Why Is My Plug-In Solar Output So Low? 8 Common Causes

Getting less power than expected? These 8 issues account for 90% of low output from plug-in solar kits in the UK.

8 Reasons for Low Solar Output

1. Shading

The #1 cause of underperformance. Even partial shading from a chimney, tree branch, or neighbouring building can halve output. Microinverters handle shading better than string inverters, but any shade is bad.

Fix: Observe your panels throughout the day. Move or trim shade sources if possible.

2. Wrong Panel Angle

Panels at the wrong tilt angle lose output. A wall-mounted panel (90°) produces ~30% less than one at the optimal 30-35°.

Fix: Use adjustable brackets to achieve the right angle for your latitude.

3. Dirty Panels

Dust, bird droppings, pollen, and tree sap reduce output by 5-25%. See our cleaning guide.

Fix: Clean panels every 2-3 months with plain water and a soft brush.

4. Cloudy Weather

UK weather is variable. A week of heavy cloud can reduce output by 70-80%. This is normal — don't compare cloudy-week output to clear-sky estimates.

Fix: None needed. Weekly and monthly averages will align with PVGIS estimates.

5. Orientation (Not South-Facing)

East or west-facing panels produce 15-25% less than south-facing. North-facing panels lose 40-50%.

Fix: Reposition panels south-facing if possible. Otherwise, accept the lower output as normal for your orientation.

6. High Temperature

Solar panels lose ~0.3-0.5% efficiency per °C above 25°C. On a 35°C summer day, output drops by about 3-5%.

Fix: Ensure airflow behind panels. Avoid mounting flat against a wall with no gap.

7. Cable Losses

Long DC cable runs lose power as heat. A 20m run with thin cables can lose 3-5% of output.

Fix: Use appropriately rated cables — 4mm² for runs under 10m, 6mm² for longer runs.

8. Inverter Limiting

If your panels produce more than 800W DC, the inverter clips output to 800VA. This isn't a fault — it's the legal limit.

Fix: No fix needed — this is normal and expected. The excess DC capacity improves performance in low light.

How do I know what my output should be?

Use our calculator with your exact postcode to get a PVGIS-backed estimate. Compare this to your actual monthly totals.

When should I contact the manufacturer?

If your output is consistently 30%+ below PVGIS estimates after accounting for shading and orientation, contact the manufacturer for warranty investigation.