Methodology and data sources

Every specification page on HotWaterAtlas is generated from a structured dataset with a documented origin. This page explains where each number comes from, how the data is cleaned, the exact formulas used in calculations, and — just as important — what the data cannot tell you.

Data sources

EPA ENERGY STAR certified water heater datasets

All 1084 water heaters in the database come from two public EPA datasets, retrieved through the official data.energystar.gov API:

• ENERGY STAR Certified Heat Pump Water Heaters — heat pump (hybrid electric) models: storage volume, first-hour rating, UEF, annual electricity use, voltage and amperage, compressor operating range, refrigerant, and dimensions.
• ENERGY STAR Certified Water Heaters — gas storage and gas tankless models: flow rates, UEF, annual gas use (therms and propane gallons), input rate, vent type and sizes, and dimensions.

Every water heater page links to its underlying EPA record in a Sources section. Coverage follows from the source: HotWaterAtlas lists only water heaters that hold a current ENERGY STAR certification and are sold in the United States. Models that were never certified, or whose certification lapsed, are not in the database — absence here is not a judgment on quality. Solar water heaters are out of scope.

Update frequency

The dataset is re-fetched from the EPA API on a recurring schedule and whenever ENERGY STAR announces certification changes. Each entity stores the date its record was retrieved; the sitemap's lastmod and this page reflect the newest retrieval in the current build. Current data snapshot: 2026-07-17 (1084 water heaters across 61 brands).

Data cleaning

• Records not marketed in the United States are excluded, and duplicate certifications of the same brand + model number keep only the most recently certified record.
• EPA certification strings embed wildcard characters (*, #) for model-number variants; these are normalized for display without changing page URLs.
• Where the EPA record carries a generic model name (e.g. literally “Gas Storage”), the model number — what retailers actually list — is used for display instead. Product lines that span several sizes under one model name are disambiguated by nominal tank size, rated flow, or model number.
• A small set of manual overrides fixes display names (part numbers → product names) and corrects attributes against manufacturer spec sheets. Overrides are reviewed, not automated.

Formulas

These are the exact formulas behind every computed number on the site:

• Estimated annual running cost: certified annual energy use × national average residential rate — $0.17/kWh for electricity, $1.45/therm for natural gas, $2.7/gal for propane (EIA averages). Certified annual use assumes the DOE test draw pattern; your usage and local rates change the dollar figure, but the ranking between models holds.
• Tankless flow at a temperature rise: input BTU/h × UEF ÷ (rise °F × 500), capped at the rated maximum flow. The constant 500 ≈ 8.34 lb/gal × 60 min; UEF stands in for steady-state efficiency. ENERGY STAR's rated GPM is measured at the DOE test's 77 °F rise, which is why certified figures are lower than marketing “max GPM” claims.
• Household fit from first-hour rating: DOE peak-hour sizing bands — under 49 gallons suits 1–2 people, 49–61 gallons 2–3, 62–74 gallons 3–4, and 75+ gallons 5 or more.
• Minimum breaker size (heat pump models): max amps × 1.25, rounded up to the next 5 A step — the U.S. National Electrical Code continuous-load rule. Always confirm against the installation manual.
• Nominal tank size: rated storage volume bucketed to how models are marketed (e.g. 46–55 gallons rated → “50-gallon” class).

Limitations

• Test-procedure numbers, not your home. Annual energy use and first-hour ratings come from the standardized DOE test; real households differ in draw pattern, inlet temperature, and setpoint.
• ENERGY STAR-only coverage. Standard-efficiency electric resistance tanks and non-certified models do not appear.
• No prices. The EPA datasets carry no pricing, so rankings never use price — see the editorial policy for how rankings are computed.
• No installation-cost modeling. Venting changes, condensate drains, gas line sizing, and electrical work vary by home and dominate the real project cost; the guides explain what to check.

Corrections

If any specification disagrees with the manufacturer's documentation or the EPA record, contact us with a link to the authoritative source. Verified corrections ship in the next build; systematic ones become ETL overrides so they survive dataset refreshes.