PV Systems: The Most Expensive Planning Mistakes

Rules of thumb instead of a calculation, an inverter that doesn't fit, shade nobody saw, and a grid connection applied for too late. That costs money on every project. The larger the array, the more expensive the mistake.

From more than 2,000 projects: the gap between an average array and one that's calculated properly is often 15 to 20 percent of yield. At the same investment.

Self-consumption is set too low, the modules face the wrong way, the battery is missing, the grid connection comes too late. On commercial sites and open land, small mistakes add up fast.

If you don't apply for the connection early, you can easily wait six months or more.

PV and storage on the same connection no longer have to meter solar electricity separately from grid supply. Since 1 October 2026 mixed operation is available: the delineation option for an exact split, and the flat-rate option for solar systems up to 30 kWp, including a bidirectional charge point. Until 30 September 2027 only with the consent of the grid operator and the metering operator. The flat-rate option only after EU state-aid approval. Both apply to the market premium and direct marketing, not the fixed feed-in tariff.