Solar panels not generating

What it usually means

A solar installation is a generator that has to follow the grid rather than lead it, and that single design requirement explains a large share of what looks like failure. The inverter takes direct current from the roof and converts it to alternating current synchronised with the mains — and it is required to stop doing so if the mains goes outside defined limits for voltage or frequency, or disappears altogether. It cannot legally or safely keep exporting into a network that engineers may be working on. So an inverter that shuts down for a few minutes in the middle of a bright afternoon is usually reporting the grid's behaviour rather than its own, and clusters of these on the same street at the same time make the case conclusively.

Before diagnosing anything, separate three situations. An inverter with no display and no lights at all is off: check its own alternating-current isolator, which is a switch near the unit, and any breaker labelled for it in the consumer unit, because these get switched off for other electrical work and forgotten. An inverter that is lit and reporting a fault or waiting to connect is a different case. And an inverter that is running perfectly while producing less than you hoped is not faulty at all, which is the most common of the three.

That last case deserves plain arithmetic. The kilowatt figure on your system is a laboratory rating measured at a light level a British afternoon rarely delivers, at a panel temperature well below what a roof reaches in July. Real output follows the season steeply: a system generating thirty units on a June day may manage three in December, and that is the system working. Panels also lose output when hot, so peak generation is often a bright cool morning rather than a hot afternoon. And shading is not proportional — one chimney shadow or one grown tree across part of a string can cut its output far more than the shaded area suggests, because the panels in a string are in series.

The measurements worth taking are cheap. Compare your generation meter reading, if you have one, against the app rather than trusting only the app, because a monitoring system that has lost its internet connection shows zero while the roof is working perfectly. Look at the panels for soiling, moss along the bottom edge, or lichen, all of which build up over years. And note the time of day the problem appears, because faults that only occur in the morning, when panels are wet with dew, are characteristic of insulation resistance problems — moisture finding a path where it should not, which some inverters report specifically and which is one of the genuine faults on this list.

One thing to be clear about regardless of the diagnosis: there is no switch that stops a panel generating. Turning the inverter off makes the alternating-current side safe and leaves the direct-current wiring from the roof live at several hundred volts for as long as it is daylight.