What is solar irradiance?
Solar irradiance is the sunlight power reaching a surface per unit area, measured in watts per square metre [S1]. Accumulated solar energy is irradiation. For illustration, an assumed constant 500 W/m² for two hours equals 1 kWh/m² of irradiation. Your roof forecast needs location and orientation as well as the correct units. Checked October 2026.
What does solar irradiance mean for your site?
Solar irradiance is the power in sunlight arriving on a surface, measured in watts per square metre (W/m²) [S1].
Why it matters for your site: this is the incoming resource used to model your roof, rather than electricity ready to run equipment. Keep the sunlight measurement separate from the generation forecast in a proposal [S2].
Irradiation is energy accumulated over time. Annual irradiation can be expressed in kilowatt-hours per square metre per year (kWh/m² per year). A kilowatt-hour (kWh) measures energy; a kilowatt (kW) measures power. PVGIS publishes annual irradiation maps separately from photovoltaic electricity-production maps [S3].
How does PVGIS report sunlight for a roof?
PVGIS means Photovoltaic Geographical Information System. Its grid-connected calculation returns annual in-plane irradiation and monthly results. Its radiation tool distinguishes horizontal surfaces from selected tilted surfaces [S2]. Record which plane your figure describes before comparing reports.
For an illustration, assume constant irradiance of 500 W/m² over two hours. Irradiation is 500 × 2 ÷ 1,000 = 1 kWh/m². These are assumed inputs, not a UK weather observation or an electricity yield.
Why does the UK location matter?
Solar-resource maps show geographical variation [S3]. Use your actual coordinates and roof geometry rather than transferring a figure from another premises. Do not multiply irradiation by roof area and label the result as electrical generation: a PV model also accounts for conversion and losses [S2].
For the wider context, read solar yield by UK region and seasonal solar output. Check capacity terminology in what kWp means.
Next step
Use the cut energy costs hub and site check, then get matched with an installer that covers your area.
Piggy Pineapple Ltd operates this referral service. We refer enquiries to installer partners and may be paid a fee. Our owner holds shares in at least one partner. Before sharing your details, we name the partner and disclose any shareholding our owner holds in it.
Common questions
Where can you find sunlight data for a business postcode?
Select your site's coordinates in PVGIS and save the radiation database, roof angle and direction with the result. Its user manual explains location-based calculations [S2].
Is irradiation the electricity your panels produce?
No. It is incoming solar energy per area. PVGIS reports electricity production separately, accounting for panel technology and specified losses [S2].
Can you use horizontal radiation data for a pitched roof?
Request an in-plane calculation for the roof's tilt and direction. Horizontal and inclined surfaces receive different radiation; PVGIS models that conversion [S1].
Do clouds remove all usable sunlight?
No. PVGIS distinguishes direct sunlight from diffuse radiation scattered by the atmosphere and clouds [S1].
Does an annual figure describe every month?
No. Download the monthly radiation values as well as the annual summary to examine seasonal variation [S2].
Ask about your site
Sources
- PVGIS data sources and calculation methods, European Commission Joint Research Centre,Last checked 19 October 2026
- PVGIS user manual, European Commission Joint Research Centre,Last checked 19 October 2026
- PVGIS geospatial data download, European Commission Joint Research Centre,Last checked 19 October 2026