Problems & fixes · Solar panels
Why a solar system may be producing less than expected
Last updated 25 October 2026 · 1 source listed · Reading time about 3 minutes
Short answer
Underperformance usually traces to weather variation against the estimate, shading that grew, soiling, inverter faults or tripping, a communication fault that hides data, or design assumptions that were optimistic. Compare like-for-like seasons, check inverter status and error codes, inspect for dirt or shade, and request the installer's modelled output and commissioning report.
Who this is for: An owner of an existing rooftop system who sees lower generation than the installer's estimate and wants to diagnose why.
What to evaluate
Weather baseline
A cloudy season can legitimately cut output without any fault.
How to check: Compare against the same months in prior years or a local irradiance dataset.
Inverter status and logs
Inverters record faults, grid trips and derating events.
How to check: Read the display or app for error codes and ask the installer to interpret them.
Shading changes
Vegetation and new structures can grow into the array's path over time.
How to check: Inspect the roof view at several times of day and compare to the original shade analysis.
Soiling and obstruction
Dust, bird droppings, leaves or snow reduce output on affected cells.
How to check: Look from ground level or use a safe viewing method; never climb unsafely.
Grid voltage and export limits
High local voltage can make inverters throttle or trip.
How to check: Check logs for voltage-related events and ask the network operator about local conditions.
Original design assumptions
An optimistic yield estimate sets expectations that the site cannot meet.
How to check: Request the design report with tilt, azimuth, loss factors and compare to the actual arrangement.
Comparison
| Cause | Sign | First step |
|---|---|---|
| Inverter fault or trip | Zero output or error code | Read the code and contact installer under warranty |
| Shading growth | Output dips at specific hours | Trim vegetation or review layout |
| Dirty panels | Gradual decline between rains | Safe cleaning per manufacturer guidance |
| Monitoring failure | Data gaps while system works | Check network connection and reset gateway |
Decision worksheet
Rate up to three options against this guide’s own criteria. It runs in your browser and saves only on this device. It is a thinking aid, not a recommendation.
1. Have you checked each criterion?
0 of 6 criteria checked.
2. Compare options (0 = fails or unknown, 3 = meets it well)
| Criterion | |||
|---|---|---|---|
| Weather baseline | |||
| Inverter status and logs | |||
| Shading changes | |||
| Soiling and obstruction | |||
| Grid voltage and export limits | |||
| Original design assumptions | |||
| Total (max 18) | 0 | 0 | 0 |
Questions to ask before you commit
- What was the modelled annual output and which assumptions did it use?
- Can you read the inverter logs remotely?
- Does my warranty cover performance shortfalls and how are they measured?
- Who is responsible for inspecting the array on site?
- Is the system tripping due to grid voltage?
- Are panels and strings measured individually?
- What cleaning method is safe and does it affect warranty?
- Is there a commissioning report with initial test results?
Typical trade-offs
Pay for professional inspection
- Upside
- Finds hidden faults such as connector damage
- Downside
- Call-out cost if nothing is wrong
Add module-level monitoring
- Upside
- Pinpoints weak panels
- Downside
- Extra hardware and cost
Trim or remove shading vegetation
- Upside
- Restores output
- Downside
- Possible disputes with neighbours or planning limits
Common mistakes
- Comparing a winter month with the annual estimate rather than the matching season.
- Cleaning panels with abrasive tools or high-pressure water against the maker's guidance.
- Ignoring a persistent inverter warning because the lights stay on.
- Climbing onto a roof for inspection without proper safety equipment.
- Waiting beyond the warranty window before raising a performance issue.
Related questions
How do I know whether a lower output is normal?
Compare the same period in earlier years and note differences in weather. A single dull month says little. A drop that persists across comparable conditions, or sudden step changes, suggests a fault. Many monitoring apps show daily patterns, so look for flat tops, early cut-offs or zero days. If in doubt, ask the installer to compare against modelled expectations.
Is cleaning the panels necessary?
In many climates rain keeps panels adequately clean. In dusty, agricultural or bird-heavy environments, soiling can matter. Follow the manufacturer's cleaning guidance, avoid abrasive materials, and prioritise safety, since roof work is hazardous. A professional cleaner with proper access equipment is often safer than doing it yourself.
Why does my inverter show an error code?
Codes indicate conditions such as grid voltage out of range, insulation faults, overheating or communication loss. Some are transient, others need a technician. Look up the code in the inverter manual, note when it appears, and send logs to the installer. Do not open the inverter yourself; there are hazardous voltages.
Does the warranty cover underperformance?
Performance warranties usually guarantee panel output over time, not the overall system yield, and have specific claim procedures. Workmanship warranties may cover installation defects. Read the terms to see what is measured and by whom, and raise issues in writing as early as possible.
Sources and further reading
These pages were reachable when we last checked. They informed definitions and rules referred to above; they do not endorse this guide. Where rules differ by country, check the authority for your location.
- Homeowner's guide to going solar — U.S. Department of Energy, retrieved 27 October 2026
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