Most people don’t realise that is a conventional solar power system if one panel is performing poorly, all the other modules it shares a string with will be dragged down too.
In a conventional system, solar panels are connected together in strings, each string is a number of solar panels connected together in series.
The current starts at A and then moves to all three panels to B. If one is shaded or underperforming, that will restrict the current that can pass through the module. Any restriction in a single panel will restrict the current through all the others.
If that does not make sense, think of the current in the wire as water in a hose. A shaded panel will pinch the hose, that means that all the panels have less water flowing through them.
The practical result of this is that if one panel is shaded or dirty or even defective all the panels will be affected in a similar way.