Why solar panels stop working, and how to work out which one it is

Work from the meter outwards

A solar installation that looks dead is usually one of five things: normal output in a month that feels wrong, monitoring that has dropped off while the hardware carries on, an inverter that disconnected because of something on the grid, a protective device that opened on the AC side, or a real fault on the array. Those five cost wildly different amounts to put right, and four of them can be separated from the ground without touching anything.

What the fault families actually mean

Manufacturers number their codes differently and the number alone tells you little. The family it belongs to tells you almost everything, including whether it is safe to clear.

Isolation faults trigger when resistance between the DC circuits and earth falls to roughly 1 megohm, against 20 to 30 megohms on a healthy circuit even in soaking wet conditions.

Isolation and arc faults are the two to treat differently. Both point at the DC wiring and both carry a fire risk. SolarEdge's own arc fault installation guide tells the user to disconnect the system from power and contact a qualified technician rather than attempt a restart. Clearing a safety stop three times a week is not a repair, it is switching off a protection device in instalments.

Grid faults are often not your system

Every grid connected inverter in Great Britain is type tested against an engineering recommendation, G98 for smaller domestic systems and G99 above that, and it carries interface protection settings it is not permitted to ignore. When the supply leaves that window the inverter disconnects. Nothing has broken.

Source: ENA Engineering Recommendation G99 Issue 2 (2025), Table 10.1, low voltage settings with 230 V as the reference. The same values appear in G98 type test reports for domestic inverters. Reconnection cannot begin until voltage and frequency have been back inside the stage 1 limits for at least 20 seconds.

Over voltage is the one that catches UK homes. Your network operator has to hold the supply inside the range set by the Electricity Safety, Quality and Continuity Regulations 2002, which permits 10 per cent above and 6 per cent below a declared 230 volts, so 253 volts at the top and 216.2 volts at the bottom. Solar pushes local voltage up while it exports, so a street carrying a lot of generation on a long cable run can sit near that ceiling on a bright afternoon. The inverter does not trip until 262.2 volts, which is above the statutory limit, so repeated over voltage trips are evidence the supply is already outside what it should be. Log the times and take it to your network operator rather than your installation team. What a DNO is tells you how to find yours, and G98 vs G99 covers which rules your installation was connected under.

Winter output is not a fault

The most common false alarm is a system running perfectly in the wrong month. Modelled in PVGIS for a 1 kWp south facing array at 35 degrees in central England, the year is deeply lopsided.

PVGIS v5.2, 1 kWp south facing at 35 degrees tilt, 52.5N 1.5W, 14% system losses, annual total 983 kWh. Accessed 20 August 2026. Your roof will differ by orientation and pitch, but the shape of the year will not.

July produces roughly three and a half times what December produces from the same roof, so a December figure that looks alarming next to a summer one is usually just December. The comparison that means something is this month against the same month last year. A slow slide of a few per cent across a decade is the degradation curve behaving as the datasheet says, covered in panel degradation and inverter life. A step change inside a week is physical, and how cloud affects output handles the rest of the weather question.

Shade that arrived slowly

Shading rarely appears overnight, which is why it gets missed. A sapling planted the year the panels went up is a real problem eight years later, a neighbour's extension quietly removes an afternoon, a new aerial or flue throws a thin hard shadow that tracks across the array, and moss creeping up from the gutter line takes the bottom row first.

What makes shade expensive is the way a module and a string handle it. A conventional 60 cell panel is divided into three groups of 20 cells, each with its own bypass diode. Shade part of one group and the diode shunts that whole group, so the module can lose around 30 per cent of its output to a shadow the size of a hand. On a plain series string every module carries the same current, so the weakest one sets the pace.

• List what has grown, been built or been fitted since commissioning, and check it at the times of day the array now underperforms.
• Look at the roof from the ground at 9am, midday and 4pm on a clear day. A shadow absent at noon can still cost you two hours either side of it.
• Moss, lichen and bird mess concentrate on the lowest row because it sits nearest the gutter, and that row is often the one dragging a string down.
• Optimisers and microinverters contain the damage to the shaded module instead of letting it set the pace, which is the real argument for them on a complicated roof.

One module down, or a whole string

If your system reports per panel, the answer is on the screen: one module or optimiser showing nothing while its neighbours look normal is a device failure, and it costs a fraction of what most people fear. If it does not report per panel, work from the shape of the loss. A drop of roughly half on an inverter with two trackers usually means one string has gone, more often at a connector or an isolator than at a panel. A persistent few per cent in good light points at one module or at soiling. A drop to zero with a grid state showing is a supply question. Servicing and repair costs covers what each fix is worth paying.

What is safe to touch, and what is not

• Safe. Reading the inverter display and the generation meter, checking the monitoring app, resetting a tripped breaker or RCD at the consumer unit once, and looking at the array from the ground with binoculars.
• Not yours. The DC isolator, the plugs and connectors, anything underneath the panels, the inverter enclosure, and the roof itself.
• Never, at any skill level. Opening a DC connection under load. A DC arc does not self extinguish, and the array cannot be turned off while there is daylight on it.
• Not a DIY job legally either. Solar PV installation is notifiable work under Part P of the Building Regulations in England and Wales, which is why it is signed off by a registered competent person.

Who to call, and what to have ready

Start with the company that installed it, whatever you currently think of them, because the workmanship warranty sits with them and so does the obligation to return. If they have stopped trading, your route runs through the paperwork instead.

• The MCS certificate, carrying the installation's unique number, your address and the system details. The MCS certificate guide covers how to get a copy if yours has gone missing.
• The inverter make, model and serial number, photographed from the label rather than quoted from memory.
• The fault code, date and time, plus how often it repeats and whether it clears on its own.
• Twelve months of monitoring history, so the drop can be dated rather than described.
• The grid connection paperwork, meaning the G98 notification or the G99 approval, which matters the moment the fault looks like a supply problem.
• Any insurance backed guarantee and the consumer code your installation team belonged to, set out in insurance backed guarantees and consumer protection schemes compared.

Clearline never carries out installations. Work is done by an MCS-certified installation team for your area, and the job here is to tell you what is worth paying for before you pay for it. The solar panels hub covers specification and sizing, and our county pages each carry a calculator built for that area.

Grid protection settings, statutory voltage limits and PVGIS output figures checked 20 August 2026. Fault code names and numbers vary by manufacturer. None of this replaces an inspection by a qualified electrician, and anything involving the DC side or the roof is their work rather than yours.