What Size Water Heater Do I Need? First-Hour Ratings vs Household Size

What size water heater do I need? Twenty gallons a person, plus five, in the busiest hour

DOE sizes tank water heaters by first-hour rating (FHR), not tank gallons. The DOE Energy Saver worksheet counts 20 gallons of hot water per shower, 2 per shave and 3 per hand dishwashing; a household where everyone showers in the same hour needs 20 gallons a person plus 5.

First-hour rating (FHR): Federal test language in appendix E: an estimate of the maximum volume of hot water a storage water heater can supply within an hour that begins with the tank fully heated. The test draws water at 3.0 gallons per minute, with 58 F supply water and 125 F outlet water.

DOE's Energy Saver page tells buyers to pick a model whose FHR at least matches peak hour demand, the most hot water the home uses in one hour. Its worksheet values are in the table below. The page's worked example (three showers, one shave, one hand dishwashing) is printed as 66 gallons; the rows add to 65 gallons, a one-gallon slip that does not change the method (DOE Energy Saver, archived 2026-07-02).

Applying the worksheet to every member of a household gives peak hours of 25 gallons for one person, 45 for two, 65 for three, 85 for four and 105 for five (KBR derived metric d1). A top-loading washer in the same hour adds 25 gallons; an H-axis washer adds 15.

The FHR also sets a tank's federal draw pattern: below 18 gallons is very small, 18 to under 51 low, 51 to under 75 medium and 75 or more high (appendix E, Table I). A label that says High-Usage therefore promises at least 75 gallons in the first hour.

Certified 50-gallon-class heat pump tanks deliver a median 67 gallons in the first hour

Across 615 US heat pump listings on the ENERGY STAR certified list, the median FHR climbs from 60 gallons in the 40-gallon class to 67 in the 50-gallon class, 79 at 65 gallons and 87 at 80 gallons for 240 V integrated units.

KBR grouped the 1197 US-market listings on the ENERGY STAR list (rows updated 2026-11-01) by rated storage volume. DOE notes that a nominal 40-gallon tank is often rated at 35 to 36 gallons, because marketed sizes may sit within 10 percent of actual storage (2024 final rule), so the classes start below the round number.

Within a class the spread is wide. 240 V units in the 50-gallon class run from 52 to 81 gallons across 135 listings and 37 distinct ratings. Split-system heat pump listings post the highest medians: 70 gallons at 50 gallons of storage and 95 at 80 gallons.

The ENERGY STAR gas storage entries (70 listings) are only 4 distinct power-vent designs from 2 manufacturers, rated 88 and 95 gallons at the 40- and 50-gallon class medians. They show what high-input gas tanks can do, not what a typical atmospheric gas tank delivers. Standard electric resistance tanks are not ENERGY STAR certified and are absent from this list.

Each extra gallon of heat pump storage adds less first-hour capacity

The median heat pump tank delivers 1.67 gallons in the first hour per gallon stored in the 40-gallon class but 1.19 in the 80-gallon class and 0.92 at 120 gallons (KBR derived metric d3).

Because the rating counts water heated during the test hour, a first hour can exceed the stored volume; that extra counts for more in small tanks than in large ones. The practical effect: moving from the 50-gallon to the 80-gallon class raises the 240 V median from 67 to 87 gallons, 20 more gallons of first-hour water for 28 more gallons of rated storage (median rated volumes of 46 and 74 gallons).

Power supply matters as much as size. The 120 V plug-in heat pump tanks in the 50-gallon class have a median FHR of 52 gallons, 15 gallons below 240 V units of the same class (derived metric d5). That is 2.6 twenty-gallon showers against 3.4 (d4). For operating cost by state, see the heat pump water heater report.

78.4% of US households fit inside that 67-gallon rating

Weighting 2023 American Housing Survey households by size, the median 240 V 50-gallon-class heat pump tank covers the worksheet rush of 78.4% of US households: every home with three or fewer people. Covering four people takes 85 gallons, the 80-gallon-class median of 87 reaches 90.9%.

This is KBR's own calculation (derived metric d2). It joins three public sources: ENERGY STAR median FHRs, the DOE worksheet's 20-gallon shower and the AHS 2023 count of people in each of 133.23 million occupied homes. Standard errors come from the survey's 160 replicate weights.

Households of four or more make up 21.6% of US homes; the 9.1% with five or more need more than 87 gallons in the worksheet scenario (d13), which only the 120-gallon class median of 110 gallons covers. A 120 V 80-gallon-class unit (84 gallons) falls 1 gallon short of a four-person rush.

