P50 and P90 yield estimates quantify the risk of below-expectation energy generation. The P50 value represents the median expected annual yield: half of all years will produce more and half will produce less. The P90 value is a conservative estimate exceeded in 9 out of 10 years, providing a downside scenario for debt service coverage calculations.
P50 and P90 yields are derived from long-term meteorological data combined with site-specific shading analysis and system performance modelling. Lenders and PPA providers typically use P90 yields for debt sizing to ensure sufficient energy income even in below-average solar irradiation years. Structural integrity, confirmed by a Solar Surveys signed report, is a prerequisite for the system to perform at modelled yield levels.