How to Size a Water Heater

Peak hour demand is the total gallons of hot water your household draws in its single busiest hour, not across a whole day. The DOE water heater sizing worksheet assigns an approximate gallon figure to common activities so you can add up a realistic worst-case hour.

Take a household of four running two showers, one shave and one load of hand dishwashing inside the same busy hour: 20 gallons for the showers, 2 for the shave and 4 for the dishes comes to a 26 gallon peak hour demand. Add a dishwasher cycle running in the same window and that climbs to 32 gallons. A heater with a published first hour rating below that number will run out before the hour is over; one at or above it will not, regardless of what the nominal tank size says on the box.

First hour rating (FHR) is the gallons of hot water a water heater can deliver in one hour starting from a full tank, and it combines two separate things: how much the tank holds, and how fast the burner or element can reheat water during that same hour. FHR is printed on the yellow EnergyGuide label required on every new water heater, and it is the single number that should be matched against your peak hour demand figure, not the nominal tank gallons printed in the model name.

Two tanks both labeled 40 gallons can carry different first hour ratings depending on element wattage or burner input, and a gas tank commonly outperforms an electric tank of the same nominal size because combustion recovers heat faster than a resistance element does. The 40 vs 50 gallon comparison and the gas vs electric comparison both walk through this in more detail; the short version is that nominal size is a starting point for narrowing shelf options, not the number that decides whether the heater fits your household.

A tank heater is sized against a one-hour demand window because it has a fixed reserve to draw down. A tankless unit has no reserve at all, so it is sized instead against the simultaneous flow rate, in gallons per minute, of every fixture likely to run at the same moment, at the specific temperature rise your incoming water needs. Two showers and a dishwasher running together is a GPM-and-rise problem, not an hourly-gallons problem, which is what the tankless sizing calculator works out.

A heat pump water heater recovers more slowly than a gas tank of similar nominal size, since it moves heat from surrounding air rather than burning fuel, so its published FHR often runs lower than a gas or high-wattage electric tank at the same gallon capacity. That slower recovery is a normal tradeoff for its efficiency, not a defect, but it means sizing a heat pump replacement against your actual peak hour demand matters even more than it does with a gas tank. Installation space and airflow requirements are a separate consideration, covered in the heat pump installation requirements guide.

Run your own peak hour through the worksheet above rather than relying on a generic household-size chart alone, since a house with teenagers showering back to back behaves nothing like a house with the same headcount on staggered schedules. The water heater size chart by household gives typical starting ranges once you have a rough headcount and usage pattern, and the size calculator turns your specific worksheet total into a target FHR and tank range.

These span a useful spread of first hour rating and fuel type. Match the specific published FHR of any model against your own peak hour demand before choosing between them.