Water Heater Sizing Guide

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Tank Size and First-Hour Rating Are Related — They Are Not Interchangeable

Nominal tank volume is how many gallons of water fit in the steel (or glass-lined) cylinder. First-hour rating is how many gallons of hot water you can pull in sixty minutes before the outlet temperature falls off the DOE test. FHR is stored hot water you can actually use, plus whatever the burner or element reheats during that same hour.

A field approximation plumbers have used for decades:

The 70% is not a law. It is a stand-in for the fact that you do not get the whole tank at 120°F. Cold inlet water mixes as you draw. By the time the top of the tank is still useful, the bottom is already diluted. Recovery is the gallons the heat source can raise through a 90°F rise in an hour. Add those two and you are in the neighborhood of the number on the EnergyGuide label. The label is the legal number. The formula is how you see why a 50-gallon gas heater and a 50-gallon electric heater are not twins.

DOE’s published instruction is blunt: pick a storage water heater whose first-hour rating at least matches your peak-hour demand. Do not pick a tank volume and hope. The calculator does the worksheet, then walks the common tank sizes until estimated FHR clears that demand for the fuel you picked.

The DOE Peak-Hour Worksheet (What This Calculator Actually Uses)

The public reference is U.S. Department of Energy, Energy Saver, Sizing a New Water Heater (energy.gov/energysaver/sizing-new-water-heater). You identify the hour of the day when the house uses the most hot water. You do not add up the whole day. You count uses in that one hour, multiply by DOE’s average gallons per use, and total them. That total is peak-hour demand. That demand is the FHR you need.

DOE’s worksheet gallons per use:

DOE’s own example is three showers (60) + one shave (2) + one hand-dish (3) = 66 gallons. That household needs a storage heater with an FHR of 66 or more. Not “a 50-gallon tank because we are a family of four.” A 50-gallon gas unit might clear 66. A 50-gallon electric unit often will not.

The calculator maps peak-usage scenarios onto that same worksheet. “Typical morning (staggered showers)” still counts showers that happen in the same hour even if they are not overlapping. “Shower + dishwasher running simultaneously” adds the dishwasher’s 7 gallons and also raises the tankless GPM line, because now two fixtures are live at once. Bathroom count caps how many showers can overlap; it does not erase sequential volume. One bathroom can still stack three back-to-back showers in sixty minutes. It cannot run two showers at 2.0 GPM at the same time. That distinction is the whole tank vs tankless problem.

Gas vs Electric Recovery — Why Fuel Changes the Tank You Buy

Recovery rate is gallons per hour the heater can raise through a given temperature rise. Physics, not marketing:

A 4,500 W electric element is 15,354 Btu/h. Through a 90°F rise that is about 21 gallons per hour. A 40,000 Btu/h gas burner at roughly 70% thermal efficiency is about 28,000 Btu/h into the water, or about 37 gallons per hour at the same 90°F rise. That is the pair the calculator uses, and it is stated in the results so you can disagree with it.

What it means on the truck:

• Gas storage can often be a size smaller than the electric tank that serves the same peak hour, because recovery carries more of the first-hour load.
• Electric resistance leans on stored volume. Families that run a full-house morning on electric usually land on 50–80 gallon tanks, not because they “use more water,” but because the element cannot restock the tank as fast.
• Heat-pump water heaters still have a tank and still publish an FHR. The compressor is slower than a 4,500 W resistance element when it is only in heat-pump mode; most units kick on backup resistance during a heavy draw. Treat the yellow-label FHR as the number that matters, and use electric as the conservative storage pick in the calculator until you are reading a specific HPWH spec sheet.
• High-input gas (50,000+ Btu/h commercial-style or power-vent) recovers faster than the 37 gal/hr assumption. If the label FHR is already 90 on a 50-gallon tank, believe the label, not this page’s generic burner.

Fuel also changes the install: venting and combustion air for gas, a 30-amp / 240-volt circuit (or more) for electric, condensate and a heat-pump’s cold-air dump for HPWH. None of that is in the gallon number. It is why a plumber still has to walk the mechanical room.

How to Read the Label on an Existing Water Heater

Two labels matter, and they are easy to mix up.

The rating plate (manufacturer’s data plate)

Usually on the side of the jacket. Model number, serial number, nominal tank volume, wattage or Btu/h input, voltage, and often a manufacture date in the serial. This is how you know whether you are looking at a 40-gallon or a 50-gallon tank, whether the electric unit is 4,500 W or 5,500 W, and whether the gas burner is 32,000 or 40,000 Btu/h. Recovery lives here as input, not as FHR.

The yellow EnergyGuide label (FTC)

DOE tells you to size storage heaters from first-hour rating. The Federal Trade Commission requires an EnergyGuide label on new conventional storage water heaters. Top-left of that yellow label is “Capacity (first hour rating)” in gallons. That is the FHR. It is not the tank volume. A 40-gallon gas heater might show 67. A 50-gallon electric might show 62. Compare that number to your peak-hour demand from the calculator.

EnergyGuide is not required on every heat-pump water heater the same way. Manufacturer literature still publishes FHR. If the yellow tag is gone on a 12-year-old tank, you are stuck with tank volume + input from the rating plate and a recovery estimate. That is a starting point. It is not the certified first-hour number.

Also look for the thermostat access panels (electric) or the gas control valve set point. Factory is often 120°F. Cranking to 140°F without a mixing valve is a scald risk. Cranking it down to 110°F to “save energy” is how a correctly sized tank feels undersized. Mixing valves let you store hotter and deliver safer. They change usable first-hour volume. They do not show up on the carton gallon number.

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