Method and sources

The calculator answers one question: how much will a solar system produce on your roof, how much of it will you use, and how many years will it take to pay for itself? Each step relies on public data cited below; no figure is typed by hand into a page, and the rules (tariffs, VAT, caps) sit in a single file, src/data/params-2026.json, checked on 22 October 2026.

1. Your commune’s output

It comes from PVGIS 5.3, the European Commission Joint Research Centre’s tool, radiation database PVGIS-SARAH3 (2005-2023). We queried it on a grid of points 0.2 degrees apart covering mainland France, for one kilowatt-peak of crystalline silicon modules tilted at 35° due south, with 14% system losses and the terrain horizon. Each of the 34,386 communes (French administrative boundaries API) takes the value of the point nearest its centre. Result: 887 to 1,589 kWh per kWp per year, median 1,185.

For other orientations and tilts, PVGIS computed the output of seven reference cities (Lille, Brest, Paris, Strasbourg, Bordeaux, Lyon, Marseille) for 8 tilts and 8 orientations; each commune uses the factor of its zone’s reference city, interpolated between grid points. In Paris, a 30° roof facing west produces 79 % of the reference, a south-facing wall 71 %.

2. The share you use yourself

This is the step that decides the return. We simulated, hour by hour over the whole of 2023, each reference city’s PVGIS output against a typical household’s consumption, taken from the profile coefficients that Enedis, the grid operator, publishes under an open licence (residential base, electric heating, and peak / off-peak profiles, year 2025). In each hour the share used on site is the smaller of output and consumption; with a battery, the excess charges it (95 % efficiency) and it discharges when output falls short (95 %). The resulting table covers 18 output-to-consumption ratios and 8 battery sizes; the calculator interpolates between them.

Example: 3 kWp in Paris for a household using 4,500 kWh, 47 % of output used on site. This matches what the regulator observed (CRE deliberation 2025-69): 69 % of 0 to 9 kWp systems self-consume between 30 and 65% of their output.

3. The money

• Saving: kWh used on site × price per kWh. By default, the Tarif Bleu of 1 August 2026 (20.01 c€ base rate, 21.42 c€ peak rate for the off-peak profile, since output falls in daytime). You can type your own price.
• Export: surplus × 1.1 c€/kWh, indexed by 2 % a year, up to 1,600 hours × system size (order of 1 June 2026).
• Costs: standing-charge surcharge for exporting, €9.84 a year before VAT, VAT at 20 %.
• Price: the one on your quote. Otherwise, an estimate interpolated between three markers found by a private comparison site on 11 quotes from September 2025 to February 2026: €6,000 for 3 kWp, €10,000 for 6 kWp, €15,000 for 9 kWp. No public body publishes an average price for 2026.
• Payback: the first year in which cumulative gains exceed the price, over 20 years, with no rise in electricity prices unless you choose one.

4. Tests

The calculation engine is covered by automated tests (src/lib/engine/solaire.test.ts): year-1 and year-20 tariff, 1,600-hour cap, Tarif Bleu, PVGIS factors at grid points, interpolation, consistency between output, self-consumption and surplus, payback. They run on every release.

5. What the calculation leaves out

• The slow loss of module efficiency with age and inverter replacement over twenty years: both lengthen the real payback slightly.
• Shade from a tree, a chimney or a nearby building, which PVGIS ignores.
• Your own consumption curve: the Enedis profile is an average; a household that is out all day uses less of its output.
• Corsica and the overseas territories, where purchase tariffs and electricity prices differ.
• Systems above 9 kWp, whose VAT and purchase terms are not those of a house.

6. Updates

The script scripts/update-data.py reads the Tarif Bleu every week from the Energy Regulatory Commission’s open data files (Base and off-peak options) and updates params-2026.json if a price has changed. The surplus purchase tariff, VAT and installation prices are reviewed by hand whenever a new text is published; 2027 tariffs are yet to be published. An error, or a figure that does not match your bill: contact@rentasolaire.fr.

Data last checked: 22 October 2026.