Solar panels and shade: what it costs and what to do about it

A shaded panel produces less electricity. What you need to know is which panels are shaded, at what times of day and in which seasons. You can often improve a design by moving a few panels, using another roof slope or changing how the panels are wired. Before paying for optimisers or microinverters, ask how many kWh each option actually gains.

In the UK, MCS-certified installers have a standard way to put a number on shade: the shade factor in your performance estimate. This guide explains how it works, so you can check it on your quote. For the basics of how much a system produces, start with solar panel output.

Work out when shade falls

A short shadow from a chimney early in the morning is very different from a neighbouring building that covers half the roof around midday. Look at the roof in mid-morning, around midday and in mid-afternoon. Note chimneys, dormers, vent pipes, aerials, satellite dishes, trees, neighbouring houses and extensions, and any hills. A deciduous tree loses its leaves, but its trunk and branches still cast shade, and it may grow. The sun's path changes through the year: a photo taken on a June afternoon says little about December, when the sun is low and shadows are long.

Tell apart near shade, such as a chimney whose shadow crosses a few panels, and a distant horizon, such as a hill or a tall building down the road. The European Commission's free PVGIS tool estimates the sunshine and output for a location, and can include a calculated horizon from terrain data. That horizon does not include nearby objects such as a house or a tree, so a site visit is still needed for your roof.

Do not go up on the roof for a better photo. Pictures from the garden or an upstairs window, a roof plan and a professional survey are safer. For how the direction and pitch of the roof affect output, see solar panel orientation and angle.

How MCS installers calculate shade

MCS-certified installers must give you a performance estimate before you sign (under MIS 3002 issue 6.0, or MCS 032 for installers on MCS's redeveloped scheme). The estimated yearly output is:

annual output (kWh) = system size (kWp) × Kk × SF

Kk is the expected kWh per kWp for your postcode zone, roof pitch and orientation, taken from MCS tables. SF is the shade factor, from 1.00 (no shade) downwards. A shade factor of 0.89 means the estimate expects shade to take 11% off the yearly output.

If the horizon is obviously clear with no near or distant shading, the installer can use 1.00. Otherwise, MCS guidance (MGD 005) sets out a simple method. The installer's steps, in outline:

1. Stand near the base and centre of the planned panels, for example at an upstairs window, with a compass and a device that measures the angle of objects above the horizon. MCS says this can usually be done without climbing on the roof; if you try it yourself, stay indoors or on the ground.
2. Draw the horizon on a sun-path chart, looking due south whatever direction the panels face. The chart shows where the sun is at different times of day and year, split into 84 segments, each worth 0.01.
3. Count every segment the horizon line touches or covers, multiply by 0.01 and take the result from 1. In the MCS example, 11 segments give 1 − 0.11 = 0.89.
4. For objects within 10 m of the panels (chimneys, vent pipes, dishes to the east, south or west), draw a shade circle on the chart with a radius equal to the object's height, and count every segment it touches. Near objects cast larger shadows for more hours, so they count for more. Near objects behind the panels (to the north) do not need to be counted, as they cast little or no shadow on them. In the MCS example, the same object moved within 10 m covers 40 segments: a shade factor of 0.60 instead of 0.89.

MCS says the method should give results within 10% of the actual annual yield for most systems, and that it is simpler, not more accurate, than specialist software. It also says near shade should be avoided, that heavily shaded locations may not be suitable, and that systems should not be sold where shade could have a severe impact.

What this means for your quote:

• If the shade factor is below 1, you should get the sun-path diagram used to work it out, along with a note saying shade will reduce output. Where another method was used instead, the installer must confirm the system will deliver at least 90% of the estimated kWh.
• If the roof was assessed remotely, the estimate must say that actual output could be significantly lower or higher.
• Optimisers and microinverters are not credited in the MCS method. An installer can show their benefit with software, modelling the system with and without them, and under MIS 3002 issue 6.0 that estimate must not be more prominent than the MCS one and must carry warning text saying it should be treated with caution if it is much better.
• Groups of panels on separate MPPTs can each get their own shade factor.

An example: the same roof, three shade factors

Take a 4 kWp system and suppose the MCS table gives 900 kWh per kWp for its postcode zone, pitch and orientation (an illustrative figure, not a value for any particular zone).

The same obstacle takes 396 kWh a year off the estimate from a distance and 1,440 kWh up close. That is why moving panels away from a chimney is often worth more than any piece of equipment.

