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Solar Panel Shading Calculator

Estimate annual kWh and revenue lost to partial shading on your UK solar array. Free calculator covering bypass-diode and microinverter topologies.

Solar Panel Shading Calculator

Annual baseline output
2,961 kWh
Annual kWh lost
133 kWh
Annual revenue lost
£36
Array output reduction
4.5%
Recommendation
Optimize string layout to isolate shaded panels.

How to use this calculator

Enter eight inputs and the calculator returns annual baseline kWh, annual kWh lost to shading, annual revenue lost (using your retail rate), and an inverter-topology recommendation.

  1. System size (kW) — total nameplate. 4 kW is the most common UK domestic install (10 × 400 W panels).
  2. Peak sun hours per day — England average 2.6, Scotland 2.2, southern England 2.8. The Energy Saving Trust and PVGIS-SARAH3 give finer-grained values.
  3. System efficiency (%) — 78% is a sensible MCS-compatible derate including soiling, wiring, inverter, and temperature losses.
  4. Electricity rate (£/kWh) — your retail unit rate. The 2026 Ofgem price cap unit rate is around 27p/kWh.
  5. Total panels in array — count from your MCS design certificate.
  6. Panels affected by shading — count of panels touched by shadow at any point in the day.
  7. Hours shaded per day — annual average; pessimistic estimates are fine because UK winter sun is low anyway.
  8. Shading severity (%) — chimneys 70–90%, trees 40–70%, vent stacks 50–80%.
  9. Inverter topology — microinverter/optimiser, modern string (G98/G99-compliant), or pre-2005 string with no bypass.

How shading really affects UK solar output

Solar panels are wired in series strings. Current is limited by the lowest-current cell. When light hits a panel, each cell generates current proportional to the irradiance striking it. A shaded cell becomes a bottleneck, restricting the entire string’s current to its level. Without protection, this is severe: a single bird dropping could halve a 10-panel string’s output.

Modern panels solve this with bypass diodes — typically 3 per panel, one per cell group of 20–24 cells. When light on a cell group drops below roughly 20% of full irradiance, that cell group’s diode shunts current around it. The string loses one-third of one panel’s output, not the whole string’s output. Every MCS-listed panel on the UK market today has bypass diodes.

Optimisers (SolarEdge, Tigo) and microinverters (Enphase IQ8, APsystems) add per-module MPPT tracking. A shaded panel no longer affects unshaded panels in the string. They don’t eliminate shading loss — they contain it perfectly.

The shading-loss math

For a string with bypass diodes (default on every install since 2005):

loss_fraction ≈ (shaded_panels / total_panels)
              × (hours_shaded / productive_hours_per_day)
              × (severity / 100)

productive_hours_per_day ≈ 8 in summer, 5 in winter (UK average ~6.5)

For an older string with no bypass, the entire string drops during shading hours:

loss_fraction ≈ (hours_shaded / 8) × (severity / 100)

Worked example for a 4 kW domestic array, 2.6 PSH, 78% derate, £0.27/kWh, 2 of 10 panels shaded 2 h/day at 60% severity, modern string inverter:

  • Baseline annual kWh = 4 × 1000 × 2.6 × 0.78 × 365 / 1000 = 2,960 kWh
  • Loss fraction = (2/10) × (2/8) × 0.60 = 0.030
  • Annual kWh lost = 2,960 × 0.030 = 89 kWh
  • Annual revenue lost ≈ £24 (split between bill savings and lost SEG export)

Typical UK shading scenarios

ScenarioSeverityHours/dayAnnual loss (modern string)
Chimney stack shadow on 1 panel80%1.51.5%
Roof vent or soil pipe on 1 panel90%0.50.6%
Telegraph pole or lamp post line on 2 panels60%11.5%
Neighbour’s tree on 3 of 10 panels50%35.6%
Dormer shading half the array80%210%
Mature oak tree, full afternoon, 5 of 10 panels70%3.515.3%
Lichen and bird soiling across whole array8%88%

UK panels in lichen-prone areas (the West Country, North Wales, Scottish Highlands) need a clean every 2–3 years or soiling losses climb. The solar panel cleaning cost calculator handles that economics.

