How many solar panels does your house in France need?
The answer depends less on roof area than on your daytime usage. Here is the size that earns most, household by household, and how to turn it into a number of modules.
The right number of solar panels is not the number that fills the roof, but the one that produces roughly what the house uses during sunny hours. Now that surplus is paid only 1.1 c€/kWh in France, each extra kWh earns 18 times less than one used on site, which saves 20.01 c€ at the regulated Tarif Bleu. Comparing thirteen sizes from 1 to 9 kWp at prices estimated from recent quotes, the best 20-year net gain in Paris comes from 2 kWp for a household using 4,500 kWh a year, 3 kWp for 8,000 kWh and 6 kWp for 15,000 kWh. The number of modules follows by dividing that size by one module’s power, shown on its datasheet: 3 kWp needs 7 modules of 450 Wp. For a 3,000 kWh household, the best size in Paris is only 1.5 kWp.
How many panels for your home?
Number of panels
5
| Best-value system size | 2 kWc |
| Output per year | 2,263 kWh |
| Net over 20 years | +€1,022 |
Best 20-year net gain among 13 sizes, at estimated prices. A module’s power is on its datasheet.
Checked by Radif Partners · Editorial policy · Method and sources
The size that earns most, by usage
| Yearly usage | Lille | Paris | Marseille | 450 Wp modules (Paris) |
|---|---|---|---|---|
| 2,000 kWh | 1 kWp | 1 kWp | 1 kWp | 3 |
| 3,000 kWh | 1 kWp | 1.5 kWp | 1 kWp | 4 |
| 4,500 kWh | 2 kWp | 2 kWp | 2 kWp | 5 |
| 6,000 kWh | 2.5 kWp | 2.5 kWp | 2.5 kWp | 6 |
| 8,000 kWh | 3.5 kWp | 3 kWp | 3 kWp | 7 |
| 10,000 kWh | 4 kWp | 4 kWp | 3.5 kWp | 9 |
| 12,000 kWh | 5 kWp | 5 kWp | 5 kWp | 12 |
| 15,000 kWh | 6 kWp | 6 kWp | 6 kWp | 14 |
The pattern is simple: useful size grows more slowly than usage, because a house uses only part of its electricity during sunny hours. The south hardly needs less capacity than the north: each kWp produces more there, but so does surplus. For small households, under 3,000 kWh a year, a fitted system pays back poorly in Paris or Lille; a plug-in kit is a better fit.
From kWp to number of modules
A module is sold with a peak power in watts-peak (Wp), measured under standard conditions. To get the number of modules, divide the target power in Wp by the module’s and round up: 3 kWp is 8 modules of 375 Wp or 6 of 500 Wp. For the same total power, the number of modules does not change output: only total power counts, along with facing and sunshine.
How much will your panels produce?
Output per year
3,395 kWh
| In July | 403 kWh |
| In December | 127 kWh |
| Value at Tarif Bleu | €679 |
Median of the département’s towns (PVGIS 2005-2023), roof at 30°, 14% losses.
Four questions before settling the size
- What is your actual yearly usage? It is on your bills and customer account.
- What share do you use between 10 am and 5 pm? Home working, a pool, a timed hot-water tank and daytime charging raise it (self-consumption).
- Does your roof face south or east-west? East-west spreads output over the day (facing and tilt).
- Will you sell surplus? Without sales, aim a little smaller.
The 3 kWp and 6 kWp pages cover two common sizes, with the usage threshold at which the larger one wins.