Two full bathrooms is the most common layout, and bigger households have more of them

32.0% of occupied US homes have two full bathrooms and 30.2% have one, per AHS 2023. Among four-person households, 15.8% still share one full bathroom.

The DOE method has no bathroom input; it counts uses in the busiest hour. Bathrooms matter because they decide how many showers can overlap. A home with one full bathroom spreads four showers across the hour, while three bathrooms let them run at once, which raises the flow a tankless unit must supply. For remodel budgets, see bathroom remodel cost and walk-in shower cost.

Half of homes with four or more people ran on a tank under 50 gallons in 2020

In RECS 2020, 50.6% of households with four or more people and their own water heater reported a main tank under 50 gallons: 12.2 million of 24.13 million homes (derived metric d6).

Tank size barely follows household size in the installed stock: 26.1% of one-person homes had a tank of 50 gallons or more, against 41.6% of four-person homes and 42.3% of homes with five or more. Tankless units were the main heater in 6.4% of homes with their own water heater.

RECS 2020 v7 is the latest release with water heater size; the preliminary RECS 2024 v1 file (May 2026) does not yet include water heating fields. Age of these units is covered in the water heater lifespan report.

Certified gas tankless units peak at a median 5.4 gallons a minute, about two showers

The 392 ENERGY STAR gas tankless listings sold in the US have a median maximum flow of 5.4 gpm at the test's 67 F rise, enough for 2.16 showers at the federal 2.5 gpm limit (d7).

Tankless units are sized by flow, not storage. DOE's Energy Saver example adds a 0.75 gpm faucet and a 2.5 gpm shower for 3.25 gpm, and assumes 50 F incoming water heated to 120 F, a 70 F rise. At that rise the median certified unit's flow works out to 5.17 gpm, or 2.07 showers (d8). Two showers plus a faucet need 5.75 gpm; 33.9% of listings reach it at the test rise (d9).

Ratings range from 3.8 to 6.0 gpm, and 66.3% of listings are at 5 gpm or more. The median listed input is 180,000 Btu per hour, near the 200,000 Btu per hour ceiling for a consumer gas instantaneous heater. Electric tankless units are not ENERGY STAR certified; DOE's page puts them at about 2 gpm for a 70 F rise.

Electric tanks over 55 gallons already need heat pump efficiency; 20 to 55 gallons follow in 2029

Under 10 CFR 430.32(d), electric storage tanks over 55 gallons have needed heat pump-level efficiency since April 16, 2015. The 2024 final rule sets 2.30 UEF for most 20 to 55 gallon electric tanks and 2.50 above 55 gallons from May 6, 2029.

The current formula for a 65-gallon medium-draw electric tank works out to a minimum UEF of 2.0456, and 2.1538 for an 80-gallon high-draw tank (d10). A 50-gallon resistance tank needs 0.9207; the 2029 level of 2.30 is 2.5 times that. DOE estimated heat pumps were about 3 percent of the electric storage market in 2023 and projected 61 percent in 2030 under the adopted standard.

For sizing, the rule keeps one resistance option: small electric storage tanks of 20 to 35 gallons effective volume in the very small and low draw patterns, which DOE describes as lowboy units with FHRs under 51 gallons. In the worksheet scenario 51 gallons covers two people. Grid-enabled resistance tanks over 75 gallons with an activation lock remain a separate class.

Status as of 2026-11-02: the eCFR text current to 2026-10-29 still carries the 2029 levels. The separate gas tankless standard was withdrawn on 2025-05-20 under the Congressional Review Act, and a gas industry petition noticed on 2026-04-27 concerns commercial water heating equipment and consumer furnaces, not consumer water heaters.

Ratios behind the comparison

Download the calculations (CSV)

