What size water heater for a family of 1?
One person is the easy case: a 30-gallon tank of either fuel covers a shower plus sink use (26 gallons of peak demand) with room to spare. The honest question is whether to spend the small difference on a 40 and stop thinking about guests. Everything below is the arithmetic.
Every version of the busiest hour, for one person
Each row is that many showers in one hour plus one shave and one round of hand dishes, priced in hot gallons off the DOE/AHRI worksheet (20 per standard shower, 13 on a 2.0 GPM low-flow head). The tank shown is the smallest standard size whose first hour rating covers the demand; our FHR figures are ~70% of tank volume plus an hour of recovery, and label values land within a few gallons of them.
| Showers in the hour | Peak demand | Gas tank | Electric tank | Demand, 2.0 GPM heads | Gas | Electric |
|---|---|---|---|---|---|---|
| 1 | 26 gal | 30 gal | 30 gal | 19 gal | 30 gal | 30 gal |
Laundry note: a hot-wash load adds 32 gallons to the hour, which moves every verdict in the table up a row or two. Warm and cold cycles barely register, and that is the cheapest sizing fix there is: wash on warm outside shower hour and buy the smaller tank.
One person, and a tank that runs all night for one shower
A single-occupant house is the one case where standing loss matters more than the first hour rating. The heater holds 120°F water around the clock for a draw that happens once a day, and the yellow EnergyGuide sticker on the box is pricing exactly that: a year of keeping the tank hot, estimated for a household bigger than yours. Read the sticker's annual figure as a ceiling and read the tank volume as the thing driving it.
That is the argument for buying small. The argument against it is guests. A 30-gallon gas unit covers the 26-gallon hour above with half its first hour rating unused, and a 40 costs very little more and covers a house full of people at Christmas. Over ten years the extra standing loss on those ten gallons is small enough that most single owners buy the 40 and stop thinking about it.
Heat pump water heaters complicate this size specifically. Most lines start at 50 gallons, so a single person shopping for one is buying storage they will never draw down. Operating cost still falls hard, because the compressor moves heat instead of making it, but the payback stretches: you are cutting two thirds off the smallest hot water bill on the street.
One thing not to do here: drop the thermostat below 120°F to save standing loss. Long gaps between draws and lukewarm storage is the exact condition Legionella likes, and a one-person house has the longest gaps of any household on this site.
Questions people ask
Is a 50-gallon water heater enough for a family of 1?
Yes, generously. A 50-gallon gas tank delivers about 78 gallons in its first hour and a 50 electric about 55, both past the 26-gallon hour one person can realistically pull. You could buy smaller; nobody regrets a 50.
Do these numbers change with teenagers?
Only in the honest direction. The worksheet's 20-gallon shower is a 10-minute shower on a 2.5 GPM head; a teenager's 25-minute shower is 50 gallons all by itself. If that is your house, count long showers as two.
What about a tankless for one person?
Feasible and usually oversized for the need; a small unit does fine. Size it to simultaneous draw with the tankless calculator, not to this page's gallons.