Tank sizing
What Size Water Heater for a 1 Person, 1 Bathroom Home?
Researched from the DOE test procedures, NFPA 54 and NEC code tables, and manufacturer specifications. Updated .
Quick answer
This page answers one question: how much hot water does a 1 person, 1 bathroom household need in the hour it needs the most? The arithmetic comes from the DOE and Energy Star first hour rating worksheet, which assigns published gallons of hot water to each activity rather than using a gallons per person figure.
The answer is 18 gallons in the peak hour. That is the number to shop against, and it is why the water heater size calculator asks for people and bathrooms rather than square footage. The tank size below it is a convention: two 30 gallon tanks with different burner ratings can differ by twenty gallons of first hour rating.
How is 18 gallons in an hour arrived at?
Each line is a published figure from the first hour rating worksheet, in gallons of hot water at the tap rather than mixed water at the fixture.
| Activity | Times in the peak hour | Gallons each | Gallons |
|---|---|---|---|
| Shower | 1 | 10 | 10 |
| Washing hands and face | 1 | 4 | 4 |
| Hair shampoo | 1 | 4 | 4 |
| Peak hour total | 18 |
Published standard Source: DOE and Energy Star first hour rating worksheet. These are gallons of hot water leaving the heater. A 10 gallon shower makes considerably more than 10 gallons of warm water at the body, because the shower mixes hot with cold.
Which tank size delivers 18 gallons in an hour?
A tank delivers the usable part of its storage plus whatever the burner or element reheats during the hour. At a typical 70 percent usable fraction and a 20 gallon per hour electric recovery rate, that puts this household on a 30 gallon tank.
| Tank size | Usable storage | Plus electric recovery | Plus gas recovery |
|---|---|---|---|
| 30 gal | 21 gal | 42 gal | 59 gal |
| 40 gal | 28 gal | 49 gal | 66 gal |
| 50 gal | 35 gal | 56 gal | 73 gal |
| 55 gal | 38.5 gal | 59 gal | 77 gal |
| 65 gal | 45.5 gal | 66 gal | 84 gal |
| 75 gal | 52.5 gal | 73 gal | 91 gal |
| 80 gal | 56 gal | 77 gal | 94 gal |
Convention Source: Usable storage at 70 percent plus one hour of recovery, computed from a 4,500 W element and a 36,000 BTU burner at a 90 F rise. A gas tank of the same size delivers noticeably more in the first hour because its burner recovers roughly twice what a standard element does. The published DOE first hour rating on the EnergyGuide label is the measured figure and governs over this estimate.
Does a tankless unit change the answer for 1 person?
Completely, because a tankless unit has no first hour rating at all. It is sized on simultaneous flow and temperature rise rather than on storage, so the question becomes how many fixtures run at once rather than how much is stored. With one bathroom the realistic worst case is one shower plus a sink, which is roughly 3.5 gallons per minute.
Run that through the tankless sizing calculator with your own inlet temperature, because the same unit delivers roughly half the flow at double the rise. The temperature rise pages show what that does across the range.
What changes this number for your house
Three things move it more than anything else. Shower head flow rate is the largest: a 2.5 gallon per minute head against a 1.75 gallon per minute head is close to a 40 percent difference over the same shower. Whether the appliances run in the peak hour is the second: moving the dishwasher to the evening removes 6 gallons from the morning. Whether showers overlap is the third, and it is the one that separates a tank decision from a tankless one.
If you want the honest number rather than the estimate, count your own busiest hour with the peak hour demand calculator.
Tanks in this size range
Verified listings around the 30 gallon mark. A 50 gallon tank replacement is often bought at a big box store with installation attached, and there is nothing wrong with that route: the parts around it are where an online order genuinely makes sense.
Rheem PROE50 M2 50 Gallon
$945.99The professional line 50 gallon electric, which is the single most common residential water heater size in the country.
Best for: A straight 50 gallon electric replacement
Check price on Amazon
GE 40 Gallon Lowboy
$522.00A lowboy is short and wide, which is the answer when the tank has to fit under a stair or in a crawlspace and the tall version physically will not go in.
Best for: Height restricted closets and crawlspaces
Check price on Amazon
Senville Hybrid Heat Pump 50 Gallon
$1,099.99The lowest price of entry into a heat pump water heater in this catalog, and Energy Star listed, which means its efficiency figure was measured against the DOE test procedure rather than claimed.
Best for: A first heat pump water heater in a warm garage or basement
Check price on AmazonFrequently asked questions
- What size water heater for a family of 1?
- A 1 person household with 1 bathroom uses about 18 gallons of hot water in its busiest hour. Shop for a first hour rating of at least that, which in practice is usually a 30 gallon tank with an ordinary recovery rate. A gas tank of the same size delivers more in the first hour than an electric one, so the fuel changes the answer as much as the household does.
- Should I size up to be safe?
- Going one size up costs standby loss every hour of every day to keep water hot that nobody draws, and on a heat pump water heater a larger tank also needs more clearance and more surrounding air. It is better to match the peak hour demand and fix the peak itself, by moving an appliance out of the morning or fitting a more efficient shower head, than to buy headroom you never use.
- Is first hour rating more important than gallons?
- Yes, and it is the number to compare between models. First hour rating is how many gallons of hot water the heater actually delivers in one hour starting from full, combining usable storage with everything the burner or element reheats during that hour. Two tanks of identical capacity can differ by twenty gallons of first hour rating because their burner ratings differ.
- Does 1 bathroom change the size?
- With one bathroom showers happen in sequence rather than at once, which spreads the same daily use across a longer window. That is easier on a storage tank than the same number of people with two bathrooms, because the burner or element gets time to recover between draws.
- How do I check my current heater is the right size?
- Find the first hour rating on the yellow EnergyGuide label, or estimate it from the tank size and the element or burner rating with the first hour rating calculator. If it is below your peak hour demand, you will run out during the busiest hour and nowhere else, which is exactly the symptom most people describe.
Researched guidance, not professional advice. These figures come from published test procedures, code tables and manufacturer specifications, and they are planning numbers rather than a sizing done for your house. Where a manufacturer publishes a figure for the unit in front of you, that figure governs over anything computed here.