Monthly Performance in Out Years

I would like to predict monthly performance of a flat-plate PV system after the first year. I know SAM has annual predictions, but is there an automated way to get monthly values, for say, year 5? I can also use the monthly distribution predicted for the first year, and generate monthly predictions based on the yearly predictions for the years after the first. Is this a validated way? Is there a better way than this that you know of ?
SAM runs hourly simulations for the first year of the analysis period, and assumes that the performance in Years 2 and later is identical to that first year. You can use an optional degradation rate to model an annual reduction (or increase) of output. The total annual kWh output in Year 1 is the sum of the hourly kWh values from the simulation. For Years 2 and later, the total annual kWh is the Year 1 value adjusted by the degradation rate.
SAM does not report hourly or monthly values for Years 2 and later. If your analysis involves a degradation rate, the monthly output would be the Year 1 output for each month adjusted by the degradation rate.
The reason this modeling approach is reasonable is that SAM is designed to use typical year weather data so that the hourly simulation is an estimate of the system's performance over a long period rather than for a single year. Using typical year data makes it possible for the financial model to make cash flow calculations based on one year's worth of simulation results.
You can run SAM with single year weather data to calculates hourly kWh output values for a specific year. If you have single year data for a range of years, you could run SAM with each weather file to generate hourly, monthly, and annual performance metrics for different years.
For more on typical year and single year data and links to source of weather data, see the Weather Data page on the SAM website:
SAM Website - Weather Data 
Best regards,

Paul.