Why a little shading causes a lot of loss
In a standard string-inverter installation, panels are connected in series — like batteries in a torch. When one panel in a string is shaded, the current through the entire string is limited by the weakest panel, because series circuits must carry the same current throughout. A single panel with 50% of its surface shaded can reduce the output of the remaining five or ten unshaded panels in the string by a significant proportion.
Modern panels include bypass diodes — typically three per panel — that allow current to flow around a shaded section of the panel rather than through it. This limits the voltage loss from shading to one-third of a panel's contribution per activated bypass diode, rather than the entire panel being dropped. But even with bypass diodes, partial shading of one panel in a string still imposes losses on the whole string.
The loss is worst with a single, concentrated shade source — a chimney casting a sharp shadow across a panel from 9–11am every morning will reduce string output during that window more than diffuse cloud cover, which affects all panels equally.
Identifying shading at your property
Shading sources to look for when assessing or reviewing an installation include: neighbouring buildings, particularly as housing density increases; chimney stacks on your own or adjacent properties; dormer windows or roof features; TV aerials and satellite dishes; and trees. Trees are a particular concern because they grow — a tree that causes no shading at installation can become a significant shade source within five to ten years.
PVGIS (the European Commission's PV estimation tool) allows you to model the generation impact of shading on a proposed installation using satellite horizon data. A good installer will have used this — or an equivalent shadow analysis tool — as part of your site survey. If they did not, and you now suspect shading is affecting performance, a post-installation shade assessment is available from specialist solar assessors.
Monitoring data is the most practical indicator. In a SolarEdge or Enphase system, per-panel data shows exactly which panels are underperforming and during what hours — shading shows up clearly as recurring output reduction patterns on specific panels at specific times of day.
Technologies that mitigate shading loss
DC optimisers (SolarEdge) and microinverters (Enphase) address the string current problem by performing maximum power point tracking (MPPT) at the individual panel level. Each panel operates independently, so a shaded panel does not drag down the unshaded ones. The system loses only the output of the shaded panel itself, not the output of the entire string.
On a south-facing, unshaded roof, the performance advantage of optimisers or microinverters over a standard string inverter is marginal — perhaps 2–5%. On a partially shaded roof, the advantage can be 20–40% in annual generation terms. If shading is present and your system uses a string inverter, retrofitting DC optimisers (which can work with many existing SolarEdge inverters) may be cost-effective.
Panel-level optimisation also improves monitoring granularity — you can see exactly what each panel is generating, which is invaluable for identifying soiling or hardware issues on specific panels.
Shading from soiling vs shading from physical objects
Localised soiling — a bird dropping on the corner of a panel, moss on the lower edge — is functionally the same as partial shading from a physical object. It reduces the output of one section of the panel, activates bypass diodes, and if severe enough affects the string.
The difference is that soiling is removable. A professional clean can eliminate soiling-induced shading loss entirely, whereas a chimney casting a morning shadow cannot be moved. In a monitoring system, recurring output reduction on a specific panel at the same time each day suggests a structural shading source; a sudden, sustained drop in a panel's output that does not correlate with time of day suggests soiling, a failing bypass diode, or a microclimatic cell failure.
For UK homeowners with string inverters on partially shaded roofs, the two highest-return interventions are: regular professional cleaning (eliminates controllable soiling losses) and, if the shading is significant, a discussion with a qualified solar technician about whether DC optimiser retrofitting is justified.