the air it pulls heat from. Cold garages and unconditioned spaces below 40 F hurt efficiency, so plan placement carefully.