Solar system losses: what the 14% in PVWatts is made of
Everything between perfect panels and your meter: dirt, wiring, mismatch, downtime, shade. PVWatts’ default is 14%; Solar Plan Builder uses the same breakdown.
Soiling 2%, shading 3%, mismatch 2%, wiring 2%, connections 0.5%, light-induced degradation 1.5%, nameplate rating 1%, availability 3% — combined, about 14%. Heat and inverter losses are modelled separately.
Change shading if trees shade your roof. Everything else is reasonable for a new, professionally installed system.
What this means for you: If an installer’s production estimate implies far fewer losses than this, ask how they modelled shade and weather.