PVGIS: a free production estimate for your roof
The Photovoltaic Geographical Information System is the European Commission's own irradiance model, free and public. Give it a location, a system size, a tilt and an orientation and it returns the annual kilowatt-hours that array would produce there.
It is a siting tool, not a survey. PVGIS models the sky, not your roof: it knows nothing about the chimney, the neighbour's tree or how your panels will degrade, and every figure it returns assumes an unshaded array.
What PVGIS is
PVGIS is an interactive tool built by the Joint Research Centre, the European Commission's in-house science service, based in Ispra, Italy. It draws on multi-year satellite records of solar irradiance, air temperature and other atmospheric conditions to work out how much energy a photovoltaic array will produce at a given point on the map.
What it will do
- Estimate production for a grid-connected or off-grid array across Europe, Africa, much of Asia and the Americas.
- Optimise the tilt angle for maximum annual output at that latitude.
- Compare orientations - south against an east-west split, which often suits self-consumption better than peak yield does.
- Export hourly data for a whole year, which is what you need if you want to model a battery or a tariff.
- Account for losses from temperature, reflection and cabling.
- Shading. Trees, chimneys and neighbouring buildings are invisible to it.
- The future. Results are the average of past weather, not a forecast.
- Degradation. The estimate is year-one output and does not decline.
- Local air quality. Fog and pollution over a valley are below its resolution.
Using it, step by step
1. Pick the location
Click the map where the array will go, type an address into the search field, or paste GPS coordinates. A few miles matters less than the shading does, but it is worth checking a couple of nearby points.
2. Choose the calculation
Grid-connected is the usual one: it returns annual and monthly production for an array feeding the grid. Off-grid models an autonomous system with storage instead.
3. Enter the system
Installed peak power is the sum of the panel nameplates - ten 500 Wp modules is 5 kWp. Slope is the angle from horizontal; tick "Optimize slope" and PVGIS will find it for you. Azimuth is the direction: 0 is south, -90 east, 90 west. Leave system loss at 14% unless you have a reason, and set the mounting position honestly - a roof-integrated array runs hotter than a free-standing one, and heat costs output.
4. Read the results
"Visualize results" gives a monthly bar chart and a summary block. The chart is where the seasonality lives; the summary is where the number you quote comes from.
Yearly PV energy production
The headline figure, in kilowatt-hours per year. This is what the array produces, and what everything else on a quote is compared against.
Specific production, in kWh/kWp/year
Production per kilowatt-peak installed. This is the figure that lets you compare two locations without the system size getting in the way, and it is the one worth writing down.
Yearly in-plane irradiation
Total solar energy arriving at the panel surface, in kWh/m². In the United States this runs from about 1,400 to 2,200 depending on where you are.
Total loss
Temperature, reflection, inverter and cabling combined - typically 20 to 25% of the energy that reached the glass.
Optimal slope and azimuth
The angle and direction that maximise the annual total. Worth knowing even if the roof gives you no choice, because it tells you what the roof is costing you.
What to expect where you are
Specific production, in kilowatt-hours per year for every kilowatt-peak installed. These are ranges for an unshaded, well-oriented array - a real roof lands inside them, not above.
| Region | Examples | kWh/kWp/year | Character |
|---|---|---|---|
| Southwest | Arizona, Nevada, New Mexico | 1,600-1,800 | Best in the country; desert climate |
| South | Texas, Florida, California | 1,400-1,600 | Excellent, with humidity on the Gulf Coast |
| Midwest | Illinois, Ohio, Missouri | 1,200-1,400 | Good; continental climate |
| Northeast | New York, Massachusetts, Pennsylvania | 1,100-1,300 | Moderate; more cloud |
| Pacific Northwest | Oregon, Washington | 1,000-1,200 | Overcast, still viable |
| Region | Examples | kWh/kWp/year | Character |
|---|---|---|---|
| Southern Europe | Spain, Portugal, Italy, Greece | 1,400-1,700 | Best in Europe; Mediterranean |
| Central-South | Southern France, Croatia, Slovenia | 1,200-1,400 | Very good; transitional |
| Central Europe | Germany, Austria, Czechia, Poland | 1,000-1,100 | Good; continental |
| Western Europe | France, Belgium, Netherlands, UK | 900-1,100 | Moderate; maritime |
| Northern Europe | Denmark, Sweden, Norway, Finland | 800-1,000 | Lower sun, very long summer days |
Getting a number you can defend
- Run two or three nearby points. A few miles can move the answer.
- Tick "Optimize slope" once to see what the roof angle is costing you.
- Download the hourly series if a battery or a time-of-use tariff is involved.
- Try an east-west split. Lower annual total, more of it used at home.
- Keep system loss at 14% or higher. Lower is optimism, not measurement.
Common questions
What is PVGIS?
How accurate is PVGIS?
What is the optimal tilt angle for solar panels?
What does kWh/kWp/year mean?
Sources
"Overview of PVGIS v5", Solar Energy, 2016 - the peer-reviewed description of the method.
Solar Energy, 2016