Many people shop for a water heater by tank size alone. That number does not tell you how much hot water the unit can actually deliver during your busiest hour. To choose the right size, you need to know two things: your household's peak-hour hot water demand and the water heater's first-hour rating.
Why size matters
An undersized water heater runs out of hot water when you need it most, usually during morning showers. An oversized unit costs more to buy and more to run. The U.S. Department of Energy notes that a storage water heater that is too large carries a higher purchase price and increased energy costs because a larger tank loses more standby heat. The goal is a unit that matches your real peak use.
What the first-hour rating means
The first-hour rating (FHR) is the number of gallons of hot water a storage water heater can supply in one hour, starting with a full tank of hot water. It accounts for the tank size plus how fast the burner or elements can reheat water during that hour.
Two tanks with the same gallon capacity can have different first-hour ratings. A unit with a larger burner or more powerful elements reheats water faster. That is why FHR is a better comparison point than tank size.
You can find the first-hour rating in the top left corner of the yellow EnergyGuide label, listed as "Capacity (first hour rating)." Manufacturer specification sheets also list it.
How to estimate your peak-hour demand
The Department of Energy provides a simple worksheet in its guide to sizing a new water heater. Follow these steps:
- Pick the one-hour period when your household uses the most hot water. For most families, this is the morning.
- Count how many times each hot water activity happens during that hour.
- Multiply each count by the average gallons per use, then add up the total.
DOE uses these average gallons of hot water per use:
- Shower: 20 gallons
- Shaving: 2 gallons
- Hand dishwashing or food prep: 3 gallons
- Automatic dishwasher: 7 gallons
- Clothes washer: 25 gallons (top-loading) or 15 gallons (front-loading, horizontal axis)
Example: Three showers (60 gallons), one shave (2 gallons) and hand dishwashing (3 gallons) during the same hour add up to a peak-hour demand of about 65 gallons. DOE's own worked example reaches 66 gallons. You would look for a water heater with a first-hour rating at or above that number.
Your actual use may differ from these averages. A long shower or a high-flow showerhead uses more. If your household has recently grown, or you added a large soaking tub, count those changes.
Sizing a tankless water heater
Tankless water heaters do not have a first-hour rating. DOE explains that they are rated by the maximum temperature rise they can provide at a given flow rate. To size one, you need two numbers:
- Flow rate: Add the gallons per minute (gpm) of the fixtures you expect to run at the same time. DOE's example adds a 0.75 gpm faucet and a 2.5 gpm shower for a required flow rate of at least 3.25 gpm.
- Temperature rise: Subtract the incoming water temperature from the temperature you want at the tap. DOE suggests assuming 50°F incoming water unless you know otherwise. With a 120°F set point, that is a 70°F rise.
In Minnesota, the water entering your home can be colder than average in winter. A larger temperature rise lowers the flow rate a tankless unit can deliver. Ask your installer to size the unit for your coldest expected incoming water, not an average day, so you are not short on hot water in February.
Other factors that affect your choice
- Fuel type: Recovery speed depends on the burner or element size, so compare first-hour ratings across gas and electric models rather than tank sizes.
- Heat pump water heaters: These need space around them and work best in an area that stays between 40°F and 90°F year-round, according to ENERGY STAR's heat pump water heater guidance. They also cool the room they sit in, which matters in a Minnesota basement during winter.
- Space and venting: A larger tank may not fit through a basement doorway or in a tight closet, and venting requirements can limit your options.
- Future plans: If you expect your household to grow or plan to add a bathroom, size for that.
What you can do before you shop
- Read the EnergyGuide label or rating plate on your current unit and write down its first-hour rating and capacity.
- Think about how often you run out of hot water and at what time of day.
- Use the DOE worksheet to total your peak-hour demand.
- Note the number of bathrooms and any high-flow fixtures, such as a large tub or multi-head shower.
Bring these numbers to your estimate. They make it easier to compare units on equal terms.
When to call a professional
Talk to a technician if you regularly run out of hot water, if you are switching between tank, tankless or heat pump models, or if you are changing fuel type. A professional can confirm your peak demand, check your gas line, venting and electrical capacity, and recommend a unit that fits your home. Learn more about water heater replacement and installation at Krinkie's, or contact Krinkie's to schedule an estimate.
Common questions
What is a water heater's first-hour rating?
It is the number of gallons of hot water a storage water heater can supply in one hour, starting with a full tank. You can find it on the EnergyGuide label.
Is a bigger water heater always better?
No. An oversized tank costs more to buy and loses more heat while it sits, which raises energy costs. Choose a unit whose first-hour rating matches your peak-hour demand.
How do I size a tankless water heater?
Add the flow rates of the fixtures you run at the same time, then find the temperature rise between your incoming water and your desired hot water temperature. The unit must deliver that flow rate at that temperature rise.