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.

Choose tank to compare peak hour demand with a storage heater's First Hour Rating. Choose tankless to total the simultaneous fixture flow from bathrooms and selected appliances, with an optional 20 percent high-demand reserve. The calculator also shows its built-in annual energy-cost range for the selected type. It does not compare purchase price, installation cost, or payback.

The calculator adds 15 percent to peak hour demand when you select a cold climate. For a storage heater, match the resulting minimum First Hour Rating. The tankless flow target comes from the selected bathrooms and appliances, not from the climate multiplier, so confirm that flow against the manufacturer's chart at your required temperature rise.

The calculator does not recommend a nominal tank volume. For four people, two bathrooms, a dishwasher, and a clothes washer, its moderate-climate model produces a minimum First Hour Rating of 79 gallons. Compare that requirement with the FHR on the EnergyGuide label because tanks with the same storage volume can have different recovery rates.

A heat pump water heater can use substantially less electricity than a resistance tank, but savings depend on climate, location, household demand, rate, and equipment. The federal Section 25C credit ended for property placed in service after December 31, 2025. For a 2026 installation, verify any state or utility rebate and compare a local installed quote with the manufacturer's placement requirements.