One patch of shade costs more than you would think

People tend to think about solar in terms of roof faces: south is good, north is useless, the rest is doubtful. Reality is both kinder and harsher — kinder about how much sun you need, harsher about what shade does.

Dutch consumer authority Milieu Centraal is reassuring on the first question: if the sun reaches a given roof section for at least half the day, you can often fit panels there perfectly well. Writing off a roof section because it sits in shade in the morning is premature.

The second question is where it goes wrong. Milieu Centraal states that shade on one of the panels reduces the yield of the whole system in most set-ups. That is the counter-intuitive part: it is not about what percentage of your roof lies in shade, but whether shade falls at all. A chimney, a dormer, an aerial or a tree that has grown for ten years costs more than its surface area suggests.

In practice: on a sunny day walk past your panels three times — morning, midday, late afternoon — and photograph the roof. If you see a hard shadow line crossing a panel, you know where your yield is going. Dirt works the same way but can be removed: a cleaning visit costs € 75-175. If yield lags with no visible cause, a yield check (€ 85-165) shows whether it is shade, dirt or a technical fault. Every rate is in the price index.