Where every number on this site comes from

The labels, and why they are not decoration

sourced means the figure is supported by the page cited beside it — either a sentence quoted from it or cells transcribed from a table it publishes. On 2026-10-05 each cited quote was re-matched by script against a copy of the source page saved that day. There are 99 of these.

derived means we did arithmetic over sourced values, and the arithmetic is printed below so you can redo it. There are 7.

assumed means nobody publishes it and we chose it. The assumptions are listed further down with their values; the tools that use them say so beside the input.

The distinction matters because the failure mode in this subject is not error, it is unearned confidence. A calculator that presents a guessed regional multiplier in the same typeface as a federal regulation has told you nothing about which parts of its answer to trust.

The derived arithmetic, written out

Therms in a thousand cubic feet of gas

Residential gas is priced per thousand cubic feet and burned per therm. 1,000 cubic feet × 1,037 Btu per cubic foot ÷ 100,000 Btu per therm = 10.37 therms per thousand cubic feet. Every gas running-cost figure passes through that conversion.

Heat the water actually needs

Gallons drawn, times the weight of a gallon of water, times the temperature rise in degrees Fahrenheit, times 365. One British thermal unit raises one pound of water one degree, so no coefficient is needed beyond the weight itself. This is the only step with no appliance in it.

Fuel actually burned

Heat delivered divided by the uniform energy factor. That single division carries standby loss, combustion loss and everything else the rating captures, which is why the rating is a divisor rather than a multiplier and why a rating above 1.0 is possible for a heat pump and impossible otherwise.

Minimum efficiency by class

Federal standards state each minimum as a straight line in rated storage volume, selected by product class and usage bin. The tools evaluate that line rather than storing a lookup table. The figure table above shows one evaluation per class, labelled derived: the medium usage bin at 50 gallons for the classes that stop at 55, at 65 gallons (the smallest priced tank above 55) for the larger classes, and at the class’s own top for the tankless classes.

• gas storage, 20 to 55 gallons: medium usage bin is 0.6483 minus 0.0017 times the rated volume; at 50 gallons, 0.6483 − 0.0017 × 50 = 0.56.
• gas storage, 56 to 100 gallons: medium usage bin is 0.7897 minus 0.0004 times the rated volume; at 65 gallons, 0.7897 − 0.0004 × 65 = 0.76.
• electric storage, 20 to 55 gallons: medium usage bin is 0.9307 minus 0.0002 times the rated volume; at 50 gallons, 0.9307 − 0.0002 × 50 = 0.92.
• electric storage, 56 to 120 gallons: medium usage bin is 2.1171 minus 0.0011 times the rated volume; at 65 gallons, 2.1171 − 0.0011 × 65 = 2.05. A minimum above 1.0 cannot be met by resistance heating, so an appliance in this class must exceed what resistance heating can reach.
• gas tankless, 0 to 2 gallons: medium usage bin is 0.81 flat; at 2 gallons, 0.81 = 0.81.
• electric tankless, 0 to 2 gallons: medium usage bin is 0.91 flat; at 2 gallons, 0.91 = 0.91.

Heat pumps at or below fifty-five gallons

In that size range the federal electric storage minimum is the resistance floor and does not describe a heat pump at all. So the site multiplies that floor by 2 and by 3, using the published statement that a heat pump can be two to three times as efficient as resistance, takes the midpoint (our choice) for the headline figure, and shows the whole band beside it. Labelled derived, because it is our multiplication rather than anybody’s measurement.

Source access check: October 25, 2026

Historical commercial estimate from the retained October 5, 2026 capture; not re-verifiable by direct fetch on October 25, 2026 (HTTP 403). The publisher does not disclose the sample period. This is not a primary-source market survey or a quote for your job. The retained figures are context only; no new national cost average is asserted.

Historical U.S. residential propane price: $2.674 per gallon, excluding taxes, for the week ending March 30, 2026. Re-verified October 25, 2026 in EIA’s historical series (release dated October 21, 2026). The rolling weekly table has moved on; this is not the latest price or a local delivery quote. The historical observation remains the model input, so this maintenance changes provenance rather than calculated prices.

Installed cost anchor

Every line the scope answers turn on contributes its own published low and high. The low total is the sum of the lows, the high total the sum of the highs, and the headline figure is the midpoint of those two rounded to the nearest twenty-five dollars. There is no multiplier anywhere in that chain.

Assumptions

These are ours. No source states them. Each is listed with the value the tools start from; where a tool uses one, it is an input you can change.

