How to Test Solar Panels: Checking Your System Is Working Properly

Solar panels are largely hands-off once installed, but they're not maintenance-free. Dust, shade, ageing inverters, and loose connectors can quietly erode output — sometimes by 20% or more — before you notice on your energy bills. These six checks give you a structured way to confirm your system is performing as it should, catch problems early, and know when to call in a professional.

1. Check Your Monitoring App Against Expected Output

Your monitoring app is the fastest health indicator. Every modern inverter (SolarEdge, SMA, Solis, Fronius, GivEnergy) ships with a companion app or online portal that logs daily, monthly, and annual generation in kWh. Compare what you actually generated against what the system was estimated to produce.

The most reliable benchmark for UK sites is PVGIS (Photovoltaic Geographical Information System), the European Commission's free irradiance database. It gives month-by-month expected output for your location, panel tilt, and orientation. Enter your system's peak capacity (e.g. 4 kWp) and compare the PVGIS monthly figure against your app's logged kWh. A shortfall of under 10% on a sunny month is normal; a consistent gap of 20% or more in good weather warrants investigation. See our full walkthrough at how to use PVGIS for your solar system.

Most apps also show individual panel or string performance if you have optimisers or microinverters fitted. A single panel reading significantly below its neighbours is a clear flag for the next step — visual inspection.

2. Read the Inverter Display and Fault Codes

Your inverter speaks in codes — learning a few common ones saves unnecessary call-outs. Most inverters show status via a combination of LED colours and a display screen or app notification:

• Green steady LED — normal operation, generating power.
• Amber/yellow flashing — warning; often a temporary grid voltage excursion.
• Red LED or fault code on screen — active fault requiring attention.

Common codes by brand:

• Fronius State 102 — AC voltage too high (grid above ~253 V). The inverter briefly disconnects for safety. If it self-clears within minutes, it is a grid issue, not a panel fault. Persistent occurrences should be reported to your DNO.
• SolarEdge "Grid Voltage Too High / Too Low" — same cause as above; logged in the monitoring portal with a timestamp.
• Solis F01 / F05 — isolation fault (leakage detected in the DC wiring). Do not ignore these: they can indicate damaged cable insulation or moisture ingress and represent a potential safety hazard. Contact an MCS installer promptly.
• SMA "Insulation Error" — equivalent to an isolation fault; professional diagnosis required.

For a full fault-code index by brand, Solar-Tech-Support maintains a detailed reference. Isolation faults should never be left unresolved — they can indicate a genuine electrical hazard.

3. Visual Inspection: Panels, Cables, and Mounting

A ground-level inspection with binoculars takes ten minutes and costs nothing. Look for:

• Cracks or hot spots — dark discolouration, spiderweb micro-cracks, or bubbling laminate on the panel surface.
• Soiling — heavy bird fouling, moss, or general grime concentrated on one area can reduce output from individual cells. Light uniform dust typically clears with rain; concentrated soiling needs cleaning. See our guide to solar panel maintenance and cleaning.
• Shading — has a tree grown since installation? A new extension or satellite dish casting shadow across even one cell of a panel in a string can drag down the whole string's output significantly.
• Loose or damaged cables — MC4 connectors should be fully seated and dry. Hanging or exposed cable runs are a fire risk and a maintenance finding.
• Mounting rails and fixings — look for lifted flashing, displaced ridge tiles, or any sign the mounting system has shifted.

According to Solar Energy UK's O&M best-practice guidance, physical inspection of panel condition and reporting damaged or broken panels is an owner-occupier task; roof access and thermal imaging of internal components should only be performed by a competent person.

4. Basic Multimeter Test: Measuring Voc and Isc

A multimeter check gives you a direct electrical reading of a panel's health — but carry out this test only on panels you can access safely without going on the roof, such as a ground-mounted array or a panel removed for a separate reason.

What Voc and Isc mean

Voc (open-circuit voltage) is the maximum voltage a panel produces when no current is flowing — i.e., with the circuit open. Isc (short-circuit current) is the maximum current the panel can deliver when the output terminals are shorted. Both figures are printed on the panel's datasheet label (usually on the rear of the panel) and are measured under Standard Test Conditions (STC: 1,000 W/m² irradiance, 25°C cell temperature). Real-world readings will be lower on a typical UK day.

Equipment and safety

• Use a CAT III 600 V (or higher) digital multimeter. Standard CAT II household meters are not rated for PV DC voltages — a typical 4-panel string produces 120–160 V DC even in overcast conditions.
• Wear insulated gloves.
• Isolate the panel from the rest of the system before testing; disconnect it from any other panels or from the inverter.
• Never test a roof-mounted, connected string from the inverter end without proper isolation equipment and training.

Voc test procedure

1. Set the multimeter to DC voltage (V⎓) on a range above the panel's rated Voc (e.g. 50 V range for a panel rated 40 V Voc).
2. Touch the red probe to the positive terminal, black probe to the negative.
3. Record the reading. In full UK summer sun, expect 90–100% of the datasheet Voc. In overcast conditions, expect 70–85%.

Isc test procedure

1. Set the multimeter to DC current (A⎓). Move the positive lead to the current input jack.
2. Touch probes to the panel terminals as before.
3. Take the reading quickly — sustained short-circuit current can stress some meters.
4. Isc scales directly with irradiance: on a bright UK summer noon, expect ~85–95% of the datasheet Isc figure. A reading below 70% of the datasheet figure in direct sun is a meaningful underperformance signal.

If measured values are significantly below expectations, compare across multiple panels in the same conditions. A consistently low Voc on one panel relative to its neighbours suggests a cell-level fault or shading; a low Isc points to soiling, shading, or degraded cells. For system-level output data, see our guide on understanding solar panel output.

5. When to Call an MCS-Accredited Installer

Some findings must be handed to a professional — don't delay on these. Contact an MCS-accredited installer if you observe any of the following:

• Persistent isolation fault codes (Solis F01/F05, SMA "Insulation Error") that do not clear after the system has run dry for 24 hours.
• Physical damage to panels — cracks, delamination, or burn marks.
• Sustained underperformance of more than 20% against a PVGIS benchmark in comparable weather across multiple months.
• Any fault code you cannot identify or that does not self-clear within one day.
• Loose, damaged, or exposed DC cabling anywhere in the array.

MCS certification means the installer has met the UK's Microgeneration Certification Scheme technical and quality standards. You can verify an installer's current MCS status at the MCS certified installer finder on mcscertified.com. The RECC (Renewable Energy Consumer Code) also provides consumer guidance and a dispute resolution service for RECC-member installations.

6. How Often to Check Your System

A simple schedule keeps your system performing well over its 25-year life.

• Monthly (5 minutes): Open your monitoring app and compare the month's kWh generation against the same month's PVGIS estimate. Flag any month where actual output is more than 15% below expected for your weather conditions. Most solar monitoring systems can send automatic alerts if daily generation drops below a threshold — enable this feature.
• Quarterly (10 minutes): Ground-level visual check with binoculars — soiling, shading, obvious panel damage, inverter LEDs all green.
• Annual: Full visual inspection of panels, mounting, and cables (by a competent person if roof access is needed); check inverter firmware is up to date via the app; review generation data for year-on-year trends. Solar Energy UK's O&M guidance recommends an annual inspection as best practice for domestic rooftop systems.

Most inverter warranties are 5–12 years; panel performance warranties typically guarantee at least 80% of rated output at 25 years (LID-adjusted). Keeping annual generation logs makes it straightforward to support any warranty claim if output degrades beyond the warranted level.