Optimisers can improve how photovoltaic elements that work under different conditions interact. However, they cannot bring light under a chimney, a tree or a neighbouring building. In an integrated roof, a sensible layout of the active areas is still the most important thing. Electronics should complement the design, not justify installing cells in a place that remains shaded for much of the day. A shaded section receives less radiation and cannot generate as much energy as a similar section in full light. How this affects the other elements depends on their construction and how they are connected. That is why two apparently similar shading situations can produce different effects. The percentage of shaded surface alone does not describe the whole phenomenon.

The mass of a covering is not a simple measure of its durability. A lightweight steel system can perform its function well if it has been properly selected and installed. A heavy material also requires a suitable structure and details. With a photovoltaic roof, the whole system must be compared, including the active layer, connections, supports and loads. Kilograms per square metre alone do not determine the choice. A lighter covering can reduce the permanent load on the structure compared with a heavier variant. This is a valuable feature, particularly in some renovations. However, it does not mean that assessment of the roof truss can be dispensed with. The condition of the timber, the spacing of the members and the way forces are transmitted still require checking by the person responsible for the design.

The capacity stated in kWp describes a photovoltaic roof under defined measurement conditions. It is not a promise of constant output, nor a number of kilowatt-hours available every day. Actual yield is determined by light, temperature, roof geometry and system losses. The benefit to the household budget also depends on when energy is generated and when it is consumed. Cell and module parameters are compared under repeatable laboratory conditions. This allows different products to be compared on a common basis. On a roof, conditions change constantly. Irradiance in the morning differs from that at midday, and cell temperature need not be the same as air temperature.