Methodology & Data Sources
Last updated Oct 18, 2026
This page lists every data source, constant and formula we use, and which numbers are our own estimates. Prices come straight from government data files; calculators combine those prices with normal weather and a simple model of your home.
Data sources
| Data | Source | Frequency | Latest period |
|---|---|---|---|
| Residential propane price ($/gal) | EIA weekly propane prices | Weekly, October–March | Oct 12, 2026 |
| Residential heating oil price ($/gal) | EIA weekly heating oil prices | Weekly, October–March | Oct 12, 2026 |
| Residential natural gas price ($/Mcf) | EIA Open Data | Monthly | Jul 2026 |
| Residential electricity price (¢/kWh) | EIA Open Data | Monthly | Jul 2026 |
| Heating degree days, normals | NOAA Climate Prediction Center, population-weighted by state | Fixed, 1991–2020 | 1991-2020 |
We never call these sources from your browser. A script downloads the data, checks it, and the site is rebuilt with the new files. If a download fails or a check fails, the previous data stays live.
How we process prices
- Natural gas: $/Mcf is converted to $/therm by dividing by 10.36 therms per Mcf.
- Gas and electricity: calculators use the average of the latest 12 months, because a single month can be unusually high or low for the season. The page also shows the latest month.
- Heating oil and propane: the latest weekly state price. If a state has no price (EIA does not survey every state), we use the regional average, then the U.S. average, and the result says so.
- EIA prices exclude taxes. We show the EIA value to three decimals in tables and round to cents in text.
- A state with no price for four weeks in a row loses its price page.
Constants
| Constant | Value | Source / status |
|---|---|---|
| Heating oil energy | 138,500 BTU/gal | EIA |
| Propane energy | 91,452 BTU/gal | EIA |
| Natural gas energy | 1,036 BTU/cf; therm = 100,000 BTU | EIA |
| Electricity | 3,412 BTU/kWh | EIA |
| Propane weight | 4.24 lb/gal | Common industry value for about 60°F; not independently sourced yet |
| Propane tank fill | 80% | Standard convention to leave room for expansion |
| Furnace efficiency (AFUE) defaults | Oil 0.83, propane 0.8, gas 0.8 | Estimates of typical equipment; you can change them |
| Heat loss factor k (BTU per sq ft per degree day) | Efficient 3.5, average 5, drafty 7 | Our estimate, checked against EIA's winter outlook (see Calibration) |
Formulas
- Heat needed (BTU) = square feet × k × heating degree days. If you enter last winter's usage, heat needed = usage × BTU per unit × efficiency instead.
- Fuel used = heat needed ÷ (BTU per unit × efficiency). Cost = fuel used × price.
- Cost per million BTU = price ÷ (BTU per unit × efficiency) × 1,000,000.
- Heat pump: each state's heating degree days are split into 5°F temperature bands. For each band, electricity = heat ÷ (3,412 × COP at that temperature). Below the unit's cutoff (0°F standard, -15°F cold-climate) electric resistance supplies the heat. Seasonal COP = heat delivered by the heat pump ÷ (3,412 × its electricity).
- Hybrid switch temperature: the outdoor temperature where the heat pump's cost per BTU equals the furnace's. Below it the furnace is cheaper.
- Heat pump size: heat loss rate = square feet × k ÷ 24 (BTU/h per °F). Design load = loss rate × (70°F − design temperature). Tons = load ÷ 12,000, rounded up to the next half ton.
- Break-even electricity price vs. propane = propane price ÷ (91,452 × AFUE) × 3,412; for a heat pump, multiply by its seasonal COP.
- Propane in a tank = water capacity × 0.80; pounds = gallons × 4.24.
Assumptions and limits
- Weather is a 30-year normal. A colder or milder winter will move your bill up or down.
- Daily average temperatures. We bin days by their average temperature, not hourly readings. That misses cold nights, so heat pump efficiency can be slightly optimistic, and our design temperatures run warmer than the 99% hourly design temperatures contractors use. The size calculator lets you override the design temperature.
- Heat pump efficiency curves are our estimates. They are illustrative values of the kind published for standard and cold-climate units, not a specific product. We plan to replace them with the median of listed models. Treat heat pump results as approximate.
- The heat loss factor is a single number per insulation level. It does not know about your windows, air leaks or ducts.
- Prices exclude taxes and fees, and a gallon of fuel costs what your dealer charges, not the EIA average.
- Alaska and Hawaii are not in the calculators (no weather normals yet). The District of Columbia uses Maryland's weather.
- The heat pump results ignore installation cost unless you enter one, and ignore rebates and financing.
Update schedule
- Heating oil and propane: after EIA's weekly release (Wednesday, later if a holiday delays it), October through March. EIA collects these weekly prices only during the heating season, so from April to September the site shows the last survey and says so.
- Natural gas and electricity: checked several times a month; EIA updates them monthly.
- Normal weather: fixed 1991–2020 values.
Calibration
We checked the heat loss factor against EIA's winter outlook, which puts the average heating oil household's winter spending near $2,115 at about $5.26 per gallon, roughly 402 gallons. For a 2,000 sq ft home with average insulation and an 83% efficient furnace, and the average winter degree days of Massachusetts, Connecticut, New York, Maine and Pennsylvania (5,352), our model uses about 466 gallons, within 25% of that benchmark. Household sizes differ, so this is a sanity check, not proof that your bill will match.
Changelog
- Oct 18, 2026: Initial release: propane and heating oil price pages, six calculators and tank pages.