Hi Mara,
You can find a list of the weather data elements required by each of SAM's performance models, including PVWatts in SAM's Help system under "Weather Data, Weather Data Elements" in the table of contents.
In addition to the solar irradiance data, PVWatts also requires ambient temperature (dry-bulb) in degrees Celsius and wind speed data in meters per second.
SAM should ignore time steps for February 29, but to be sure, I would exclude it from your weather file.
If you are constructing your own weather files in the SAM CSV format, you can find the format description in Help under "Weather File Formats, SAM CSV Format for Solar." We have also posted that section of Help as a PDF on the SAM website at
sam.nrel.gov/weather-data/weather-data-publications.
Best regards,
Paul.