Revision History

Revision Log of PVWATTS Changes

Effective September 12, 2006, PVWATTS calculates performance for 1-axis trackers that have a rotation limit about their axis of ± 45° instead of ± 90°. This reduces calculated output by 2-4%, but is more representative of available 1-axis trackers. Additional explanatory information in the help material section "Changing System Parameters" has also been added to provide guidance on assigning the derate factor for shading when PV arrays consist of multiple rows and one row shades another.

Effective 4/14/2006, PVWATTS version 2 electric costs for the US have been updated to the 2004 residential rates. The electricty rates were calculated from PowerDat, a database produced by RDI/FT Energy, Inc., and supplemented with data from EIA.

Effective 3/29/2006, PVWATTS version 1 can be run for 360 sites outside the United States. Sites are chosen from seven regional drop-down menus. A CGI pulls site location information and local electric costs into a form similar to that PVWATTS version 1 users are accustomed to for sites within the U.S. and its Territories. Users may then set desired system parameters including array DC rating, derate factor, array type, tilt and azimuth. Where electric costs are not available, the user may supply one. If the cost is not provided no energy value is calculated. Hourly performance data can be output for the international sites, as well. PVWATTS version 1 electric costs for the US and its Territories have been updated to the 2004 rates.

Effective 7/18/2005, the PV System Specification input for system size was changed from an AC power rating to a nameplate DC power rating because information for determining the nameplate DC power rating is more readily available and is less open to interpretation. An input for an overall DC to AC derate factor was added for calculating a reference AC power rating by PVWATTS. The user may also have PVWATTS calculate a new overall DC to AC derate factor by specifying individual PV system component derate factors. This offers more transparency for the loss factors used by PVWATTS, and permits loss factors to be changed, if desired, to better match system-specific components or loss mechanisms. The state average residential electric rates were updated for Version 1 using 2003 data, while electric rates for Version 2 use 2002 data. The results page was revised to include monthly and yearly solar radiation values for the PV array. Version 2 may now be launched with NREL's IMS PVWATS Version 2 site which provides a simpler user interface.

Effective October 2004, an option was added for Version 1 that alows the user to have PVWATTS output values of AC energy produced by the PV system for each hour of the year.

Effective September 2001, PVWATTS Version 2 went on-line. Version 2 employs interactive mapping on NREL's Internet Map Server (IMS), allowing the user to select any location within the contiguous 48 states, Alaska or Hawaii. Because Version 2 uses addtional information to run the calculations, results may vary slightly from those obtained using PVWATTS Version 1.

Effective September 1999, PVWATTS: A Performance Calculator for Grid-Connected PV Systems went on-line. This original version allowed the user to choose a location from any of 239 locations in the contiguous US, Alaska, Hawaii, Puerto Rico and Guam. User-selected inputs included AC rating, array type, array tilt and azimuth, and electric rate.