• Cold-water inlet temperature: 55 °F assumed — Our starting value, not a published figure. Every tool that uses it lets you change it.
• Delivered hot-water temperature: 120 °F assumed — The setting we chose; change it to yours.
• Daily hot water: 64 gallons a day assumed — Our default for the running-cost and payback tools, not a published figure. Enter your own.
• Flow needed at once from a tankless unit: 4 gallons a minute assumed — Our default for the sizing tool, not a published figure.
• Usage bin for running cost: tank volume = first-hour rating; tankless as 40 gallons assumed — The running-cost tool treats a tank’s rated volume as its first-hour rating and a tankless unit as if rated at 40 gallons, to pick the federal usage bin. That puts a fifty-gallon tank in the low-usage bin. It is our rule, not a federal one.
• Heat pump efficiency at or below 55 gallons: midpoint of 2–3× the resistance floor assumed — Federal guidance says a heat pump can be two to three times as efficient as resistance; taking the midpoint is our choice. The result is labelled derived.
• Repair-or-replace scoring: points rule assumed — Symptom points (a tank leak decides it alone); +4 at or past the expected life and +2 past three quarters of it; −3 under warranty; +1 when maintenance was skipped; +3 when a quote is at least half of a replacement and −2 at a fifth or less. Replace at 5 or more, repair at 1 or less, otherwise get a second quote. Our rule, not a published one.
• Service life for heat pump candidates: 15 years assumed — Neither source gives a separate heat pump life, so the payback tool uses the federal storage-tank figure.
• Daily-use sensitivity: ±25% assumed — The running-cost tool shows the answer with a quarter less and a quarter more hot water; the size of that swing is our choice.
• Added yearly servicing in the payback tool: $150 assumed — Chosen for descaling a tankless unit, which no source we checked prices. It applies to whichever upgrade is chosen, so set it to what yours needs.
• Default extra work in the payback tool: first-time vent and larger gas line assumed — The payback tool starts with both selected for a gas tankless upgrade; change them to match your house.
• Propane appliances: gas efficiency class; generic tank price bands assumed — Propane tanks use the gas storage efficiency minimum and the same capacity price bands as other tanks; the cost guide does not split tanks by fuel.
• Awkward-access hours: starts at 0 assumed — The hourly premium is sourced; the hours are not. The tool starts at zero; enter the hours your installer quotes.

Known limitations, stated rather than hidden

A discrepancy inside the federal sizing worksheet

The worked example on the federal sizing page lists three showers, one shave and one hand dishwashing, then prints a total of 66 gallons. Those line items sum to 65. We add the line items, because the per-use gallons are the operative data. Anyone checking this site against that page will find the one-gallon difference, so it is recorded here rather than left to look like our error.

Efficiency ratings are measured on a laboratory schedule

A uniform energy factor is measured against one of four standard draw patterns, not against your household. Two appliances rated in different bins are not directly comparable at all. The running cost figures on this site are therefore a comparison instrument on fixed assumptions, not a prediction of your bill, and they exclude the standing charge you pay regardless of usage.

Installed-cost bands are commercial, historical and from one guide

The physics, the efficiency standards, the draw-pattern bins and every energy price here come from federal primary sources. The installed-cost bands do not. All of them come from one commercial cost guide (HomeGuide), cited through the Internet Archive copy of August 8, 2025, which does not publish its sample. A second publisher (HomeAdvisor) supplies only the national average and range quoted on the home page, checked on its live page on 2026-10-05. The bands are historical estimates and weaker evidence than the rest of this page, and should be read that way.

There is no regional cost adjustment, on purpose

Labor plainly costs more in some metros than others. No source we could verify publishes a defensible state-by-state installed-cost index for this trade. Rather than invent a multiplier that would look authoritative and be fabricated, the installed-cost tool prices awkward access by the hour from a published hourly premium and lets you set the hours. Energy prices, by contrast, are state-specific and are treated that way.

Service life sources disagree

Federal guidance gives a tankless unit more than 20 years and a storage tank 10 to 15. A commercial guide starts its ranges at 15 and 8. Both are shown wherever service life affects an answer, and the payback verdict uses the longer figure, which gives an upgrade the most time to repay its premium; the shorter figure appears in the sensitivity table.

The energy prices are historical

Electricity: EIA Electric Power Monthly Table 5.6.A, May 2026, from the archived July 2026 issue, marked preliminary by EIA. Natural gas: EIA annual residential price, 2025, from its release of September 30, 2026 (retrieved 2026-10-05). Propane: EIA weekly residential series, U.S., week ending March 30, 2026, excluding taxes. Different fuels therefore carry different periods; none of them is today’s rate. Every figure also carries a refresh date six months past the day it was checked, listed in the table above.

Update log

• 2026-10-05. Sources re-checked. HomeGuide figures, the USGS water-weight figure and the state electricity table are now labelled historical and linked to the dated copy that shows them. The HomeAdvisor national average was re-sourced from its live page ($1,334 to $1,348), and the state natural gas table moved to EIA’s release of September 30, 2026 (Louisiana and Mississippi up one cent, Oklahoma added). The repair tool now tests a quote against half of a replacement without rounding first. Table rows are shown as transcribed cells rather than quotations. Several statements that went beyond their sources were narrowed or removed, the payback page’s description of the service-life choice was corrected, the efficiency-class figures were relabelled derived, and the assumptions list above was completed.