What it yields where you are
How much solar produces in Roma
In Roma a well-oriented system produces 1,503 kWh a year for every kWp installed. A 6 kW system — a home-sized one — makes about 9,020 kWh a year. The figures below come from PVGIS, the European Commission's database.
The three sizes actually installed on a house
| kW | Produces each year | 450 W modules |
|---|---|---|
| 3 kW | 4,510 kWh a year | 7 modules |
| 6 kW | 9,020 kWh a year | 13 modules |
| 10 kW | 15,034 kWh a year | 22 modules |
Roma, with PVGIS figures
- 1,503kWh a year for every kWpPVGIS, 2005–2023 average
- 9,020kWh a year from a 6 kW system
- 37°the best tilt, facing south
Month by month
Month by month in Roma
A yearly total doesn't say when the energy arrives, and that's the part that decides whether you need storage.
A 6 kW system yields most in July, with 985 kWh, and least in December, with 507: 1.9 times as much. That's why the summer bill goes to zero and the winter one doesn't.
| Month | Output of 6 kW | Sunshine | Cloud | Temperature | Days above 30°C |
|---|---|---|---|---|---|
| January | 545 kWh | 216 h | 51% | 7.6 °C | — |
| February | 611 kWh | 235 h | 50% | 9.2 °C | — |
| March | 761 kWh | 288 h | 51% | 11.0 °C | — |
| April | 827 kWh | 327 h | 49% | 14.0 °C | — |
| May | 883 kWh | 365 h | 50% | 18.1 °C | 1 |
| June | 907 kWh | 398 h | 34% | 23.3 °C | 13 |
| July | 985 kWh | 428 h | 17% | 26.9 °C | 28 |
| August | 951 kWh | 395 h | 23% | 26.5 °C | 25 |
| September | 806 kWh | 327 h | 41% | 21.8 °C | 6 |
| October | 712 kWh | 272 h | 49% | 17.5 °C | — |
| November | 531 kWh | 211 h | 55% | 12.9 °C | — |
| December | 507 kWh | 213 h | 47% | 9.0 °C | — |
Heat costs yield: above 25 °C of cell temperature a panel loses about 0.4% per degree, and in summer the cell runs some twenty degrees above the air. Here there are 72 days above 30 °C a year, nearly all between June and September. That loss is already inside the figures above — don't subtract it twice — but it is why July, which gets the most sun of any month, never yields as much as the extra sun would suggest.
Output: our own calculation on PVGIS irradiation, calibrated against PVGIS city by city and month by month. Sunshine, cloud, temperature and hot days: ten-year averages 2015-2024 from the ERA5 reanalysis, via Open-Meteo. Two different sources, in separate columns on purpose.
Orientation
And if the roof doesn't face south
Everyone asks this, and the answer in Roma is: compared with a south-facing roof you lose about 19% facing east and 21% facing west, at the same tilt. Less than people expect — an awkward roof is still a good roof.
South-facing, 30°
1,495 kWh/kWp
The benchmark
East-facing, 30°
1,205 kWh/kWp
−19% compared with south
West-facing, 30°
1,188 kWh/kWp
−21% compared with south
The years
And if next year is cloudy?
Fair question: the sun isn't the same every year. Here is how much it actually varied, from 2005 to 2024.
Over twenty years the poorest was 2018: 5% below average. The most generous was 2022: 4% above. Every other year sits in between. That's why a solar estimate uses the average of many years and not the last one — a bad year doesn't change the maths of a system that lasts twenty-five.
Yearly irradiation from 2005 to 2024 according to the ERA5 reanalysis, via Open-Meteo, against its own average. This is a different database from the one behind the output figures above, which comes from PVGIS and stops at 2023 — which is why the two periods don't match.
For designers
The technical figures
The ones a designer needs. If you don't know them, you don't need them: everything is above.
Yield per installed kW
1,503 kWh/kWp
How many kWh each kW of panels gives in a year, on the best possible roof.
Best tilt
37°
The angle worth using here, facing south.
Sunlight reaching the ground
1,651 kWh/m²
Solar energy landing on one flat square metre in a year.
Other cities
On your roof
Your roof isn't an average.
These figures are for an ideal roof in Roma. The estimate on your address accounts for your roof, the shade and your consumption: two minutes, without leaving your contact details.
Free, no sign-up and no phone number.