Methodology

This page explains how the WaterHeaterCalcs calculators and the reference datasets are derived and verified — and why they need no ongoing maintenance to stay correct. It is the method behind the size-by-household reference and its hot-water-use-by-fixture companion, our signature data assets.

1. Timeless thermodynamics, stable conventions

Every tool computes from a closed-form identity: it takes 8.33 BTU to raise a gallon of water 1 °F, so heat energy = gallons × 8.33 × ΔT; recovery rate = input BTU/hr × efficiency ÷ (8.33 × ΔT); First-Hour Rating = usable storage (≈ 0.70 × gallons) + recovery; reheat time = gallons × 8.33 × ΔT ÷ (input × efficiency); annual energy = daily gallons × 365 × 8.33 × ΔT ÷ 3412 ÷ UEF (kWh) or ÷ 100000 ÷ UEF (therms); and cost = (unit/parts + labor + add-ons − discount) ×(1 + contingency). The only baked-in numbers are stable physical identities and labeled published typicals. These do not drift, so the statements stay true over time.

2. Sizing as first-hour-rating math; energy as thermodynamics; cost as a quantity × unit-price sum

Water-heater sizing is first-hour-rating and flow/temperature math — the peak-hour demand is the sum of the hot-water uses in your busiest hour, and you pick a tank whose First-Hour Rating meets it, or a tankless whose GPM meets your simultaneous flow at your temperature rise. Energy is closed-form thermodynamics. Cost is a quantity × unit-price sum — the parts or unit plus labor and add-ons, with a contingency. Round sizing up and leave headroom so you don’t run out of hot water at peak.

3. The signature size & hot-water-use references

The size-by-household reference tabulates the LABELED tank gallons, First-Hour Rating and tankless GPM by household, and the companion gives the LABELED DOE hot-water use per fixture (gallons per use + GPM). Both are dated snapshots (currently 2026-07-13), not live feeds: they hold only stable identities and clearly labeled published planning ranges, so they never need maintenance. Assumptions and limits are stated on the page.

4. Where the conventions come from

The physical constants (8.33 lb/gallon, 3412 BTU/kWh, 100000 BTU/therm, 91500 BTU/gallon propane) are definitions. The DOE hot-water use per fixture, the gallons + FHR by household, the typical UEF and lifespan by type, the inlet water temperature by region, the cost bands and the contingency (~10%) are LABELED published planning typicals, cited in Sources and user-adjustable.

5. No prices, no rates, no feeds

There is deliberately no live utility rate, no equipment or labor price, no regional cost index, no product catalog, no plumber directory and no live rate. Every cost tool works on the prices you enter from your own quotes and bills (unit $, parts $, labor $, add-on $) and every energy tool on your own $/kWh and $/therm. Labeled cost bands are shown only as a sanity guide. That is why the site is correct regardless of what equipment, labor or utility prices do.

6. Numeric self-check

Every formula is asserted against a worked example with known numbers (for instance: a busy morning of three showers, a shave, two hand washes and food prep peaks at 75 gallons, so a 50-gallon tank at a First-Hour Rating near 75 fits; a 50-gallon electric tank (4.5 kW, ΔT 70) recovers about 26 gph and reheats in about 1.94 hours; the same delivered heat costs about $694/yr on electric resistance at $0.16/kWh vs about $264/yr on gas at $1.20/therm; a heat-pump model cuts the electric figure by about $512/yr; a like-for-like tank replacement is about $1,650, a heating-element repair about $220). A release gate runs all of these and fails on any mismatch, so “verification” here is mathematical correctness plus accurate conventions — not a time-based check.

7. Estimate or sizing guide, not a design

The contingency %, UEF, inlet temperature, daily hot-water use, recovery efficiency, fixture flow, labor multipliers and cost bands are labeled planning typicals — a starting point, not a spec. Every result is a planning estimate or a sizing / energy guide: get itemized written quotes from licensed, insured plumbers, confirm your unit’s rated First-Hour Rating, GPM and UEF on its EnergyGuide label, and follow the manufacturer’s instructions and local code. Gas, venting, combustion, the temperature-&-pressure relief valve, the scald / Legionella tradeoff and code certification are set by a licensed professional, the manufacturer’s instructions and local code; they are out of scope. Nothing here is an installation procedure, a combustion or venting verdict, a medical scald / Legionella verdict, or a certified design.