Short answer: The most common causes are a failed or tripped inverter, a tripped breaker in the consumer unit, a monitoring app that has lost Wi-Fi, or a normal winter drop. Check the inverter screen for an error code and the solar breaker in your fuse box. Don't go on the roof or touch the DC cables.
Safety first
Solar panels generate DC electricity whenever light falls on them, and it can’t be switched off at the fuse box. So:
- Stay off the roof.
- Don’t unplug, cut or touch the DC cables between the panels and the inverter.
- If anything smells of burning, shows scorch marks or crackles, keep people away and call 999 if there is smoke or fire.
Everything below can be checked from the ground, inside the house.
Check 1: the inverter
The inverter is the box, usually in the loft, garage or under the stairs, that turns the panels’ DC power into household AC. It is the part most likely to fail. The Energy Saving Trust says it needs replacing after around 12 years, while panels last 25 years or more.
Look at the screen or lights in daylight:
- Normal or green, with power showing: the system is generating. The problem may be the monitoring, or the output may just be low for the season.
- A fault or error code, or a red light: write down the code and the wording exactly. It tells an engineer where to start.
- Completely blank in daylight: the inverter has lost power from one side, or has failed.
- Beeping: usually an alarm. Note the code on the screen.
Some inverters switch off for a short time when the local grid voltage runs high, often on sunny afternoons, and show a grid or voltage message. If it happens now and then and clears itself, note when it happens. If it happens often, call someone out.
Check 2: the breaker and the AC isolator
In your consumer unit there should be a breaker labelled for the solar system. If it has tripped, you can switch it back on once. If it trips again, leave it off and call an engineer, because something is causing it.
There is usually an AC isolator switch on the wall near the inverter or the consumer unit. Check it hasn’t been knocked off.
Check 3: is it the monitoring?
If your app says the system is offline or showing zero, look at the inverter before assuming the worst. A new router, a changed Wi-Fi password or a weak signal in the loft can cut the monitoring while the system carries on generating. If the inverter shows power, the panels are fine.
Check 4: is it just the season?
Solar output in Birmingham swings a long way through the year. These are PVGIS figures for each kWp of panels on a south-facing roof:
| Month | kWh per kWp | Month | kWh per kWp |
|---|---|---|---|
| January | 37 | July | 119 |
| February | 51 | August | 106 |
| March | 85 | September | 91 |
| April | 107 | October | 62 |
| May | 118 | November | 43 |
| June | 116 | December | 34 |
Multiply by your system size to get a rough guide. A 4 kWp system making around 140 kWh in December is doing what it should. The fair test is the same month in an earlier year.
Other causes of a slow decline
- New shade: trees grow, and a neighbour’s extension can change the afternoon sun.
- Dirt or moss: rain cleans most pitched roofs, but not shallow ones. Solar panel cleaning covers when it helps.
- Pigeons: nesting under the array can damage cables. Bird proofing stops it.
- A failing panel or connector: one bad panel can pull down others wired with it.
When to call someone
Call if the inverter shows a fault code, the breaker keeps tripping, the inverter is blank in daylight, or output is clearly lower than the same month last year. Have the error code, the inverter make and roughly how old the system is. Solar panel repair explains how faults are traced, and solar inverter replacement covers the most common fix.
Bottom line
Start with the inverter screen and the solar breaker, check the monitoring isn't just offline, and compare the month's output with what's normal for the season. If the inverter shows a fault, the breaker keeps tripping or output has dropped for no reason, call us with the error code. We repair systems from any installer. Call 0121 330 1151, 9am to 7pm, 7 days.