Why does a fixed energy surcharge never match actual consumption?

A fixed charge per booking is based on average consumption that rarely occurs in practice, because consumption per guest can differ by a factor of two to three. Quiet guests pay too much, heavy users too little, and you as the landlord silently make up that difference. Raising or lowering the amount does not solve this, because the problem lies in the idea of a fixed amount itself. Only actual metering and billing of the real consumption aligns with what a guest actually uses, even if that is not worthwhile for every rental situation.