What is a first-hour rating?

A first-hour rating (FHR) is the gallons of hot water a storage heater can deliver in the first hour of use, starting with a full tank. It combines the stored volume with how fast the unit reheats. The U.S. EPA lists it as a key product criterion for residential water heaters (see energystar.gov/products/water_heaters/residential_water_heaters_key_product_criteria).

Is first-hour rating the same as tank size?

No. Two tanks with the same gallon capacity can have different first-hour ratings depending on their burner or element output and recovery rate. First-hour rating reflects usable hot water during peak use, not just how much the tank holds.

How do I use first-hour rating to choose?

Estimate your peak demand — the most hot water your home uses in an hour — and choose a unit whose first-hour rating meets or exceeds it. That match is more useful than comparing tank gallons alone.

First-Hour Rating vs Tank Size

Tank size is capacity stored. First-hour rating is capacity delivered. For peak use, the delivered number is the one that matters.

What Each Number Means

Metric	What It Measures	Why It Matters
Tank size (gallons)	Volume the tank holds	Sets a ceiling on stored hot water
First-hour rating	Hot water delivered in the first hour	Reflects stored volume plus recovery
Recovery rate	How fast the unit reheats water	Affects back-to-back use

Why First-Hour Rating Can Differ Between Same-Size Tanks

Two 50-gallon tanks can have different first-hour ratings. A tank with a higher burner or element output reheats faster, so it delivers more usable hot water in that first hour. That's why two units with identical gallon capacity can perform differently under load.

The U.S. EPA includes first-hour rating as a key product criterion for residential water heaters, because it reflects real-world delivery better than tank gallons alone.

How to Use It When Comparing

Estimate your peak one-hour hot-water demand
Compare first-hour ratings, not just gallon capacity
Consider recovery rate if you use hot water back-to-back
Check that the fuel type and connections fit your home

Comparing Water Heater Ratings?

We translate first-hour ratings and recovery into a recommendation that fits your actual demand — not just a bigger tank.

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First-Hour Rating vs. Tank Size

What Each Number Means

Why First-Hour Rating Can Differ Between Same-Size Tanks

How to Use It When Comparing

Comparing Water Heater Ratings?

Frequently Asked Questions

Related Reading

Water Heater Installation

Sizing a Water Heater for Household Demand

Why Hot Water Runs Out Quickly

Planning for Simultaneous Hot-Water Use

ENERGY STAR — Residential Water Heaters Key Product Criteria