Mitigation strategies, ranked by cost-effectiveness

  1. Trim or remove trees — £200–£800 for a typical UK domestic tree. If you’re recovering 8% on a 4 kW system at 27p/kWh, that’s £64/yr — payback under 10 years even for the higher quote. Check Tree Preservation Orders before cutting; if the tree is in a conservation area, you’ll need council consent.
  2. String layout optimisation — wire unshaded panels in one string, shaded panels in another. Costs nothing at install time. MCS installers should do this by default on any partially shaded roof.
  3. DC optimisers (SolarEdge, Tigo TS4-A) — adds £400–£900 to a 4 kW install. Best for partial shading and mixed-orientation roofs.
  4. Microinverters (Enphase IQ8) — adds £600–£1,200 to a 4 kW install. Best for heavy shading, complex roofs, and arrays that will be expanded later.
  5. Move the array — if the chosen face has 15%+ shading and another face is shade-free, the extra cable run usually pays for itself in year-one production.

Common mistakes

  • Confusing winter low light with shading. UK winter overcast is global diffuse irradiance — already in your peak sun hours figure. Don’t double-count.
  • Underestimating tree growth between quote and install. A tree quoted as “5 m” might be 5.5 m by install day six months later. Ask the installer to model with a 10% margin.
  • Ignoring chimney shadow direction. A chimney on the south-east of a south-facing roof shades early morning panels; one on the south-west shades late afternoon. Different mitigation strategies.
  • Treating soiling as shading. Soiling is constant low-grade reduction, not a directional shadow. Cleaning fixes soiling; only string layout or optimisers fix true shading.

Sources

Frequently asked questions

How much does partial shading reduce output on a UK rooftop array?
On a modern domestic array with bypass diodes — wired to BS 7671 Section 712 standards — shading 1 of 10 panels for 2 hours at 60% blockage costs roughly 1.5% of annual output. On a 4 kW system producing 3,400 kWh/yr, that's 51 kWh worth about £14 at the 2026 average unit rate. Older pre-2005 arrays without bypass diodes lose much more because the shaded panel limits current for the whole string. UK MCS-certified installers now default to optimisers (SolarEdge) or microinverters (Enphase) on shaded roofs.
Does shading kill my SEG payments?
The Smart Export Guarantee pays you for kWh exported to the grid, so anything that cuts generation directly reduces SEG income. If you self-consume during peak shading hours, the impact on SEG is smaller — you simply import less, and the unshaded morning sun still generates exportable surplus. Octopus, Good Energy, and EDF SEG tariffs in 2026 range from 4p to 15p per exported kWh. On a 4 kW array with 5% shade loss, expected SEG loss is roughly £8–£25/yr depending on your tariff and self-consumption pattern.
Should I get a SolarEdge optimiser system on a Yorkshire roof?
If you have meaningful shading (chimney, dormer, neighbouring tree, telegraph pole, satellite dish), optimisers add about £400–£900 to a 4 kW install but typically recover £150–£300/yr in additional production over a string inverter. Payback is 3–6 years, well inside the panel warranty. If your roof is shade-free south-facing with no obstructions, save the money. Use the calculator: if projected annual loss exceeds 4% on a modern string inverter, optimisers pay back.
Are there free UK tools to check shading on my roof?
Google Project Sunroof is now available across most UK postcodes — it uses Ordnance Survey LiDAR and aerial imagery to estimate annual sun hours by roof section. The Energy Saving Trust solar calculator is conservative but useful. For a DIY check, take phone photos from your roof at 9 AM, noon, and 3 PM in mid-winter (worst case) and mid-summer (best case). Compare them. MCS-certified installers will do a full shade analysis with an MCS003 Shading Module or Solar Pathfinder as part of the design quote.
What's the difference between hard shade and diffuse shade in the UK?
Hard shade is a solid object blocking direct sunlight — a chimney casts a sharp-edged shadow that blocks 80–95% of light. Diffuse shade is the soft penumbra around hard shadows or light scattered through tree leaves — typically 30–50% blockage. UK winter overcast counts as global diffuse irradiance and isn't 'shading' in the calculator sense; it's already baked into your peak sun hours figure (~2.6 PSH/day in much of England, 2.2 in Scotland).

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