Your solar monitoring app is the most powerful diagnostic tool you own, yet most homeowners only open it to check today's generation total. Solar monitoring for early warning signs is a different discipline — one that involves looking at patterns, ratios, and anomalies rather than headline numbers. Here are seven specific signals worth knowing.
1. Persistent String Voltage Drop on One Input
Modern string and hybrid inverters report input voltage per MPPT channel. If one string consistently reads 10–20V lower than usual at the same irradiance level, you likely have a failing bypass diode, a corroded MC4 connector, or a degrading cell in that string. This differs from normal shade dips, which track the sun's position and resolve within minutes. A persistent, irradiance-independent voltage deficit points to a physical fault.
What to do: Log three consecutive clear-sky days of readings and report the pattern to your installer. Do not attempt to inspect connectors yourself — DC circuits at string voltage are dangerous.
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2. Morning Ramp That Lags Behind Historical Baseline
Most monitoring platforms (SolarEdge mySolarEdge, Enphase Enlighten, GivEnergy portal) show historical generation curves. If your current morning ramp is consistently slower to reach peak output than the same week in previous years — at matched irradiance — this can indicate early degradation, soiling, or a partial shading obstruction that has grown (a tree branch, new satellite dish, or roof obstruction).
3. Inverter Derating Events
Derating occurs when an inverter reduces its output to protect itself — typically due to excessive heat or a grid voltage excursion. Derating events appear as sudden flat-tops on your generation curve on hot, high-output days. Occasional derating in peak summer is normal for some inverter models. Frequent derating (more than four or five times per summer fortnight) suggests inadequate ventilation around the inverter cabinet, or a firmware issue. Check that the inverter's ventilation clearances match the manufacturer's specifications — typically 200mm above, below, and to the sides.
4. MPPT Channel Mismatch at Matched Irradiance
If your inverter has two or more MPPT inputs and one string is consistently producing 15% less power than another of the same size and orientation under clear sky, you have a mismatch fault. Common causes include: one panel in the underperforming string has an internal failure, a micro-crack that has propagated, or an incorrect string wiring configuration post-installation. On microinverter systems, this shows as one module persistently underperforming its neighbours.
5. Unexplained Overnight Battery Drain
If your system includes a battery and you notice the state of charge falling during the night faster than your monitored loads account for, this points to a parasitic draw — either a misconfigured export setting (the inverter is exporting stored energy to grid overnight during a tariff window you've forgotten), or an auxiliary load connected to the battery output that isn't captured in your monitoring setup. Check your inverter's export schedule against your current tariff.
6. Export Power That Exceeds Configured DNO Limit
Most grid-connected residential systems in the UK are G98 or G99 connected with a defined export limit — commonly 3.68 kW single-phase. If your monitoring shows export regularly hitting exactly this cap but generation continues to rise above it (your self-consumption doesn't account for the gap), the inverter is clipping correctly but you may be wasting yield. This is a design issue rather than a fault, but it's worth discussing battery storage sizing with an installer if you're seeing regular clipping of more than 10% of daily generation.
7. Performance Ratio Drift Over Multiple Months
Performance Ratio (PR) is the ratio of actual system output to what PVGIS predicts for your location and system size. A healthy residential system runs at PR 75–85%. Most monitoring platforms calculate this automatically. A PR that drops more than 5 percentage points year-on-year — after correcting for irradiance — indicates cumulative degradation beyond the manufacturer's warranted rate (typically 0.4–0.7% per year for TOPCon and HJT panels). Flag this to your installer, as it may trigger a warranty claim.
Key Takeaways
- String voltage drops that are persistent and irradiance-independent are a clear sign of a physical fault, not shading.
- Inverter derating more than five times per fortnight in summer usually means a ventilation or firmware problem.
- MPPT mismatches between equally sized, same-orientation strings indicate a panel-level failure in the underperforming string.
- Check Performance Ratio annually against your PVGIS baseline — a 5+ percentage point drop warrants investigation.
- Overnight battery drain beyond metered loads points to misconfigured export schedules or an unmonitored parasitic load.
Spotting these patterns early means smaller repair bills and more generation over your system's lifetime. Use Comparisun to find accredited installers who offer ongoing monitoring review as part of their maintenance packages.