Method and sources

The calculator chains six steps: your département’s climate, the heat your home needs, what that heat costs today and with a heat pump, the installation price, the grants, and what is left to pay. Each parameter is written once, in src/data/params-2026.json, with its source; the pages quote the figures the engine works out and change with them. Only three parameters do not come from an official text or a public measurement: the insulation coefficient G, the ceiling height and the heat pump price without a quote. They are marked “estimate” below, with their method and origin, and each can be replaced by your own figure in the “Refine” panel (real consumption, quote).

1. Climate: Météo-France degree days

Heating degree days (DJU) add up, day after day, the gap between 18 °C and the mean outdoor temperature ((minimum + maximum) / 2) when it is lower. We computed them for 1,361 mainland stations from Météo-France’s basic daily climate data (Licence Ouverte 2.0), averaged over 2015-2024 for stations with at least eight complete years, and for 2025 alone. Each département uses by default the station nearest its prefecture; you can pick another station in the département. Degree days range from 933 (Bastia_Sapc) to 2,736 (Mende-Ville). Départements with no qualifying station, which borrow the nearest: Hauts-de-Seine (Longchamp). Corsica is split between its two départements by nearest prefecture.

For output, we compute at each station the mean temperature of the three coldest consecutive days of each winter from 2014-2015 to 2023-2024, then average those ten values. That is a “typical winter” cold, milder than the NF EN 12831 design temperature the installer uses: the output we show is an order of magnitude, not a sizing.

2. Heating need (estimate)

Need (kWh a year) = G × floor area × 2.5 m × DJU × 24 / 1,000. G is the home’s heat-loss coefficient per cubic metre and per degree of difference; its value depends on insulation. Both parameters are the site’s estimates, not official figures. The G coefficient comes from the 1974-1982 French thermal regulations, which no longer exist; no text in force gives a typical value per insulation level. We therefore use four orders of magnitude of our own choosing: 1.6 W/m³·K for poor insulation, 1.1 for average, 0.75 for good, 0.5 for a very efficient home. The 2.5 m ceiling height is the usual height of recent homes; the only regulatory threshold is a 2.20 m minimum (decree 2002-120 on decent housing). If you know your real consumption, enter it: the need then becomes consumption × efficiency of your current system, and these estimates are no longer used.

Output in cold spells uses the 19 °C set point of the official DPE method (3CL-DPE 2021, § 9.1.1).

3. Consumption and bill, before and after

  • Current boiler: need ÷ 86 %. That efficiency is the minimum European Regulation 813/2013 (annex II) has required of new boilers up to 70 kW since 26 September 2015; an old boiler does worse, so the real saving is often larger than shown. Electric heaters: 100% efficiency, the DPE method’s value for direct resistance heating (§ 12.4.1).
  • Heat pump: need ÷ seasonal efficiency. By default, ADEME’s field-measured values: 2.9 for an air-to-water unit, 4.1 for ground-source. These are averages: from 1.8 to 4.5 across homes for air-to-water, from 2.4 to 7.5 for ground-source. For air-to-air there is no comparable public measurement: we use the official DPE method’s default for a unit fitted since 2015, 3 in zones H1 and H2 and 3.3 in zone H3 (§ 12.4.2), a conventional value, not a measurement.
  • Prices: electricity €0.1985 per kWh incl. tax (regulated Base tariff 9 kVA, CRE, from 1 August 2026); gas €0.1347 (Eurostat, households, 2026-S1); heating oil €1,931 per 1,000 litres (Weekly Oil Bulletin, week of 19 October 2026), converted at 10 kWh per litre (net calorific value), the ratio of ADEME’s two Base Carbone factors for heating oil in mainland France: 3.25 kgCO2e per litre ÷ 0.325 kgCO2e per kWh. Standing charges are not counted.

4. Installation price (estimate)

No public body publishes heat pump installation prices. The price shown without a quote is therefore an estimate, never an official price: we start from the median of real quotes published by the ScoreSolaire observatory (data.gouv.fr, CC BY 4.0 licence, updated 7 August 2026): €15,788 incl. VAT for an air-to-water unit (only 22 quotes, so indicative), €7,618 for air-to-air (148 quotes). These ranges come from a set of real, anonymised quotes published under an open licence; they are not public statistics. No open source publishes ground-source prices: as an example, the calculation starts from the MaPrimeRénov’ expense ceiling (€18,000), which is not a price; enter your quote.

5. Grants

Income category. Reference tax income compared with the Anah’s 2026 ceilings, Île-de-France or elsewhere depending on the département, ceiling included.

Per-item MaPrimeRénov’. Flat rate from the 2026 scale (Service-Public page F35083, checked 1 September 2026, re-read by us on 22 October 2026 with the France Rénov’ scale: flat rates, expense ceilings and income ceilings unchanged), provided the home is over 2 years old and a main residence. Air-to-air: no grant. Pink: no grant. Since 17 August 2026, applications are made on the France Rénov’ website, the Anah’s single entry point.

CEE bonus. kWh cumac = sheet amount (BAR-TH-171 for air-to-water, BAR-TH-172 for ground-source, version A82.5) × area factor × zone factor × 5 if the Coup de pouce applies (main residence, coal, oil or gas boiler replaced, approved model since 1 September 2026). Value in euros = kWh cumac ÷ 1,000 × Emmy average monthly price (€8.82/MWh, or €14.64 for fuel-poverty certificates from blue and yellow households). The registry states this price is not a benchmark: we show it as a market value. Example: 100 m² house in zone H1, Coup de pouce, 545,400 kWh cumac, i.e. €4,810.

Cap. Decree 2020-26, article 3: MaPrimeRénov’ ≤ (1 − minimum out-of-pocket share) × min(price incl. VAT, expense ceiling) − CEE bonus, with a minimum share of 10 %, 25 %, 40 % and 60 % by colour.

Payback. Out-of-pocket cost ÷ yearly saving, at constant energy prices, with no servicing or replacement.

Tests

Each update runs the engine’s tests (src/lib/engine/pac.test.ts): flat rates and kWh cumac copied from the texts, income ceilings at their bounds (an income equal to the ceiling stays in the category), area factors at the 35, 60, 70 and 90 m² thresholds, a cap that never goes negative, 96 départements with plausible degree days and zones, and consistency of the full simulation. A failing test blocks publication.

What the site does not do

  • It does not calculate the CEE bonus for an air-to-air unit (sheet BAR-TH-129) or a ground-source unit in a flat: we have not reread the text in force.
  • It leaves out domestic hot water, yearly servicing, the gas standing charge you drop and any change of electricity meter rating.
  • It does not cover the overseas départements, whose scales differ.
  • It does not replace the installer’s heat-loss study or the sizing note required for the CEE bonus.

Updates

The script scripts/update-data.mjs rereads the CEE price (Emmy registry, monthly), the regulated electricity tariff (CRE), the gas price (Eurostat) and heating oil (Weekly Oil Bulletin), then rewrites the parameters and the date; a scheduled job runs it, replays the tests and rebuilds the site. Scales: whenever the texts change. Climate: every year, with the year just ended. An error reported to contact@prixpac.fr is checked against the source, corrected and turned into a test. See the editorial policy.

All sources

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Publisher of a neutral calculator of the cost, grants and savings of a heat pump in France, with no quote request

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