For locations in the U.S. and its territories, PVWATTS uses TMYs from the 239 station TMY2 data base (http://rredc.nrel.gov/solar/old_data/nsrdb/tmy2/). For locations in regions outside the U.S., PVWATTS uses TMYs from three sources: the Solar and Wind and Energy Resource Assessment (SWERA) project (http://swera.unep.net/swera/index.php?id=6); the International Weather for Energy Calculations (IWEC) weather files^*; and the Canadian Weather for Energy Calculations (CWEC) weather files^**.

PVWATTS v.2 provides the same output as PVWATTS v.1, but incorporates NREL's 40 km resolution solar resource data to permit site-specific calculations. Accessed through the PVWATTS Version 2 IMS site on NREL's Internet Map Server at http://mapserve2.nrel.gov/website/PVWATTSLITE/viewer.htm, or through NREL's Unites States Solar Atlas at http://mapserve2.nrel.gov/website/L48NEWPVWATTS/viewer.htm, the user identifies the desired grid cell location to run PVWATTS v.2. PVWATTS v.2 calculates performance using hourly data for a nearby TMY2 site that is climatologically similar, and then the output is adjusted based on differences between the TMY2 site and the grid cell with respect to the solar resource (direct, diffuse horizontal, and global horizontal radiation) and daily maximum temperature. For more information regarding PVWATTS v. 2, please see "PVWATTS Version 2 - Enhanced Spatial Resolution for Calculating Grid-Connected PV Performance" (Marion, et al., 2001).

For both PVWATTS v.1 and PVWATTS v.2, default system parameters are used, but users have the option to change the following:

• DC rating
• DC to AC derate factor
• type of PV array (fixed or tracking)
• PV array tilt angle
• PV array azimuth angle
• local electric costs
.

Users cannot change the folowing system parameters:

• installed nominal operating cell temperature (INOCT) of 45°C
• power degradation due to temperature of 0.5%/°C
• angle-of-incidence (reflection) losses for a glass PV module cover^***

Using the TMY weather data for the location selected, PVWATTS calculates the solar radiation incident the PV array and the PV cell temperature for each hour of the year. The DC energy for each hour is calculated from the PV system DC rating and the incident solar radiation, and then corrected for the PV cell temperature. The AC energy for each hour is calculated by multiplying the DC energy by the overall DC to AC derate factor and adjusting for inverter efficiency as a function of load. Hourly values of AC energy are then summed to calculate monthly and annual AC energy production.

^* User's Manual - International Weather for Energy Calculations (IWEC Weather Files) Version 1.1, Atlanta, GA: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, 2002.

^**User's Manual - Canadian Weather for Energy Calculations (CWEC Files), Waterloo, ON: Numerical Logics Inc., 1999.