Here is how we turn your answers into a recommendation, which data we use, and how far you can trust the figures. Everything is calculated again for each home; nothing is copied from a table.
In short
- We find the home from the address you type. If you choose the roof, Google's aerial survey gives us its slope, the direction it faces and how much space it has.
- We take a typical year of sunshine and weather for that spot from the European Commission's solar database (PVGIS).
- For every hour of that year, we work out what panels of different power would produce there.
- We spread your electricity use over the same hours, then follow, hour by hour, how much solar electricity you would use yourself, how much a battery would store and how much would go to the grid.
- We compare several combinations and choose the smallest one that meets the goal you put first.
Sunshine and weather
PVGIS builds a typical year from real measurements: for each month it picks the most typical one from 2005–2023, using satellite data for sunlight and weather records for temperature and wind.
It also includes the shade cast by hills and mountains on the horizon. It does not include shade from nearby trees or buildings.
A real year can be sunnier or cloudier than the typical one, so a single year's production can be higher or lower than our figure.
From sunshine to electricity
We use pvlib, an open-source library used by researchers and the solar industry, to follow the sunlight onto the panels and into your home, hour by hour:
- how much sunlight reaches panels at your slope and direction, counting light from the whole sky and the light the glass reflects away when the sun is low;
- how heat lowers output: panels lose about 0.4% of their power for every degree they are warmer than 25 °C;
- everyday losses in cables, from dirt, from small differences between panels and in the inverter: 14% in total, the same default as the European Commission's own calculator;
- shade: for a roof face we found, from Google's sunlight data for each panel position; for a roof you describe or panels on the ground with some shade, an assumed 10% over the year;
- the inverter's limit: in the sunniest hours it can't pass on more than its rated power.
Your electricity use
If you give a yearly or monthly figure, we keep your monthly totals and spread each day's use over the hours in a typical pattern: a small morning peak and a larger evening peak.
Whether someone is at home during the day matters a lot for a battery, so we calculate three patterns: at home during the day, mixed, and out during the day. The headline uses the one you chose; the report shows the other two as a range.
The battery
The battery charges only from solar electricity your home isn't using, and gives it back when the panels make less than you use. It doesn't charge from the grid, and we don't model special tariffs.
- the usable part of a battery is about 90% of the capacity it is sold with;
- it can charge or supply about half its usable capacity each hour: 2.5 kW for small batteries, up to 5 kW for large ones;
- it keeps about 95% of the energy each way, going in and coming out;
- it never runs below 10% of its usable capacity.
How we choose the sizes
- Panels: we compare several panel powers around the one that would cover your yearly use (67%, 100%, 133% and 167% of it), plus the lowest panel power that produces at least 90% of what you use in a year.
- Space: we count 5 m² of panels for each kWp, and never suggest more than fits or more than the home needs. Below 1 kWp there is no home system to plan, so we say so instead of giving a result.
- Size limit: we never suggest more than 20 kWp; larger systems are outside the scope of a home check.
- Two roof faces: panels fill the face you chose first, the second face takes only what doesn't fit, and both share one inverter.
- Battery: we add 5 kWh at a time, up to 15 kWh, and stop when the next step adds less than 15% more battery energy over a year, or when each extra kWh the battery can hold would give you less than 40 kWh a year.
- Inverter: the smallest common inverter (from 3.6 kW) that isn't too small for the panels: the panels' power may be at most 1.3 times the inverter's.
How accurate it is
We check our calculation against the European Commission's own solar calculator (PVGIS) on a test system with the same losses: our yearly figure is about 3.8% higher.
On a real 5 kWp home, our estimate for 2025 was about 12% above what the panels actually produced (about 5% from April to September). Part of the gap comes from the panels' age, snow and winter shade, which we don't model. We explain such gaps; we don't adjust the model to hide them.
So treat the figures as careful estimates for planning, not a promise. Shade, dirt, snow, the exact equipment and how you really use electricity can all change the result.
What we don't take into account yet
- how shade changes through the day and the year: we take off one yearly shading loss, spread evenly;
- more than two roof faces, or panels on the roof and on the ground together;
- some battery types that connect differently (DC-coupled);
- prices, savings, payback or tariffs;
- whether your grid connection allows the system: the installer and the network operator decide that;
- specific brands or products.
Data sources and credits
- Sunshine and weather: PVGIS, European Commission Joint Research Centre, © European Union. Used with acknowledgement of the source.
- Solar calculations: pvlib python, open-source software (BSD licence).
- Address search, location and roof shape: Google Maps. We use Google only for the roof's geometry; the energy figures are always our own calculation.
- Installer qualifications in France: ADEME's list of RGE companies (data.ademe.fr), Licence Ouverte 2.0.
- Calculation version 0.1.0.