Water Heater Sizing: First-Hour Rating, Peak Demand, and Tank vs. Tankless

Estimating Peak-Hour Demand

Add up the hot water you might use in the busiest hour. Typical hot water use (the hot portion of the mix) is about: shower of 8 minutes at 2.0 gallons per minute, 16 gallons total mixed water, but the hot part is about 10 to 12 gallons; the figures below use mixed volume to be conservative. Clothes washer 7 to 20 gallons depending on the cycle; dishwasher 4 to 6 gallons; shaving 2 gallons; hand washing or a kitchen sink 2 to 4 gallons. A peak hour with two showers (32 gallons), a dishwasher (6), a sink (4), and a laundry load (20) comes to 62 gallons.

A tank with an FHR of 70 gallons would handle this household; one with an FHR of 50 might run out of hot water during back-to-back showers.

Reading the Label

The EnergyGuide label lists the first-hour rating and the uniform energy factor (UEF), the efficiency metric. Choose a model whose FHR equals or exceeds your peak hour demand. A 50-gallon gas heater might have an FHR of 70 to 80 gallons because the burner heats quickly, while a 50-gallon electric tank may have an FHR of 55 to 65 gallons. Heat pump water heaters are much more efficient (UEF of 3 or more versus about 0.9 for standard electric resistance) but have slower recovery in heat pump mode, so they often have hybrid modes for peak demand.

Standby losses also matter; insulating the tank and the first few feet of pipe saves energy.

Tankless Water Heaters

Tankless units heat water on demand and are rated by flow rate in gallons per minute at a given temperature rise. To size one, add up the flow of fixtures that may run at the same time and compute the temperature rise: the difference between the desired temperature (about 120°F) and the incoming cold water (for example, 50°F in northern climates, giving a rise of 70°F). Two showers at 2.0 gpm each plus a sink at 1.0 gpm total 5 gpm. A gas tankless unit that can deliver 5 gpm at a 70°F rise would handle that load. Colder inlet water reduces the maximum flow. Tankless units avoid standby losses and can last 20 years, but they cost more to buy and install, and gas units may need larger venting and gas lines.

Energy Cost and Efficiency

Water heating is typically 15% to 20% of home energy use. A family using 64 gallons of hot water a day heats about 23,000 gallons a year; raising the temperature by 70°F requires 8.34 × 70 = 584 BTU per gallon, or 13.6 million BTU a year, which is 3,990 kWh at 100% efficiency. At $0.16 per kWh, electric resistance costs about $640 a year; a heat pump water heater at a COP of 3 would cost about $213 a year, saving about $430. Setting the thermostat at 120°F saves energy and reduces scald risk compared with 140°F. Low-flow fixtures, shorter showers, and fixing leaks reduce demand.

Practical Recommendations

For a household of four with typical use, a 50-gallon tank with an FHR of 65 to 75 gallons is a common choice. Consider the installation space, venting, fuel type, and upfront versus operating costs. Check utility rebates and incentives for efficient models. A plumber can verify sizing, installation requirements, and local code.