How to Use PVGIS to Estimate Solar Output (UK Guide)

Before you commit to a solar quote, it helps to have an independent estimate of what your roof should generate each year. PVGIS — the Photovoltaic Geographical Information System — is a free tool published by the European Commission’s Joint Research Centre (JRC) that does exactly that. It uses satellite irradiance data to estimate annual and monthly electricity production for any location, with no registration, no sales agenda, and no paywall. This guide explains every step, so you can enter your own numbers and understand what comes back.

What is PVGIS and why use it?

PVGIS was developed by the JRC and is the reference tool used by solar researchers and many professional engineers across Europe. It draws on the CMSAF SARAH satellite dataset, which covers Europe (including the whole of the UK, well below the 65° N latitude limit) at a spatial resolution of roughly 5 km × 5 km. For UK homes, the tool gives annual yield estimates that are broadly consistent with monitored system performance across the country.

The main advantage over a quote from an installer is independence: PVGIS has no commercial interest in making your roof look more productive than it is. Use it alongside, not instead of, a proper MCS-certified site survey — but it is an excellent sanity-check on the figures you are given.

Step 1: Go to the tool and enter your location

Open your browser and go to re.jrc.ec.europa.eu/pvg_tools/en/ and select “Grid-connected PV”. This is the standard option for a home solar system connected to the electricity grid.

To set your location, click on the map or type your postcode or town into the search field. The tool will drop a pin and display the latitude, longitude, and elevation. For a UK address, check the pin lands on your actual property rather than the centre of a postcode area — for densely built streets, a small offset can make little practical difference, but it is worth checking in rural areas where the nearest neighbour may be a kilometre away.

The tool will automatically select the most appropriate solar radiation database for your location. For most of England, Wales, and Scotland, this will be SARAH-3 (the most recent satellite dataset). If you are near the far north of Scotland or Shetland, ERA5 reanalysis data may be used as a fallback — both are reliable for planning purposes.

Step 2: Set your system size in kWp

Enter the peak power of your intended system in kilowatt-peak (kWp) under “Installed peak PV power”. The average domestic solar installation in the UK was 4.6 kWp in 2024, though most homes are quoted systems between 3.5 kWp and 6 kWp depending on roof area and electricity demand.

If you are not yet sure of your system size, our guide to solar panel dimensions and roof layout explains how to work out how many panels will physically fit on your roof, which directly determines the maximum system size before any demand calculation. As a rough starting point: a 4 kWp system uses around eight to ten 400–430 W panels, each roughly 1.76 m × 1.13 m, occupying about 16–20 m² of usable roof area.

For PVGIS purposes, you can run the calculation with your target size (say, 4 kWp), then scale the output up or down proportionally for other sizes — the annual kWh figure is directly proportional to the installed kWp.

Step 3: Choose mounting type and set tilt

Leave the mounting type as “Fixed” for a standard pitched roof installation. Tracking systems (which rotate to follow the sun) are not used in UK domestic installations.

For “Slope” (the tilt angle of your panels from horizontal), enter the pitch of your roof in degrees. The typical UK domestic roof pitch is in the range of 30°–40°, with 35° being the most common for Victorian, Edwardian, and post-war semi-detached houses. If you are not sure of your roof pitch, you can look it up from your original planning drawings, measure it with a digital angle finder (around £10 at a builders’ merchant), or estimate it visually — 35° is a reasonable default for most UK terraced and semi-detached homes.

If you click “Optimise slope”, PVGIS will calculate the angle that maximises annual output for your latitude. In southern England this is typically around 35–38°; in Scotland it is closer to 40–42°. These optimal angles happen to match typical UK roof pitches reasonably well, which is one reason south-facing pitched roofs perform so effectively in the UK.

Step 4: Set the azimuth (direction your roof faces)

This is the step where the biggest mistakes are made. In PVGIS, azimuth is measured from due South, not from North as in a compass bearing. The convention is:

• 0° = South (optimal for UK homes)
• −90° = East
• +90° = West
• ±180° = North

If your roof faces south, enter 0. If it faces south-east, enter roughly −45. If it faces south-west, enter +45. A common mistake is entering a compass bearing directly (for example, 135 for south-east) — PVGIS will interpret this as a near-west-facing roof, completely wrong for your situation.

East- and west-facing roofs lose roughly 15–20% of annual output compared with a south-facing roof at the same tilt. North-facing roofs lose 50% or more and are generally not viable for a UK domestic system. If your property has both east- and west-facing roof planes, run PVGIS twice (once for −90°, once for +90°) and add the outputs together to estimate a split-array system.