Why shade costs more at some times than others

kWp is the panels' rated power under test conditions; kWh is the electricity they actually produce. Shade reduces the light reaching the shaded cells, and so their output at that moment. The yearly loss depends on how often it happens and at what time of day. Shade in the middle of a bright day usually costs more than the same length of shade when the sun is low and weak.

Wiring matters too. In a string of panels connected to one inverter, the panels' characteristics and the string layout affect how the whole string responds to shade. Fraunhofer research on partial shading shows why panel design and bypass diodes matter: the diodes can route current around a shaded section of a panel, but they cannot replace the missing light. With microinverters, each panel or small group can be tracked and converted separately; optimisers with a compatible inverter can reduce some mismatch between panels. None of them turns a shaded panel into a sunny one.

Choose where the panels go first

Often the simplest fix is not to put panels in the area that is shaded for long periods. An installer can move a row, use another slope, fit slightly fewer panels or arrange them differently. The best design is not always the one with the most kWp on the quote: a few badly shaded panels add equipment and fitting cost for few useful kWh.

Then look at the electrics. A string inverter can suit a uniform, well-exposed array very well. Microinverters or optimisers become more interesting when slopes face different ways, when local shade cannot be avoided or when per-panel monitoring would make fault-finding easier. To choose, ask for the same layout and the same shade assumptions in each design, and compare the full extra cost, the kWh gained, access for repairs and the warranties on the extra devices. Microinverters vs string inverters covers that choice in detail.

Pruning a tree can reduce shade, if the tree is yours and can be maintained safely. In England, you need your council's consent to prune or fell a tree covered by a tree preservation order, and you must give the council six weeks' notice before most work on other trees in a conservation area. Rules differ in Scotland, Wales and Northern Ireland, so check with your council. Do not base the whole case for the system on pruning that you do not control, especially a neighbour's tree.

Read the estimate, not just the total

A good quote shows where each panel goes, names the panel and inverter models and the number of panels, and includes the MCS performance estimate with its shade factor. Questions to ask at the survey:

1. Which parts of the roof have you left out, and why?
2. Which near objects did you measure on site? Does the sun-path diagram show them, including their effect in winter?
3. What is the shade factor, and what is the estimated yearly output with and without shade?
4. If you are proposing optimisers or microinverters, how many extra kWh a year do you expect compared with the cheaper design, on the same layout?
5. How will the panels be monitored, and who will act if output drops?

Comparing solar quotes shows how to compare offers on more than price per kWp, and the MCS certificate explains what certification covers. An installer who tells you part of the roof is poor and leaves a panel off may be serving you better than one who quotes the largest system that fits.

Keep an eye on it after installation

Keep the original estimate, the layout drawing, the sun-path diagram and access to your monitoring. Compare like with like: a month against the same month in another year, not a cloudy day against a sunny one. If a tree grows or a new extension goes up next door, photograph the shadow from the ground and note when it reaches the panels. If a string or panel produces clearly less than expected, the installer can check your notes against the inverter data. Solar panel monitoring explains what to look for.

Never unplug a solar connector or remove a panel to "test" shade. Panels in daylight can stay electrically live. Checks on the roof or the DC wiring are a job for a qualified professional. With clear records, you can tell ordinary shade from a fault and decide whether changing the system is worth the cost.

Frequently asked questions

How much does shade reduce solar panel output?

It depends on what casts the shade, how close it is and when it falls. In the MCS method, each shaded segment of the sun-path chart takes 1% off the yearly estimate. In the MCS example, one object takes 11% off from a distance and 40% within 10 m.

What is a shade factor on a solar quote?

It is the figure, from 1.00 down, that the MCS estimate multiplies the expected output by to allow for shade. A shade factor of 0.89 means an 11% reduction. If it is below 1, you should get the sun-path diagram used to work it out.

Will one shaded panel affect the whole system?

It can affect the other panels on the same string, depending on the wiring, the panels' bypass diodes and the inverter's shade handling. Putting shaded panels on their own MPPT, or using optimisers or microinverters, can limit the effect, but the lost light is not recovered.

Are microinverters worth it for a partly shaded roof?

Sometimes. Ask for two designs with the same layout and shade model and compare the extra kWh a year with the extra cost. The standard MCS estimate does not credit them, so any benefit must come from a separate, clearly labelled estimate.

Can I put solar panels on a north-facing or heavily shaded roof?

An obstacle to the north of the panels casts little or no shadow on them, but a north-facing roof gets less sun overall; solar panel orientation and angle compares directions. On a heavily shaded roof, MCS guidance says installation may not be appropriate and systems should not be sold where the impact of shade could be severe.