Where the numbers diverge

1. The median FHR of ENERGY STAR certified 240 V heat pump tanks sold in the US is 67 gallons in the 50-gallon class (135 listings, November 2026).^[1]
2. The same median is 60 gallons in the 40-gallon class, 79 in the 65-gallon class and 87 in the 80-gallon class (US, November 2026).^[1]
3. 120 V plug-in heat pump tanks in the 50-gallon class have a median FHR of 52 gallons, 15 below 240 V units (US, November 2026).^[1]
4. Split-system heat pump tanks post a median FHR of 70 gallons at the 50-gallon class and 95 at the 80-gallon class (US, November 2026).^[1]
5. First-hour gallons per rated gallon fall from 1.67 for 40-gallon-class heat pump tanks to 1.19 for the 80-gallon class (KBR, November 2026).^[1]
6. DOE's Energy Saver worksheet assumes 20 gallons of hot water per shower, 2 per shave and 3 per hand dishwashing (2026 capture).^[4]
7. 78.4% of US households in 2023 had three or fewer people, so their worksheet peak hour of up to 65 gallons fits a 67-gallon FHR (KBR, AHS 2023).^[11][4][1]
8. 21.6% of US households in 2023 had four or more people and a worksheet peak hour of 85 gallons or more (AHS 2023).^[11]
9. 32.0% of US households had two full bathrooms in 2023 and 30.2% had one (AHS 2023).^[11]
10. 50.6% of US households with four or more people and their own water heater reported a main tank under 50 gallons in 2020 (RECS 2020).^[14]
11. The median ENERGY STAR gas tankless unit sold in the US is rated 5.4 gpm at a 67 F rise; 66.3% of 392 listings reach 5 gpm (November 2026).^[1][6]
12. Federal rules have required heat pump-level efficiency for electric storage tanks over 55 gallons since April 16, 2015 (10 CFR 430.32(d)).^[5][7]
13. From May 6, 2029, most electric storage tanks of 20 to 55 gallons must reach 2.30 UEF and those over 55 gallons 2.50 UEF (10 CFR 430.32(d)(2)).^[5][7]
14. DOE estimated heat pumps held about 3 percent of the US electric storage water heater market in 2023 (89 FR 37778).^[7]
15. In 2023, 46.5% of US occupied homes heated water with electricity and 46.8% with piped gas (AHS 2023).^[11]

Summary

• DOE's worksheet counts 20 gallons of hot water per shower; a household where everyone showers in one hour needs a first-hour rating of 20 gallons a person plus 5.
• The median certified 240 V heat pump tank delivers 60 gallons in the 40-gallon class, 67 in the 50-gallon class, 79 at 65 gallons and 87 at 80 gallons.
• KBR finds the 67-gallon median covers 78.4% of US households (three people or fewer); the 87-gallon median of the 80-gallon class covers 90.9%.
• 120 V plug-in heat pump tanks trail 240 V units by 15 gallons in the 50-gallon class.
• 50.6% of 2020 households with four or more people had a main tank under 50 gallons.
• Certified gas tankless units have a median 5.4 gpm at a 67 F rise: two showers, not three.
• Electric tanks over 55 gallons need heat pump-level efficiency now; most 20 to 55 gallon electric tanks follow on May 6, 2029.

Frequently asked questions

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Reproduce this analysis

Every figure comes from the files below. The scripts that pulled and computed them are kept with the report and can be re-run.

Data quality and exclusions

• ENERGY STAR: removed 179 Canada-only listings, 55 solar systems and 64 residential-duty commercial gas tanks; 1 tankless listing with no max GPM was dropped from flow statistics (392 kept).
• ENERGY STAR gas storage: the 70 listings reduce to 4 distinct rated designs, so gas medians describe those designs, not the gas tank market.
• AHS: dropped 7142 vacant or usual-residence-elsewhere records; no occupied record had a missing NUMPEOPLE. Household sizes of 7 or more are grouped.
• RECS: excluded 1888 records (13.8% of households, weighted) whose water heater serves several units; 16608 records kept, no missing WHEATSIZ.
• DOE worksheet: the worked example prints 66 gallons although its rows sum to 65; the row values were used.
• DOE CCMS (regulations.doe.gov/certification-data) returned HTTP 403 on 2026-11-02 and was not used; no workaround was attempted.

Methodology

Source selection: federal datasets only for figures (ENERGY STAR certified product list, eCFR, Federal Register, Census AHS, EIA RECS, DOE Energy Saver). Three contractor and publisher pages appear only as pages to beat.

Ratings: the ENERGY STAR Certified Water Heaters dataset (pbpq-swnu) was downloaded on 2026-11-02 (rows updated 2026-11-01). KBR kept listings whose markets include the United States and grouped tanks by rated storage volume into 40, 50, 65, 80 and 120 gallon classes. Medians and ranges are of listings; brand variants of one design count separately, so distinct-rating counts are shown beside listing counts.

Households: AHS 2023 national PUF v1.1, occupied interviews (INTSTATUS = 1), weighted by WEIGHT, with standard errors from the 160 replicate weights (variance = 4/160 x sum of squared differences). RECS 2020 v7, weighted by NWEIGHT, with jackknife standard errors (59/60 factor), excluding households whose water heater serves several units.

Signature metric (d2): each household's worksheet peak hour is 20 gallons times the number of people plus 5 gallons; a tank class covers the household when its median FHR is at least that figure. The covered share is the weighted share of households meeting the test.

Conflicts: tank-size rules of thumb on contractor pages (for example about 15 gallons of tank per person) were not used; the federal FHR method was. DOE's worked example prints 66 gallons where the rows add to 65; KBR used the row values.

