Hi Gabriel,
The answer to that question depends on the weather data you use for the simulation. The NSRDB PSM data that SAM downloads by default is instantaneous data at the minute indicated in the weather file (30 minutes past the hour by default for hourly data), so that would result in the system power generated data being instantaneous.
Note that you can use the "Solar Resource Interpolation" macro to convert the hourly data to 15-minute time steps, and that the NSRDB data for specific years is available in 30-minute time steps if that better suits your analysis than the hourly data.
Best regards,
Paul.