Hi Gita,
The default losses are reasonable values to use if you do not have more information about losses for your system.
PVWatts and the Detailed Photovoltaic models use a different method to model soiling losses:
PVWatts models it as part of the total system losses, which is applied to the total AC electric output of the system.
The Detailed PV model applies the soiling loss to the plane-of-array beam irradiance value in each time step, so the soiling loss decreases the solar energy available to the array.
The PVWatts default loss values are based on the values used for NREL's online PVWatts Calculator at
pvwatts.nrel.gov. The Detailed PV model default loss values are adapted from those values.
Best regards,
Paul.