3 August 2026

Disappointing yield: dirt, shade or a fault?

Your app shows less than last year. How to read from the pattern where to look, without going up on the roof.

The symptom is nearly always the same: the monitoring app shows fewer kilowatt-hours than you are used to. But "less" has a pattern, and that pattern gives away the cause. So first compare the same month with a year earlier: an overcast August is not a fair comparison with a clear-blue July.

Three causes explain most dips. Soiling shows as a gradual decline over weeks or months, across the whole system at once: the glass gets dirtier slowly, so the curve sinks slowly with it, often most clearly after pollen season or a long dry spell. Shade shows as a dip returning around the same time every day, or appearing in a particular season: a tree that has grown taller, the neighbours' new dormer, or a chimney that suddenly falls across the panels in winter when the sun is low. A fault shows as a sudden break: sharply less from one day to the next, an inverter dropping out or an error message on the display, or, if you have per-panel monitoring, one panel structurally lagging the rest: that points to a failing optimizer or localised soiling such as a splat of bird droppings.

What can you do yourself, with both feet on the ground? More than you think. Read the inverter and write down any error code literally, with date and time: the professional can use that straight away. Put the day curves of a sunny day now and a sunny day last year side by side in the app and see where the curve deviates: lower all day points to dirt, a bite out of the morning or afternoon points to shade. And look at the panels from the street or a roof window: a dirt edge, green haze or obvious droppings are often visible from below.

What you do not do: climb onto the roof to "take a quick look", and certainly not tinker with the inverter, optimizers or cabling: that is work for someone with fall protection and electrical knowledge. If you cannot settle it from the ground, a yield check is the logical next step: the professional inspects the panels visually, reads out the inverter and compares actual yield with what your system should deliver. You receive a report with photos and advice: clean, repair or do nothing, and many companies offset the € 85 to € 165 inspection fee against a follow-up job.

If soiling is the verdict, a cleaning visit costs € 75 to € 175 for an average home; if it turns out to be a loose cable or failing optimizer, minor repairs fall under € 95 to € 250. All rates are listed in the price index.