Skip to content

Do solar panels work in shade, and how much do you lose

Panels keep producing in shade, from diffuse light. The expensive question is different: what one small opaque shadow does to the rest of the array. Bypass diodes decide the answer, and they work in thirds.

Performance9 min read

Photograph to accompany: Do solar panels work in shade, and how much do you lose

The short answer

Shaded panels still generate, because a large share of the light reaching a British roof is diffuse rather than direct. The damaging case is a small, hard shadow such as a chimney, a flue or a bird dropping. Modern modules carry three bypass diodes, so shading a single cell typically knocks out one third of that module rather than all of it. Without per-module electronics the rest of the string still operates, but the inverter has to find the right operating point on a distorted curve, and cheaper equipment sometimes does not.

Two different questions get asked with the same words, and they have opposite answers.

Does a panel produce anything when the sky is overcast? Yes, comfortably. Does a panel produce anything useful when a chimney casts a hard shadow across one corner of it? That depends on the module's internal wiring, and the cost is far higher than the shaded area suggests.

Overcast is not shade

Sunlight reaching a panel arrives two ways. Direct beam radiation comes straight from the disc of the sun. Diffuse radiation is scattered by cloud and atmosphere and arrives from the whole sky dome. On a heavily overcast British day the direct component is close to zero and the diffuse component is doing all of the work.

That is why the reference array in these guides still delivers 134 kWh in December. The sun is barely above the horizon, the days are short, and much of the month is cloud. It generates anyway, at roughly 29% of its July output.

A panel under general cloud is uniformly lit. Every cell in it is receiving about the same reduced amount, so the module behaves normally at a lower power. Nothing pathological happens.

Hard shade is the problem, and the reason is wiring

Cells inside a module are wired in series. Current has to pass through every cell in turn, which means the current through the string is set by the cell passing the least. Shade one cell heavily and, without protection, it throttles every other cell wired with it. A module that is 2% shaded can lose far more than 2% of its output, and the shaded cell heats up as it absorbs the power the others are trying to push through it.

Bypass diodes exist to stop that. A typical 60 or 72 cell module carries three, each spanning roughly a third of the cells. When one cell is shaded enough to hold back its group, the diode across that group conducts and routes current around it.

The diode turns a disproportionate loss into a proportionate one, and it stops the hot spot. It does not give you the third back.

What it costs the rest of the array

With bypass diodes working, a shaded module in a string of ten drops its own contribution by a third. The string keeps running. The array loses roughly one thirtieth, not a tenth and certainly not everything.

Two things can make it worse than that.

  • Partial shading distorts the string's current-voltage curve so that it has several local power peaks instead of one. The inverter's maximum power point tracker has to find the highest. Good equipment sweeps the full curve periodically and finds it. Weaker equipment settles on a lower peak and stays there, and the loss is invisible unless someone looks at the data.
  • Shade moves. A chimney shadow crosses the array through the day, so the affected sub-strings change hour by hour. Averaged across a year the total can be substantial even though no single moment looks disastrous.

This is the case that per-module electronics are sold for. Microinverters and DC optimisers give each module its own maximum power point, so a shaded module's problem stops at that module instead of influencing the string. Whether that is worth the extra cost is set out in microinverters against a string inverter.

Close crop of blue polycrystalline modules showing the individual cells and the busbars running between them
Each square is one cell. A shadow across a few of them triggers the bypass diode for that whole sub-string.

Where shade comes from, in rough order of cost

Shading sources and what makes each one expensive
SourceCharacterWhy it costs what it costs
Chimney or flueHard, close, moves through the daySweeps across sub-strings for hours; present every day of the year
Neighbouring buildingHard, fixed geometry, seasonalWorst at low winter sun, when output is already scarce
Deciduous treeHard, seasonal, growingLighter in winter when bare; gets worse every year as the tree grows
Evergreen treeHard, year-round, growingNo seasonal relief at all
Overhead cable or aerialThin, hard, sweepingSmall area but can hold back a sub-string as it crosses
Bird droppings, lichen, leaf litterHard, static, localisedOpaque over a few cells; the one case where cleaning is clearly worth it
General cloudSoft, uniformReduces output smoothly; no sub-string effect

Everything above the last row is opaque and localised. That is what triggers a bypass diode. Cloud does not.

Working out what your own shade costs

The only honest way is measurement, and an installer who is not doing it should be asked why.

  1. 1A shading survey from the roof plane, with an instrument that maps the skyline and the sun paths across the year. This produces a percentage, not an adjective.
  2. 2A yield model with that shading applied. PVGIS accounts for the terrain horizon from elevation data, so it already knows about hills. It does not know about your chimney, and the difference between those two is exactly the number you need.
  3. 3Module-level monitoring once the system is running, if the array has per-module electronics. It shows which modules underperform and when, which is the only way to catch an MPPT settling on the wrong peak.

One practical point on layout. If part of a roof is reliably shaded at the times of day that matter, putting fewer panels on it is often better than putting panels there and adding electronics to manage the consequences. That decision belongs in the design, before anything is bought.

Our calculator models an unshaded roof and says so. If your array has significant object shading, the figure it produces is an upper bound rather than an estimate.

Monthly output figures modelled with PVGIS 5.2 (European Commission Joint Research Centre), PVGIS-SARAH2 radiation database, 2005–2020 average. 4 kWp crystalline silicon, roof-mounted, 14% system loss, Birmingham, UK. Retrieved 2026-09-12. Bypass diode counts and module wiring are standard for 60 and 72 cell crystalline modules; confirm against your own module datasheet. Source

Interactive Schematic

Bypass Diode Action Under Shade

Toggle shaded section:
Active (Producing)SHADED (Bypassed)Active (Producing)
Module Output Capacity
67% Power Output
Bypass diode #2 routes current around the shaded sub-string, saving 67% of full capacity.

Common questions

Do solar panels work in the shade?
Yes, but at much reduced output. Panels convert diffuse light scattered by cloud and sky as well as direct sunlight, which is why a British array still generates through overcast winter months. The reference array in these guides produces about 134 kWh in December against 461 kWh in July.
How much output is lost if one solar panel is shaded?
A typical 60 or 72 cell module carries three bypass diodes, each covering about a third of the cells. Shading a single cell causes that third to be bypassed, so the module loses roughly one third of its output rather than all of it. In a string of ten modules that is about one thirtieth of the array.
Does one shaded panel affect the whole string?
Far less than it used to. Bypass diodes route current around the shaded section, so the rest of the string continues to operate. The residual risk is that partial shading creates several power peaks on the string's current-voltage curve and the inverter tracks a lower one, which is a loss that shows up only in monitoring data.
Do microinverters or optimisers solve shading?
They contain it. Per-module electronics give each panel its own maximum power point, so a shaded module no longer influences how the rest of the string operates. They do not recover the output of the shaded cells themselves, and they do not change what a bypass diode does inside the module.
Is a bird dropping on a solar panel a real problem?
It can be, out of proportion to its size. A dropping is opaque and sits over a few cells, which is exactly the condition that makes a bypass diode conduct and takes out a third of the module. This is the clearest case where cleaning a panel is worth the effort, as opposed to general dust, which rain handles.