PVsyst Loss Tree Explained: How to Read Your Simulation Report
PVsyst generates a loss tree diagram that traces global horizontal irradiance down to the energy injected into the grid. Every step between those two numbers represents a loss — and understanding those losses is what separates a generic simulation from a design tool.
The tree starts with the global horizontal irradiance at the site, measured in kWh per square metre per year. The first adjustment is the transposition gain or loss — the effect of tilting and orienting the array away from horizontal. A south-facing array in the northern hemisphere typically shows a transposition gain; an east-west split or a low tilt shows less. This is the first place to check whether your layout assumptions are reasonable.
Next come the shading losses. PVsyst separates near shading — from nearby obstructions like trees, parapets, or adjacent rows — from horizon shading caused by distant terrain. Near shading losses above 3 to 4 percent on a commercial rooftop usually signal a layout problem: row spacing too tight, or modules placed in areas that should have been excluded.
IAM losses (incidence angle modifier) represent the reflection of sunlight off the module glass at steep angles. These are physics, not design — typically 2 to 3 percent and not adjustable. Soiling losses are site-dependent and set by the designer. A clean suburban roof might justify 2 percent; a dusty industrial site or an agricultural ground mount might need 5 percent or more.
The electrical losses are where designers have the most control. Module quality loss, mismatch loss, wiring loss, and inverter efficiency each appear as separate lines. Wiring losses above 2 percent suggest the DC home run is too long or undersized. Inverter clipping — energy lost when the DC array output exceeds the inverter's AC rating — appears as a separate line and is a direct indicator of the DC-to-AC ratio. A small amount of clipping (1 to 3 percent) is normal and often economically optimal; above 5 percent, the designer should revisit the inverter sizing.
The final number — energy injected into the grid in MWh — is the one that matters to the project owner. But the path from irradiance to that number is where the engineering decisions live. Read the loss tree, challenge any line that looks unusually high, and use it to iterate the design before you commit to a layout.