Comparing two mounting structure quotations by steel weight is common, understandable and misleading. Weight tells you what a structure costs to make. It tells you very little about whether it will hold a specific array on a specific site for twenty-five years.
What actually governs the design
A mounting structure resolves several load cases at once. Dead load from the modules and the structure itself is the predictable part. The governing case is almost always wind — uplift on the underside of a tilted array, drag across a long row, and the local pressure increase at the edges and corners of the array field.
- Basic wind speed for the site's classification zone
- Terrain category — open ground behaves very differently from built-up surroundings
- Array geometry — tilt angle, row pitch and module dimensions
- Position within the field — edge and corner rows see higher local pressures
- Height above ground and local topography
Change any of these and the required section changes with it. A structure engineered for a sheltered inland site and deployed on an exposed coastal one is not conservative by a small margin — it is designed for a different problem.
A structure that is heavier than it needs to be in the wrong places is still under-designed in the right ones.
Where weight comparison breaks down
Two structures of identical weight can distribute that material very differently. One may put steel into a member that carries little load while under-sizing the connection that carries most of it. Weight is an aggregate; load paths are specific.
The more useful question at tender stage is not what the structure weighs but what it was designed against: which wind speed, which terrain category, which module, which row position. If those inputs are not stated, the design has not really been done — it has been selected.
What to ask for
- The design wind speed and terrain category used
- The module dimensions and weight the geometry assumes
- Whether edge and corner rows are treated differently
- The design life assumed for the coating system
- The foundation condition the design assumes
A supplier who can answer those five questions has engineered your structure. One who responds with a weight per kilowatt has quoted you a commodity.
