Solar PV systems are reliable, but performance problems do occur - and most of them show up in your monitoring data before they become serious. Knowing what to look for saves you money in lost generation and, in some cases, avoids a more expensive failure down the line.

What Your Monitoring Data Shows

Most modern inverters include a free app that logs daily generation, inverter status, and often individual string performance. The three numbers to track are:

Daily generation (kWh). Compare against the same date in previous years. A 10% year-on-year drop in a settled period of similar weather warrants investigation. A 30% drop needs attention now.

Performance ratio. This is actual output divided by expected output under your local conditions. A healthy system sits above 75-80%. Consistent decline over several months - more than 2-3% annually - points to a developing problem rather than weather variation.

String voltages and currents. Systems with multiple strings (circuits of panels) should show similar readings across strings. A string running significantly lower than the others has a fault.

Gradual Performance Decline

A slow, steady drop in output over weeks or months is usually one of three things.

Soiling. Dust, pollen, and bird droppings accumulate on the panel surface and reduce light reaching the cells. This is common in spring and during dry spells. A generation dip that recovers after rainfall confirms it. Clean panels two or three times a year - a soft brush and water is enough, or hire a professional for roof-mounted systems.

Shading from new growth. Trees that weren't an issue at installation can grow to shade panels within a few years. Check whether generation drops at the same time of day consistently - that pattern points to a shadow falling across a specific panel group. If one panel in a string is shaded, it drags down the whole string's output.

Natural degradation. Panels lose output at around 0.3-0.5% per year for quality monocrystalline panels. Over 10 years that's a 3-5% reduction - noticeable in the data but not a fault. If your system is declining faster than this, something else is causing it.

Sudden Production Drops

An abrupt fall in generation - one day the system produced normally, the next it's significantly down - points to a specific fault rather than gradual wear.

Inverter failure. A complete shutdown of generation with an error code on the inverter display is the most common sudden fault. Most inverters display fault codes that your installer can interpret. Don't attempt to open or repair an inverter yourself.

String fault. If your system has multiple strings and generation drops by roughly half, one string has likely failed. Common causes are a loose DC connector, a faulty bypass diode in a panel, or a blown string fuse. An electrician with solar experience can trace and fix these quickly.

Protection device tripping. Your system includes safety devices - ground fault protection, arc fault protection, and AC isolation - that shut the system down if they detect an abnormal condition. A tripped protection device needs a qualified engineer to investigate before resetting, because the protection activated for a reason.

Grid supply issues. Inverters shut down if grid voltage or frequency falls outside acceptable limits, which happens during grid faults. If your inverter went offline at the same time as a local power cut and restarted normally afterward, no fault is likely.

Irregular Daily Patterns

A system that generates well some hours but poorly at others - and it's not explained by weather - points to intermittent shading or a specific panel fault.

Micro-cracks in solar cells reduce output without being visible to the naked eye. Thermal imaging during a professional inspection reveals them. Hot spots - localised overheating in a cell or junction box - show the same pattern and can eventually cause panel failure. Both require a professional to diagnose.

If your monitoring shows one string consistently underperforming compared to others, that's either a wiring or connector issue, a failed bypass diode, or a panel problem in that specific string. Individual panel-level monitoring (available with microinverters or power optimisers) makes this much easier to pinpoint.

DIY vs. Professional

Sort yourself: cleaning panels, checking the app for error codes, verifying the inverter display shows no faults, checking the AC isolator switch is on, resetting the inverter per the manufacturer's instructions after a power cut.

Call a professional: anything involving wiring or DC connectors, persistent error codes, any tripped protection device, physical panel damage, inverter replacement, and any situation where you're not sure what you're looking at. DC solar systems carry live voltage even when the inverter is switched off - the panels are always generating when light hits them.

Monitoring Routines That Actually Help

Weekly: Open the app. Check yesterday's generation against a similar recent day. If there's a significant unexplained gap, note it.

Monthly: Compare this month's total against the same month last year. Adjust mentally for weather differences - a memorably cloudy July will show lower generation than a sunny one. A consistent 15%+ underperformance on similar weather months is worth investigating.

Annually: Most monitoring platforms let you export a year's data. Review the annual total against the year before. Your installer may offer a performance review - useful if you've had any equipment changes or noticed unexplained dips.

When Monitoring Data Doesn't Tell the Full Story

Monitoring data tells you something is wrong; it rarely tells you exactly what. A string showing lower voltage might be a faulty connector, a bad panel, or a poorly routed cable picking up interference. Getting from "this string is underperforming" to "this specific connector is corroded" requires physical inspection, sometimes thermal imaging, and a qualified engineer.

If your system is underperforming and you can't find an obvious cause, document the monitoring data with dates and share it with your installer when you call. A clear record of when the problem started and how severe it is cuts diagnostic time significantly.

FAQ

My inverter is showing an error code. What should I do? Note the code, check your inverter manual or manufacturer's website for its meaning, and call your installer. Some codes resolve on their own after a reset; others indicate a fault that needs investigation before the system restarts.

Generation is lower than I expected. Is something wrong? Compare against last year's data for the same period rather than against the system's theoretical maximum. If you don't have last year's data yet, check whether the shortfall tracks with cloudy weather in your monitoring app. A genuine underperformance issue shows up consistently across multiple similar-weather days, not just one bad week.

Can I clean my panels myself? For ground-mounted or easily accessible flat-roof panels, yes. Use a soft brush or low-pressure water and avoid abrasive materials. For panels on a pitched roof, the fall risk isn't worth it - hire a professional.

How often should my system be professionally inspected? Every five years is a reasonable baseline for a healthy system. If you've had any significant weather events (hail, storms), noticed unexplained performance drops, or the system is approaching 10 years old, bring that forward.

My system stopped generating completely. The inverter has no error lights. What's wrong? Check the AC isolator switch on the wall near the inverter first - it sometimes gets switched off accidentally. Then check your consumer unit for a tripped breaker. If both look normal and the inverter display is completely dark, the inverter may have failed or lost power. Call your installer.

Contact us to discuss a performance review or maintenance visit. We'll respond within one working day.