Storage and heat-pump tanks
A Storage Tank Keeps Hot Water Ready for Your Next Use
The number on a tank tells you how much water it holds. It does not tell you exactly how much hot water it can provide during a busy morning. While you use hot water, the heater is making more.
That is why this calculator gives you a first-hour rating. It is the number to compare with when you look at a storage or heat-pump heater. Think about your busiest hour, such as showers and dishwashing close together, rather than every fixture in the house.
Tankless heaters
A Tankless Heater Warms Water Only When You Turn It On
A tankless heater has no stored hot water. It warms water while the water is flowing through it. Start by counting the showers, taps, or appliances that may need hot water at the same time.
Next, enter the temperature of the water coming into your home and the temperature you want. Colder incoming water takes more work to heat, so the same heater can provide less hot water at once. Check the result against the manufacturer’s chart for the same temperature difference.
How the result helps you choose
Two Simple Examples
A busy morning with a tank
Two showers, hand dishwashing, and one dishwasher cycle can use 30 gallons from a 125°F tank in the same hour.
Calculation: 2 showers × 10 gal + 1 hand-dishwashing use × 4 gal + 1 dishwasher cycle × 6 gal = 30 gal.
Decision: a 28-gallon first-hour rating is short by 2 gallons in the first hour. A 35-gallon first-hour rating exceeds the target by 5 gallons. Compare first-hour rating, not tank volume alone.
Two showers with a tankless heater
In this starter example, each shower is entered as 2 GPM of hot water supplied by the heater. Together, that is 4 GPM. If the water enters at 50°F and you want 120°F water, it needs to warm by 70°F.
Find 70°F in the candidate heater’s flow chart and confirm that it can supply at least 4 GPM on that row. If you only know a shower’s total mixed-water flow, use the manufacturer’s sizing method or adjust for the cold-water share.
For tanks and heat pumps
Look at First-Hour Rating, Not Just Tank Size
Why equal-size tanks can feel different
Two 50-gallon tanks can give you different amounts of hot water in a busy hour. One may warm replacement water faster than the other. First-hour rating includes both the hot water in the tank and the hot water it can make during that hour.
What number to look for
Add the hot-water uses that can happen in your busiest hour. Then choose a model with a published first-hour rating at least as high as your result.
Before choosing a heat-pump tank
Check the model’s room-temperature range, required clearances and airflow, noise, and where condensate water will drain. These details can affect comfort, efficiency, and how quickly the heater makes more hot water.
For tankless heaters
Match Flow and Temperature Difference
Use winter water, not a summer guess
Colder incoming water needs more heating. A heater that keeps up in summer may provide less hot water during colder weather, so use the lowest incoming-water temperature you reasonably expect.
Compare the same two numbers
Your result is a required GPM at a specific temperature difference. Find that same temperature difference in the manufacturer’s flow chart, then check that the listed flow meets or exceeds your result.
Capacity is not the whole installation
Heater capacity does not tell you how long hot water takes to reach a shower. Pipe route, heater location, water quality, venting, gas supply or electrical service still need to suit the selected model.
Need to size the hot-water pipe for the flow you enter? Use the Pipe Size Calculator after selecting your heater requirement.
Where the ratings come from
Sources and Important Limits
Sources checked September 2026
For a storage or heat-pump model, compare the published first-hour rating on its EnergyGuide label with your target. For tankless, match the same GPM and temperature difference in the manufacturer’s flow chart. Then confirm installation requirements in that model’s manual.
- ENERGY STAR - Water Heater Criteria: explains first-hour rating and tankless flow ratings.
- AHRI Directory: look up certified product ratings before you buy.
- ENERGY STAR - Heat Pump Water Heater Guide: check the manufacturer’s guidance for the place where the heater will be installed.
- Starter-value basis: storage and heat-pump activity values represent water from a 125°F tank so they can be compared with first-hour rating. Tankless flow values represent hot water supplied by the heater. They are planning estimates, not a single published standard, and should be replaced with your own fixture information or a manufacturer’s sizing method.
Quick answers
Water Heater Size FAQs
Should I get a tank or a tankless heater?
Choose a tank or heat-pump heater when you want enough hot water for your busiest hour. Choose tankless when you want to cover showers, taps, or appliances that may need hot water at the same time. Compare each result with the same rating in the product information.
Why does this calculator use water from a 125°F tank?
First-hour rating is measured using water supplied from a 125°F tank, so the storage values here use the same basis. That lets you compare your result fairly with the rating on a product label. If you plan to run the tank at a substantially different setting, follow the manufacturer’s sizing guidance.
Why might one 50-gallon tank give more hot water than another?
A tank size tells you how much water it holds. It does not tell you how quickly the heater can make more hot water. Look at the first-hour rating on the product label to compare how much hot water each tank can provide during a busy hour.
What should I include in my busiest hour?
Count uses that can happen in the same 60 minutes, such as showers, dishwashing, or laundry. Do not count every fixture in the house just because it is installed. Change the starter values to match your normal busy time.
Should I plan for every shower running at once?
Plan for the number of hot-water uses that can realistically overlap in your home, not every fixture that could possibly be turned on. Tankless needs the flow used at the same time. A tank or heat-pump heater needs the total use in the busiest hour.
Why can a tankless heater supply less hot water in winter?
Water entering your home is colder in winter, so it needs more heating. That means the same heater can provide less hot water at once. Check the manufacturer’s chart at the same temperature difference shown in your result.
Can I use this for a heat-pump water heater?
Yes. Use the busiest-hour option, then compare your result with the model’s first-hour rating. Also check that the model is suitable for the room where you plan to install it.
Does UEF tell me which size to buy?
No. UEF helps compare energy use. For a tank or heat-pump model, use first-hour rating to check size. For tankless, use the flow the heater can provide at your required temperature difference.