Heat pump sizing from your own bills
Your bills already know what size heat pump your house needs.
Quotes for the same house often differ by a ton or more. Here is what a year of bills says., and the standard calculation often comes out about twice what homes really use. Rightsize fits a year of your gas, oil, propane or electric bills against real hourly weather for your town. It shows how much heat your house actually needed, extends that to the coldest hours of the year, and puts every contractor's quote on the same chart.
- Each dot is one bill. The colder the month, the more fuel was burned.
- The line runs through them and on to your town's coldest hours.
- Each coloured line is a quote. Too far above the dot, and the unit is oversized.
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The numbers behind this
Size your own house
About five minutes with a year of bills. Most utilities show past bills online. Every number stays on this page; only your town's name goes to the weather service.
Where is the house?
For the weather. A town or postcode is enough.
How is it heated now?
Efficiency matters: a furnace that's 80% efficient turns 100 units of gas into 80 units of heat. Forgetting this is the most common mistake in hand calculations.
Your bills
A full year is best: the summer bills measure hot water and cooking, so they can be taken out of the winter ones. Paste rows straight from a spreadsheet or your utility's billing history, or upload a usage CSV ("Green Button" / "Download my data").
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Your quotes (optional)
Tons are on the quote (1 ton = 12,000 Btu/h). If you can, look up the model's spec sheet or the NEEP cold-climate list for its maximum output at 5°F and 17°F, and its minimum output at 47°F. Then Rightsize can tell whether it keeps up on cold days and how often it would cycle on mild ones.
How it works
Your house as its own test
Every bill is a measurement: this much fuel kept the house warm through this much cold. Rightsize downloads hourly temperatures for your town and works out how cold each billing period really was, measured against your thermostat setting. Bills in cold months sit higher and further left. They fall close to a straight line, because heat loss grows steadily as it gets colder outside.
The coldest hours
Heat pumps are sized for a "design temperature", which only the coldest 1% of hours fall below. Rightsize calculates it from ten years of hourly weather for your location. It then extends your line to that temperature. The shaded band is the 90% range from re-fitting the line 2,000 times with different weights on each bill (a Bayesian bootstrap).
Where the method comes from
It follows the "bill method" from a 2026 study by Purdue University and Trane Technologies. The study compared it with professional Manual J calculations in 74 homes. Manual J came out about 2.3 times higher on average, and the homes' existing furnaces were about 3 times bigger than needed (Lee, Pergantis & Kircher, arXiv:2609.07619, a preprint). UK trials of measured heat loss found the same pattern.
Questions to ask your contractor
- "What heat loss did you calculate, and at what outdoor design temperature? Can I see the Manual J (or heat loss survey) inputs?"
- "What is this unit's maximum output at my design temperature, not its nominal tons?"
- "What is its minimum output at 47°F? How often will it cycle on and off in mild weather?"
- "My fuel bills suggest a load of about X. Why is your number different?"
What this can't do
- It's a cross-check, not a design. Duct, radiator and room-by-room sizing still need a proper survey, and UK grants require an MCS one.
- It assumes your current system is your only heat. A wood stove, space heaters, a vacant month or big setbacks will skew it, and so will planned insulation or an extension.
- It measures what your house used at the thermostat setting you enter. If you plan to keep it warmer, the load will be higher.
- Weather comes from Open-Meteo's reanalysis grid, which can run a couple of degrees milder than a local airport in cold snaps. Your contractor's design temperature may be slightly colder.
- The line is extended past your coldest month. Weekly or daily data (a usage CSV) gives more points and a tighter range.