Not advice: this report summarizes federal data and is not plumbing, electrical or purchasing advice. Last updated 2026-11-02.

This report presents published data with its limitations. It is not legal, financial or investment advice.

Sources, ranked by quality

Tier 1: primary and official sources (18)

• [1] U.S. EPA ENERGY STAR. ENERGY STAR Certified Water Heaters (dataset pbpq-swnu), rows updated 2026-11-01. Certified models listed as of 2026-11-01. Accessed 2026-11-02.
• [2] U.S. EPA ENERGY STAR. ENERGY STAR Certified Heat Pump Water Heaters (dataset v7jr-74b4), used as a cross-check. Certified models listed as of 2026-11-01. Accessed 2026-11-02.
• [3] U.S. EPA ENERGY STAR. ENERGY STAR Certified Gas (Storage and Tankless) Water Heaters (dataset 6sbi-yuk2), used as a cross-check. Certified models listed as of 2026-11-01. Accessed 2026-11-02.
• [4] U.S. Department of Energy. Sizing a New Water Heater, Energy Saver (Internet Archive capture of the energy.gov page, 2026-07-02; live URL returned HTTP 404 on 2026-11-02). Page captured 2026-07-02. Accessed 2026-11-02.
• [5] U.S. Government Publishing Office, eCFR. 10 CFR 430.32, Energy and water conservation standards and their compliance dates (eCFR, current to 2026-10-29). Regulation text as of 2026-10-29. Accessed 2026-11-02.
• [6] U.S. Government Publishing Office, eCFR. Appendix E to Subpart B of 10 CFR Part 430, Uniform Test Method for Measuring the Energy Consumption of Water Heaters (eCFR, current to 2026-10-29). Regulation text as of 2026-10-29. Accessed 2026-11-02.
• [7] U.S. Department of Energy via Federal Register. Energy Conservation Program: Energy Conservation Standards for Consumer Water Heaters, final rule, 89 FR 37778. Published 2024-05-06. Accessed 2026-11-02.
• [8] U.S. Department of Energy via Federal Register. Energy Conservation Standards for Consumer Gas-Fired Instantaneous Water Heaters, withdrawal of disapproved final rule (2025-09030). Published 2025-05-20. Accessed 2026-11-02.
• [9] U.S. Department of Energy via Federal Register. Energy Conservation Program: Notification of Petition for Rulemaking (2026-08145). Published 2026-04-27. Accessed 2026-11-02.
• [10] U.S. Department of Energy. Consumer Water Heaters, Appliance and Equipment Standards product page. Rulemaking history 2024 to 2025. Accessed 2026-11-02.
• [11] U.S. Census Bureau and HUD. American Housing Survey 2023 National Public Use File v1.1 (household.csv: NUMPEOPLE, BATHROOMS, HOTWATER, WEIGHT, REPWEIGHT1 to 160). 2023. Accessed 2026-11-02.
• [12] U.S. Census Bureau. AHS 2023 National Public Use File page and Value Labels Package. 2023. Accessed 2026-11-02.
• [13] U.S. Census Bureau. Quick Guide to Estimating Variance Using Replicate Weights: 2009 to Current (AHS). 2009 to 2023. Accessed 2026-11-02.
• [14] U.S. Energy Information Administration. 2020 Residential Energy Consumption Survey public use microdata, version 7 (WHEATSIZ, NHSLDMEM, H2OAPT, NWEIGHT). 2020. Accessed 2026-11-02.
• [15] U.S. Energy Information Administration. RECS 2020 Codebook for Public File, v7. 2020. Accessed 2026-11-02.
• [16] U.S. Energy Information Administration. Using the 2020 RECS microdata file to compute estimates and standard errors (RSEs). 2020. Accessed 2026-11-02.
• [17] U.S. Energy Information Administration. 2024 RECS microdata page (preliminary v1 file, May 2026) and RECS 2024 Codebook v1. 2024. Accessed 2026-11-02.
• [21] U.S. Department of Energy. Compliance Certification Database (consumer water heaters), access attempt. Not retrieved: HTTP 403 on 2026-11-02. Accessed 2026-11-02.

Tier 3: journalism and expert commentary (3)

• [18] Bob Vila. What Size Water Heater Do I Need? (updated Sep 23, 2020). 2020. Accessed 2026-11-02.
• [19] Mr. Rooter Plumbing. What Size Water Heater Do I Need?. Undated. Accessed 2026-11-02.
• [20] CoolToday. What Size Water Heater Do I Need?. Undated. Accessed 2026-11-02.