How to Choose a Water Heater? | Sizing By Demand, Not Tank Size

Choose a water heater by first-hour rating for tanks or GPM for tankless, not by tank size alone.

Picking the wrong water heater usually means cold showers at peak times or an oversized unit that wastes energy for years. The right way to choose a water heater starts with one decision — storage tank or tankless — and then matches the unit’s delivery capacity to your household’s peak hot-water demand. Tank gallons alone tell you almost nothing useful; the number that matters is printed on the EnergyGuide label.

Storage Tank vs. Tankless: The First Decision

The choice between a storage tank and a tankless unit changes how you size everything else. Storage tanks keep 30 to 80 gallons heated and ready, while tankless units heat water on demand as it flows through. Each has a different sizing metric, and each carries its own installation constraints.

Storage tanks cost less upfront and deliver a large burst of hot water at once, which suits households with heavy simultaneous use. Tankless units never run out of hot water, last longer with proper maintenance, and take up far less space — but they cost more to install and deliver a limited gallons-per-minute flow. Consumer Reports’ buying guidance notes that size depends on the number of people in the home and peak usage patterns.

How To Size A Storage Tank Water Heater

For storage tanks, the first-hour rating (FHR) is the correct sizing metric, not the tank’s gallon capacity. The FHR tells you how many gallons of hot water the unit delivers in the first hour starting from a full tank — which accounts for both the stored water and how fast the burner reheats it.

Here is how to size one:

• Estimate peak demand: Count the fixtures likely to run in the busiest hour — showers, dishwasher, washing machine. A family of four with two showers back-to-back plus a dishwasher run needs a higher FHR than a couple taking separate showers.
• Find the FHR number: Look for the first-hour rating on the yellow EnergyGuide label. It is listed in gallons and is the number to compare across models.
• Match demand to FHR: Choose a unit whose FHR meets or slightly exceeds your busiest hour’s usage. A 40-gallon tank can have a higher FHR than a 50-gallon tank if its burner reheats faster.
• Check the EnergyGuide label: The label also lists estimated annual operating cost, which is the number to compare for long-term expense.

A larger tank does not automatically mean a higher FHR — a fast-recovering 40-gallon unit often beats a slow 50-gallon model on delivered hot water.

Tankless Sizing: GPM And Temperature Rise

Tankless water heaters are sized by gallons per minute (GPM) at a specified temperature rise, not by tank capacity. The GPM rating only holds at a particular incoming-water temperature, so the same unit delivers less hot water in winter when the incoming water is colder.

To size one accurately:

• Count simultaneous fixtures: Add the flow rates of everything you might run at once — a shower uses about 2.5 GPM, a faucet about 1.5 GPM, a dishwasher about 1.5 GPM.
• Know your region’s incoming water temperature: The temperature rise is the difference between incoming ground water and your target of about 120°F. Northern homes need a larger rise than southern homes.
• Check the rating at your rise: A unit advertised at 8 GPM may only deliver 5 GPM at the 70°F rise a cold climate requires.
• Confirm winter performance: A unit sized for summer inlet temperatures will underperform in winter because the required temperature rise is larger.

If you’re replacing a failing tank and want better efficiency, ENERGY STAR’s sizing guidance for heat pump water heaters also relies on the first-hour rating rather than tank size alone.

Compatibility, Efficiency, And Common Mistakes

The remaining factors are fuel source, installation compatibility, efficiency, warranty, and budget. Verify fuel availability first — electric, natural gas, propane, or heat pump — because venting and electrical service requirements differ sharply between them.

Installation checks: Confirm venting options, electrical capacity, clearances, seismic strapping, and expansion tank needs against local plumbing codes. Combustion units also carry a backdrafting risk that a qualified installer should evaluate.

Efficiency labels: Compare the Energy Factor (EF) or Uniform Energy Factor (UEF) on the EnergyGuide label; higher numbers mean better efficiency. The estimated annual operating cost on that same label is more useful than purchase price alone.

Common mistakes: Buying by tank gallons instead of FHR, ignoring simultaneous fixture use, trusting advertised GPM without checking temperature rise, skipping fuel and venting compatibility, and comparing purchase price instead of EnergyGuide operating cost.

Once you’ve matched a unit to your demand, protecting it matters — a sacrificial anode rod is what keeps the tank from rusting from the inside. Our tested roundup of the best anode rods for water heater maintenance will help you extend its life.