What Size Water Heater Do I Need? Tank and Tankless Sizing

Picking a new water heater by copying the old label seems easy, but it can leave you short on hot water or paying to heat water you never use. If you are replacing a tank soon, our water heater replacement service in Newark sizes each install to the household, not just the old unit. This guide shows how to size a tank, a heat pump model and a tankless unit using the U.S. Department of Energy's method. Bring your old heater's label and a rough idea of your morning routine.

What size water heater do I need? Start with your peak hour

The U.S. Department of Energy (DOE) sizes storage and heat pump water heaters by their first-hour rating (FHR). That is the number of gallons of hot water a heater can deliver in one hour, starting with a full tank.

The FHR depends on three things:

• Tank size in gallons
• The fuel or heat source
• Burner or element size, which sets how fast it reheats

A gas tank with a big burner can have a higher FHR than a larger electric tank. That is why two "50-gallon" heaters can perform very differently.

DOE's advice is simple: find your household's busiest hour of hot water use. Then pick a heater with an FHR within a couple of gallons of that number, or higher.

How to estimate your peak-hour demand

Use DOE's average gallons per use:

Now list what happens in your busiest hour. Mornings are typical.

1. Count showers in that hour and multiply by 20.
2. Add other uses from the table.
3. Total the gallons. That is your peak-hour demand.

Example: three showers (60), one shave (2) and some hand dishwashing (3) add up to about 65 gallons. That home needs a heater with an FHR of at least the mid-60s.

Bathtubs are not on DOE's worksheet. A large soaking tub can use far more hot water than a shower, so plan extra if someone fills one during the busy hour.

Where to find the first-hour rating

Look for the yellow EnergyGuide label on a new tank. DOE notes it lists capacity as the first-hour rating. You can also find the FHR in the spec sheet.

DOE adds that heat pump water heaters are not required to show it on the EnergyGuide label. For those, check the maker's spec sheet. Many list FHR in both efficiency and hybrid modes, and the hybrid number is usually higher.

Typical tank sizes by household

This is a starting point, not a rule. Always check the FHR against your own peak hour.

Two things shift these numbers. Gas tanks usually recover faster than electric ones of the same size. Large tubs, body-spray showers or teens with long showers push you up a size.

What size tankless water heater do I need?

Tankless units do not store water, so they are sized by flow rate in gallons per minute (GPM) at a given temperature rise. DOE's method has two steps.

Step 1: Add up flow for fixtures used at the same time

DOE's example: a faucet at 0.75 GPM plus a shower at 2.5 GPM needs at least 3.25 GPM.

Step 2: Find the temperature rise

Subtract the incoming water temperature from the temperature you want. DOE assumes 50°F incoming water unless you measure it. To reach 120°F, that is a 70°F rise.

Then pick a unit that delivers your total GPM at that rise. Makers publish flow charts for each model. Winter inlet water is colder, so a unit that is fine in summer may fall short in January. Our tankless vs. tank water heater guide covers costs and trade-offs.

What size electric or on-demand water heater do I need?

Electric tankless units need very high power. A whole-house model can need several large breakers and heavy wiring. Many older homes cannot support one without a panel upgrade. DOE's tankless measure guide also notes gas tankless models often need a larger gas line and new venting.

For electric homes, a heat pump water heater is often the better whole-house choice. For a far-off bathroom, a small point-of-use electric unit can make sense.

Sizing a heat pump water heater

Heat pump models heat more slowly than standard electric tanks in efficiency mode. Many installers size up one step, such as from 40 to 50 gallons, to cover busy mornings. Heat pump models also need room around them: enough air volume, clearance for service and a drain for condensate. ENERGY STAR's design guidance covers these points, and our guide to what a heat pump water heater is explains the space rules.

Size limits: fit, fuel and the 2024 standards

Before you upsize, check:

• Space. Taller or wider tanks may not fit a closet or under a stair.
• Fuel. DOE's selecting guide notes that switching fuels can add the cost of a new breaker, gas line or vent. Our gas vs. electric water heater guide compares them.
• New efficiency rules. DOE's 2024 standards will push most electric tanks of 35 gallons or more toward heat pump designs starting in 2029. Keep that in mind if you plan to stay in the home a long time.
• Code. A bigger tank may need a bigger drain pan, expansion tank or vent. A water heater permit review will catch these.

Sizing tips for Newark and Delaware homes

• Older Newark townhomes and rowhomes often have tight utility closets. Measure height and width before choosing a larger tank.
• Homes on well water may have colder inlet water in winter, which matters for tankless sizing.
• Families in new construction around Middletown and Bear often have large soaking tubs. Plan for that in your peak hour.

How to avoid the wrong size

• Write down your busiest hour before shopping.
• Compare first-hour ratings, not just gallons.
• Check recovery and fuel, since gas reheats faster.
• Measure the space, including door width.
• Plan for change, such as a new baby or teens.

Local note: In Middletown and Bear, many newer homes have three or more full baths, which can outgrow a 40-gallon tank. In older Wilmington homes, the challenge is often fitting a larger tank through a narrow basement stair.

Related reading: Plan your budget with water heater replacement cost in Delaware and see whether your old unit is nearly done in how long do water heaters last?

When should you call a water heater professional?

Call a pro if:

• You run out of hot water most mornings
• You want to switch from tank to tankless or heat pump
• You plan to change fuels or vent type
• The new heater is a different size than the old one
• You are not sure your panel or gas line can handle a bigger unit

Wondering what size water heater you need in Newark? Call (302) 732-0874 for water heater replacement from Newark Heater Pros.