Chapter 01Records · Know-how
Reading the water heater label and the data plate
A tank water heater carries two labels worth reading. The yellow EnergyGuide label gives the tank size and the first hour rating, which is the number to size by, plus a band from very small to high. The data plate on the heater gives the maker, model number and serial number. Copy all four onto your heater record. You can read both from where you stand, without opening anything.
Source: 16 CFR 305.17, EnergyGuide label content, checked October 20, 2026; City of Palo Alto, Check Your Water Heater's Age, checked October 20, 2026; PNNL Building America, DHW First Hour Rating, checked October 19, 2026.
- Capacity on a tank label: tank size and first hour rating2 numbers 16 CFR 305.17, EnergyGuide label content, October 20, 2026
- Very small, low, medium and high4 bands 16 CFR 305.17, EnergyGuide label content, October 20, 2026
- What a tankless label shows instead of a first hour ratinggpm 16 CFR 305.17, EnergyGuide label content, October 20, 2026
Read only, open nothing
Everything on this page is read from the outside of the heater. Don't remove covers, panels or insulation to find a number. If a label is hidden behind the heater or a wall, note "label not reachable" on the record and ask the plumber or landlord to read it at the next visit.
Two labels, two jobs
The yellow EnergyGuide label is there to compare heaters: federal rules set what it shows, including the model number, the capacity, an estimated yearly energy cost and an estimated yearly energy use. The data plate is the maker's own label on the heater. The City of Palo Alto describes it as the label that lists the manufacturer, model, serial number and other specifications.
Source: 16 CFR 305.17, EnergyGuide label content, checked October 20, 2026; City of Palo Alto, Check Your Water Heater's Age, checked October 20, 2026.
Tank size vs first hour rating
On a storage (tank) heater the label has to show capacity two ways: the rated storage volume, written as tank size (storage capacity), and the first hour rating. The tank size is how much the tank holds. ENERGY STAR defines the first hour rating as an estimate of the most hot water the heater can supply in an hour that starts with the tank fully heated. It depends on the tank size and on how fast the heater recovers, and it's the number the PNNL sizing tool compares with your busiest hour.
Source: 16 CFR 305.17, EnergyGuide label content, checked October 20, 2026; ENERGY STAR Residential Water Heaters spec, Version 5.0, checked October 20, 2026; PNNL Building America, DHW First Hour Rating, checked October 19, 2026.
The PNNL tool's rule is to pick a heater whose first hour rating is equal to or greater than your peak hour demand. Two heaters with the same tank size can have different first hour ratings, so compare the rating, not the gallons on the tank.
Source: PNNL Building America, DHW First Hour Rating, checked October 19, 2026.
The four bands
- Very small: a first hour rating under 18 gallons.
- Low: 18 to 50.9 gallons.
- Medium: 51 to 74.9 gallons.
- High: 75 gallons or more.
Source: 16 CFR 305.17, EnergyGuide label content, checked October 20, 2026.
The band decides which cost range the label prints. It's a rough size class, so write the exact first hour rating on your record, not only the band.
Source: 16 CFR 305.17, EnergyGuide label content, checked October 20, 2026.
Tankless heaters show gallons per minute
An instantaneous (tankless) heater has no tank and no first hour rating. Its label puts it in a gallons per minute band instead: very small 0 to 1.6, low 1.7 to 2.7, medium 2.8 to 3.9 and high 4.0 or more. Copy that figure, and use the tool's tankless check to add up the taps you run at the same moment.
Source: 16 CFR 305.17, EnergyGuide label content, checked October 20, 2026; PNNL Building America, DHW Instantaneous Sizing, checked October 19, 2026.
Let the tool do the arithmeticWater heater capacity estimatorType the first hour rating from the label into the estimator. It adds up your busiest hour and shows whether the rating covers it, then prints a heater record with the model and serial lines ready to fill in.Example: 59 gal in the busiest hour (label says 62 gal, 3 to spare)Open the toolCopying the data plate
Find the plate
Look on the outside of the heater for the maker's label with the model and serial number. Use a flashlight in a dark closet.
Take a photo first
A clear phone photo of the plate and the yellow label means you can copy the numbers at a table instead of squinting in the closet.
Copy four lines
Maker, model number, serial number, and the tank size with the first hour rating (or the gpm figure on a tankless label).
Check the hard characters
Read the serial twice. Letters like O and Q and the digit 0 are easy to mix up, and the serial is what the maker asks for.
File it
The numbers go on the heater record. The photo goes in the house binder or a phone album named for the heater.
Source: City of Palo Alto, Check Your Water Heater's Age, checked October 20, 2026.
The serial number also holds a date
Many makers code the month and year of manufacture into the serial number. The install date guide shows how to read it for three common brands.
