Those options provide access to different solar resource data sets.
In general, for analyses of solar projects for locations in the U.S., a TMY3 weather file is a reasonable choice if one is available. The TMY3 data set represents the "typical" solar resource over a relatively recent period (1991-2005). The TMY2 data represent a longer, but older period (1961-1990). Both TMY datasets are from NREL's database of solar resource data (NSRDB) database, which you can read more about here:
rredc.nrel.gov/solar/old_data/nsrdb/. The user manual for each dataset describes the data and methods used to develop it.
SAM comes with the complete set of TMY2 files. To use a TMY2 file, you simply choose it from the Location list on the Climate page. To use a TMY3 file, you have to download it by clicking the appropriate link on the Climate page, and then use the Add/Remove button on the Climate page to tell SAM where to find your weather file folder.
Location Lookup provides access to a database of solar resource data for the U.S. developed from satellite-derived data using the Perez State University of New York (SUNY) model. The typical year data represents the period 1998-2005. That option is reasonable for locations that are not included in the TMY3 or TMY2 databases. To use it, you type a street address or latitude/longitude, and SAM automatically downloads the appropriate file from the database.
The Perez SUNY method estimates the solar resource from satellite images, while the NSRDB method estimates the resource from ground observations of cloud cover (and measurements of radiation available at a few of the sites).
EPW weather files are designed for use with the EnergyPlus building simulation software. SAM can read the EnergyPlus weather file format. EPW files are best for modeling projects outside of the U.S. The EPW files come from a variety of sources described on the EnergyPlus website. To use an EPW file in SAM, follow the appropriate link on the Climate page to download the file to your weather file folder. (
apps1.eere.energy.gov/buildings/energyplus/cfm/weather_data.cfm)
For a description of the different methods and discussion of using weather data for modeling solar power systems, see the Best Practices Handbook for the Collection and Use of Solar Resource Data:
www.nrel.gov/docs/fy10osti/47465.pdf. Although the handbook was written with CSP project modeling in mind, it is also useful for PV modeling.
For instructions on using the different weather file formats (or creating your own) in SAM, see the Climate page's Help topic.