Step 5: Check the system loss figure

PVGIS defaults to a system loss of 14%, and this is a reasonable starting point for a new UK installation. System loss represents everything that reduces the power actually delivered to the grid compared with what the panels produce at their rated peak power. The JRC user manual lists the main components as cable losses, inverter inefficiency, soiling (dust, lichen, bird droppings), and module degradation over time.

For a brand-new installation with a high-efficiency modern inverter, a figure of 10–14% is appropriate. Over time, as panels age and efficiency gradually declines (typically 0.3–0.5% per year), the effective loss increases. MCS-certified installers in the UK commonly use 14% as a conservative default when producing system yield estimates for customers, which aligns with the PVGIS default. Do not be tempted to reduce this figure to make the numbers look better — 14% is already on the optimistic side for a system that has been in service for several years.

If your roof has significant shading — from a chimney, a nearby tree, or a neighbouring building — add extra loss beyond the default. Even moderate shading can cut output by 10–30% in a string inverter system. Read our detailed guide to how shading affects solar panel output before assuming PVGIS’s clean-panel figures apply to your roof.

Reading the output: what the numbers mean

Once you click “Visualise results”, PVGIS shows annual and monthly energy production in kWh, plus the in-plane irradiation and a breakdown of losses. The headline figure is the “Yearly PV energy production” in kWh.

To put the number in context for a UK home:

• A well-sited 4 kWp system in London or the south-east will typically show around 3,400–3,800 kWh per year (roughly 850–950 kWh per kWp).
• The same system in Brighton or the south coast may reach 3,800–4,200 kWh per year (950–1,050 kWh per kWp).
• In Manchester or the Midlands, expect 3,200–3,600 kWh (800–900 kWh per kWp).
• In Inverness or northern Scotland, a 4 kWp system might produce 2,800–3,200 kWh (700–800 kWh per kWp).

The monthly bar chart shows seasonal variation: output in June and July is roughly four to five times higher than in December and January. This seasonality matters for battery sizing and for working out how much of your electricity you can realistically cover with solar through the winter months.

To translate kWh into £ savings, multiply the annual production by the share you expect to self-consume (typically 30–50% for a home without a battery, higher with one), then multiply by your electricity unit rate. For the remainder exported to the grid, apply your Smart Export Guarantee rate. The Energy Saving Trust estimates that a south-facing 4 kWp system in the UK saves a typical household between £160 and £430 per year on electricity bills, depending on location and self-consumption.

Common mistakes to avoid

Wrong azimuth entry. The most frequent error. Remember: PVGIS uses a south-relative convention (0° = South), not a compass bearing. Double-check by confirming that a south-facing roof shows 0° and that the output is highest at that value.

Ignoring shading. PVGIS assumes an unobstructed view of the sky. It has no built-in shading modeller. If your roof has a chimney stack, a dormer, or a nearby tree that catches the low winter sun, the real output will be lower — sometimes significantly so. An MCS-certified site survey is the only way to quantify this accurately.

Using wrong tilt. Entering 0° (horizontal) gives the output for a flat roof without any tilt compensation. Entering 90° gives the output for a vertical wall. For a standard pitched roof, measure or estimate your actual pitch and enter it.

Over-optimising system loss. Setting system loss below 10% assumes a perfect system. Real-world losses — cabling, inverter efficiency at partial load, soiling, and degradation — typically push the combined figure to 12–16% over the system’s lifetime. The 14% default is a sensible conservative estimate; only reduce it if you have specific data (e.g., a premium inverter with published efficiency curves).

Limitations of PVGIS

PVGIS is an excellent planning tool, but it has real limitations you should understand:

• No shading modelling. As noted above, PVGIS assumes a clear sky view from every panel. Professional shading analysis tools (PVsyst, Aurora Solar) model horizon obstructions and moving shadows through the year.
• Historical average, not forecast. PVGIS figures are based on long-term satellite-derived averages (2005–2023 for SARAH-3). Actual output in any given year will vary by ±10–15% depending on weather.
• No module-level detail. PVGIS treats the array as a single unit. It does not model the impact of mismatched panels, micro-cracks, or partial shade on individual modules. For detailed design, use PVsyst.
• Grid connection losses not modelled. The export limit imposed by your DNO (Distribution Network Operator) — typically 3.68 kW for a single-phase connection — can curtail output on the sunniest days. PVGIS does not account for this.

Next steps

Once you have a PVGIS estimate, you have an independent benchmark to compare against installer quotes. If a quote’s stated annual output is more than 10–15% higher than your PVGIS figure, ask the installer to explain the difference — it may reflect a lower system loss assumption or a different shading assessment. If it is unexplained, treat it as a yellow flag.

From there, our Solar Planner takes your PVGIS output figure and combines it with your electricity tariff, battery options, and Smart Export Guarantee rate to produce a full financial estimate for your home. It is free, takes about five minutes, and requires no personal details to get started.