Checklist to print
Tick these off at the heater:
- Photo of the yellow EnergyGuide label
- Photo of the data plate
- Maker and model number copied
- Serial number copied and read twice
- Tank size copied
- First hour rating copied (or gpm on a tankless label)
- Band noted: very small, low, medium or high
- First hour rating typed into the estimator
- No covers or panels opened
Tips and tricks
- Tip 01
Photograph the yellow label early
The label must say federal law bars removing it before the sale. After the sale, years in a closet can fade or peel it, so take the photo while it's readable.
Source: 16 CFR 305.17, EnergyGuide label content, checked October 20, 2026.
- Tip 02
The model number appears on both labels
The EnergyGuide label carries the model number too. If the data plate is hard to read, check that the model matches on both.
Source: 16 CFR 305.17, EnergyGuide label content, checked October 20, 2026.
- Tip 03
Don't size by the tank alone
A bigger tank number doesn't always mean more hot water in your busiest hour, because the rating also depends on how fast the heater recovers. The first hour rating is the figure to compare.
Source: PNNL Building America, DHW First Hour Rating, checked October 19, 2026; ENERGY STAR Residential Water Heaters spec, Version 5.0, checked October 20, 2026.
- Tip 04
Renting? Copy it anyway
A record with the model, serial and rating helps when you report a hot water problem to the landlord, since it's the first thing a plumber asks for.
What we would do
Copy both labels today
Take one photo of the yellow label and one of the data plate. Copy the maker, model, serial number, tank size and first hour rating onto the heater record, then run the estimator with that first hour rating to see whether it covers your busiest hour. Leave any covers and panels shut; anything that needs opening is a job for a licensed plumber or the landlord.
Source: 16 CFR 305.17, EnergyGuide label content, checked October 20, 2026; PNNL Building America, DHW First Hour Rating, checked October 19, 2026.
Questions
What is the first hour rating on a water heater?
ENERGY STAR defines it as an estimate of the most hot water a tank heater can supply in an hour that starts with the tank fully heated. Federal label rules require it on the EnergyGuide label next to the tank size, and the PNNL sizing tool says to pick a rating at least as big as your busiest hour.
Source: ENERGY STAR Residential Water Heaters spec, Version 5.0, checked October 20, 2026; 16 CFR 305.17, EnergyGuide label content, checked October 20, 2026; PNNL Building America, DHW First Hour Rating, checked October 19, 2026.
Where is the model number on a water heater?
On the data plate, the maker's label on the heater, along with the serial number and other specifications. The yellow EnergyGuide label shows the model number too.
Source: City of Palo Alto, Check Your Water Heater's Age, checked October 20, 2026; 16 CFR 305.17, EnergyGuide label content, checked October 20, 2026.
What does medium mean on a water heater label?
It's the label's capacity band. For a tank heater, medium means a first hour rating of 51 to 74.9 gallons. For a tankless heater, medium means 2.8 to 3.9 gallons per minute.
Source: 16 CFR 305.17, EnergyGuide label content, checked October 20, 2026.
My tankless heater has no first hour rating. Is the label wrong?
No. Tankless labels show a gallons per minute rating band instead, because there's no tank to empty in an hour.
Source: 16 CFR 305.17, EnergyGuide label content, checked October 20, 2026.
The yellow label is gone. Where else can I find the first hour rating?
Look up the model number from the data plate on the maker's website and check the spec sheet, or ask the installer. Write where you found it on the record.
Part of a system
This page is one part of a bigger job. The system page puts the parts in order.
The tool and more guides
Chapter 01Water heater capacity estimatorAdd up your busiest hour of hot water, check it against the first hour rating on the EnergyGuide label, and print a record of your heater.Open the tool
ProblemNo size printed on my air filter: how to find itThe print on the frame is gone or the slot is empty. Measure the opening, round to the nearest inch, and know when you need a custom size.Read the guide
Practical noteWhat to change on move-in day: locks, garage opener and codesNew house, old keys. Here's the order to work in on move-in week: count what you got, change the locks, clear the garage opener, reset codes, then cut copies.Read the guide- Chapter 01All records toolsPrintable home records with a calculator behind each one: appliance and warranty log, HVAC filter sizes and change dates.Open the chapter
Sources
- 16 CFR § 305.17: Television labeling and water heater label content (EnergyGuide), Federal Trade Commission, 16 CFR 305.17 (Energy Labeling Rule), via Cornell LII. Checked October 20, 2026.
- Check Your Water Heater's Age, City of Palo Alto Utilities. Checked October 20, 2026.
- DHW First Hour Rating (RED Calc tool), Pacific Northwest National Laboratory, Building America Solution Center (U.S. Department of Energy). Checked October 19, 2026.
- ENERGY STAR Program Requirements, Product Specification for Residential Water Heaters, Final Draft Version 5.0, ENERGY STAR (U.S. Environmental Protection Agency). Checked October 20, 2026.
- DHW Instantaneous Sizing (RED Calc tool), Pacific Northwest National Laboratory, Building America Solution Center (U.S. Department of Energy). Checked October 19, 2026.
Every outside number on this page names its source and the date we checked it. See them all on the sources page. Spotted an error? Tell us.