How to Use This Calculator
Select each source of shade that affects your roof, specify its direction and severity, and we’ll estimate the annual generation loss. You can add multiple shade sources to see their combined impact, plus get recommendations on whether you need optimisers or microinverters.
Why Shading Matters for Solar Panels
Shading is the single biggest factor that can reduce your solar panel output — and it’s often underestimated. Even partial shading on a small section of your array can have a disproportionate impact on total generation, depending on your inverter setup.
Understanding your shading situation before installation helps you make informed decisions about panel placement, system design, and whether to invest in shade-mitigation technology like optimisers or microinverters.
Trees
The most common cause of shading. Can range from light dappled shade to complete blockage. Deciduous trees change seasonally.
Buildings
Neighbouring properties, extensions, or taller structures nearby. Impact depends on height, distance, and direction.
Chimneys & Dormers
Obstacles on your own roof cast shadows that move throughout the day. Usually affects 1-3 panels depending on size.
Aerials & Vents
Small obstacles with minimal impact. Often can be relocated during installation if they’d shade panels.
The String Inverter Problem
Most residential solar systems use a string inverter — a single device that converts DC power from all your panels into AC power for your home. Panels are connected in series (a “string”), and the entire string’s output is limited by its weakest panel.
This creates a problem: if one panel is shaded and producing only 50% of its capacity, it can drag down the output of every other panel in the string — even those in full sun.
How String Inverters Are Affected by Shading
In this example, one shaded panel (40% output) forces the entire 6-panel string down to 40% — even though 5 panels are in full sun. Total output: 40% instead of the expected 90%.
This Is Why Shading Impact Can Be Severe
A single shaded panel doesn’t just lose its own output — it can reduce the entire system’s generation. This is why even small amounts of shading need to be taken seriously, and why optimisers or microinverters are so valuable for shaded roofs.
Typical Shade Impact by Source
The actual impact depends on the severity of shading, the direction (south-facing obstacles are worst), and when during the day the shading occurs. Here are typical ranges:
| Shade Source | Light Impact | Partial Impact | Heavy Impact |
|---|---|---|---|
| Trees (nearby) | 5-10% | 15-20% | 25-35% |
| Neighbouring building | 8-12% | 18-25% | 30-40% |
| Chimney | 2-4% | 5-8% | 8-12% |
| Dormer window | 3-5% | 6-10% | 10-15% |
| Aerial/satellite dish | 1-2% | 2-4% | 4-6% |
| Roof vent/pipe | 1% | 2-3% | 3-5% |
Direction Matters
Shade from the south has the biggest impact because south-facing panels receive the most direct sunlight. Shade from the north has minimal impact since it rarely falls on south-facing roof sections.
- South-facing obstacle: +30% more impact
- East or West: Standard impact
- North-facing obstacle: -50% less impact
When Does Shading Occur?
The time of day when shading occurs significantly affects its impact on total generation:
| Time Period | Solar Production | Impact Multiplier | Notes |
|---|---|---|---|
| Morning (6am-10am) | ~15% of daily total | Lower impact | Sun is low, less generation anyway |
| Midday (10am-2pm) | ~50% of daily total | Highest impact | Peak production hours — shade here hurts most |
| Afternoon (2pm-6pm) | ~30% of daily total | Moderate impact | Still significant production time |
| All day | 100% | Severe impact | Constant shade is the worst scenario |
Seasonal Variation
Shading impact changes throughout the year because the sun’s path across the sky varies:
Winter (Worst)
Sun is low in the sky, casting long shadows. Trees and buildings that don’t shade in summer may shade significantly in winter. However, winter production is lower anyway.
Summer (Best)
Sun is high overhead, shadows are short. Most obstacles have minimal impact. This is also when ~60% of annual production occurs, so summer shading matters most.
Deciduous Trees: A Double-Edged Sword
Deciduous trees (oak, beech, etc.) lose their leaves in winter, which sounds helpful — but remember, winter is when shadows are longest. Even bare branches can cause significant shading when the sun is low.
The good news: in summer when production is highest, leaf cover may provide only dappled shade rather than complete blockage.
Solutions: Optimisers vs Microinverters
If you have shading issues, two technologies can significantly reduce the impact by allowing each panel to operate independently:
Power Optimisers
DC-DC converters attached to each panel. Still uses a central string inverter, but each panel optimises its own output.
- Panel-level optimisation
- Panel-level monitoring
- Works with string inverter
- SolarEdge, Tigo, Huawei
Microinverters
Best for Heavy ShadeSmall inverters attached to each panel. Converts DC to AC right at the panel — no string effect at all.
- Complete independence per panel
- No single point of failure
- Easy system expansion
- Enphase, AP Systems
When Are They Worth It?
| Shade Impact | Recommendation | Why |
|---|---|---|
| 0-10% | Standard string inverter | Losses too small to justify extra cost |
| 10-20% | Consider optimisers | Optimisers can recover 40-60% of losses |
| 20-30% | Optimisers recommended | Clear ROI improvement with optimisers |
| 30%+ | Microinverters essential | Maximum independence needed for viability |
The Economics of Shade Mitigation
Example: A 10 panel solar system with 20% shading losses:
- Annual loss without mitigation: ~680 kWh = ~£165/year
- Cost of optimisers: ~£500-600
- Recovery with optimisers: ~50% = 340 kWh = ~£82/year
- Payback on optimisers: ~6-7 years
With 25-year panel warranties, that’s 18+ years of improved returns after payback.
Other Ways to Reduce Shade Impact
Tree Management
Pruning, crown reduction, or selective removal can be more cost-effective than equipment upgrades. One good trim might save thousands in lost generation over 25 years.
Strategic Panel Placement
A skilled installer can position panels to avoid the worst shading. Sometimes fitting fewer panels in better positions outperforms more panels in shaded spots.
Multiple Strings
Splitting panels into separate strings (shaded vs unshaded) prevents shaded panels from affecting clean ones. Requires compatible inverter with multiple MPPT inputs.
Relocate Small Obstacles
TV aerials, satellite dishes, and some vents can sometimes be moved during installation to avoid shading panels. Usually a minor additional cost.
Frequently Asked Questions
Key Takeaways
- Shading disproportionately affects string inverter systems — one shaded panel can reduce the entire string’s output
- South-facing obstacles have the highest impact; north-facing obstacles matter least
- Midday shading (10am-2pm) is most damaging as this is peak production time
- Winter shading is worse than summer due to low sun angle and long shadows
- Under 10% impact: standard string inverter is usually fine
- 10-25% impact: power optimisers are worth considering
- Over 25% impact: microinverters strongly recommended
- Tree management can be more cost-effective than equipment upgrades
- Always get a proper site survey — satellite images don’t tell the whole story
