The P50/P90 calculations involve two timeframes. The first is the simulation timestep, which by default is one hour. The second is the number of weather files in the set you use for your analysis, which determines the number of years involved in the analysis.
The "default weather database file" contains file sets for locations in the U.S. For some locations, the file set has 15 files representing 15 years worth of data, and for other locations the set contains 30 years' worth of data.
For a given location, SAM runs an hourly simulation for each year for each of the years in the file set to calculate that year's total electricity output in kWh and other metrics. For example, for a location with 30 weather files, SAM would calculate 30 annual electricity output values.
After running the simulations, SAM then makes the statistical calculations on those annual values. For the location with 30 files, SAM would report the minimum, maximum, P50 and P90 annual output values of the 30 values.
I'm sorry that we do not have the resources to provide telephone support, but please don't hesitate to post follow-up questions if you have them.
Best regards,
Paul.