What Is the Shading Factor?
The shading factor measures how much sunlight is blocked by nearby obstacles at a given point on a roof or panel, directly impacting your solar panels' energy output. A lower shading factor means more sunlight reaches your panels, maximizing energy production.
For example, if a tree or building blocks 20% of the sunlight at a given point, the shading factor at that location is 20%. The goal is to minimize this value to optimize system performance.
Shading Factor vs. Shading Loss
It's important to distinguish between the shading factor and shading loss:
Shading factor is the fraction of irradiation blocked before sunlight is converted into energy.
Shading loss refers to the actual electricity loss due to shading after sunlight has been converted into energy.
While these values are often treated as linearly connected for simplification, real-world shading losses may vary. Factors such as partial shading of strings and the use of optimizers can influence shading losses beyond what the shading factor alone suggests. In Expert, we use a straightforward linear approach to keep calculations clear and predictable: you enter the shading loss as a percentage on the Shading / snow losses page, and the simulation reduces the yield by exactly that percentage.
Where You'll See the Shading Factor
The shading factor is calculated as part of the Shading analysis in the 3D Panel Layout. Once enabled, it is visualized in two ways:
Heatmap: A color-coded shading factor overlay on every roof you've drawn, making it easy to spot the more and less shaded areas of a roof. Orientation and tilt don't count here: an unshaded north-facing roof shows the same value as an unshaded south-facing roof.
Per-panel values: In the 2D view, each panel is colored and labeled based on its own annual shading factor, so you can compare panels directly against each other.
Note: Panel-level shading factor values are only meaningful for comparing panels on the same roof — don't compare shading factor values across different roofs of a project.
To check the shading factor at any single point without placing panels there, use the Shading factor selector in the Shading analysis control panel in the 2D view: click the button, then click on the roof. Eturnity reads the value from the heatmap color; from 20% upwards it shows “≥20%”. For the next point, click the button again.
The Shading analysis is a planning aid: its values do not flow into the yield calculation automatically. For shading from neighboring buildings, trees or obstacles to be part of the simulated yield, enter the loss as a percentage on the Shading / snow losses page (Design group; in the older navigation under Photovoltaics) — for the whole year or per month, together with any snow losses. Shading from the terrain (horizon) is already included in the yield calculation via the PVGIS irradiation data. For projects in the UK that fall within MCS scope, Eturnity instead calculates the annual yield according to MCS, using the shading factor from the MCS Shading tab.
How Is the Shading Factor Calculated?
Shading factor calculation involves three key steps:
Data collection: The system gathers details about buildings and obstacles you've modeled, while horizon data and direct and diffuse irradiation come from PVGIS — a trusted solar data source developed by the European Commission's Joint Research Centre, ensuring accuracy based on decades of validated satellite and ground measurements. Expert itself estimates the share reflected from the ground. The terrain horizon serves as the reference: what mountains or hills already block does not count as shading.
Point-by-point calculation: The software calculates the shading factor for each point on the roof, then aggregates it per panel.
Visualization: These outputs drive the shading heatmap and per-panel values described above.
A couple of things worth knowing about how the calculation behaves:
When you move panels or adjust panel fields, Eturnity updates the panel values automatically as soon as you let go. You change roofs and obstacles with other tools; switching tools turns the Shading analysis off, and when you turn it on again, it calculates with the new state.
Panels themselves are not treated as obstacles for each other in the calculation, so on a flat roof without surrounding obstacles, panels won't show shading purely from being close together.