PVWatts Legacy Calculators (Version 1 & 2) will no longer be supported after June 2014.
Please use the most current version PVWatts®.
Legacy Calculator Revision History
Revisions to the legacy PVWatts calculators are explained in the entries below.
August 27, 2012
PVWatts Site Specific Data (Version 1) U.S. electric rates were updated to 2010 rates.
May 8, 2012
Version 1 for the U.S. electric costs were updated to 2010 rates.
June 6, 2011
The PVWatts website was redesigned.
May 19, 2008
Version 1 users can now output their results to a text format that can be be imported to an MS Excel Spreadsheet.
September 25, 2007
The PVWatts website was redesigned. No changes were made to the PVWatts calculator.
September 12, 2006
The rotation limit about the axis of one-axis trackers was changed from ±90° to ±45°. This reduces calculated output by 2%–4% but is more representative of available one-axis trackers.
Additional information was added to How to Change PVWatts Parameters to provide guidance on assigning a derate factor for shading when PV arrays consist of multiple rows.
April 14, 2006
The PVWatts Grid Data Calculator (Version 2) electric costs for the United States were updated to 2004 residential rates. The rates were calculated from PowerDat, a database produced by RDI/FT Energy Inc., and supplemented with data from the Energy Information Administration.
March 29, 2006
Version 1 calculator was expanded to cover 360 sites outside the United States. When electric cost is not available, the user may supply one. If cost is not provided, no energy value is calculated.
Version 1 calculator electric costs were updated to 2004 rates.
July 18, 2005
The PV system specification input for system size was changed from an AC power rating to a nameplate DC power rating. Information for determining the nameplate DC power rating is more readily available and less open to interpretation.
An input for an overall DC-to-AC derate factor was added. The PVWatts calculator can also determine an overall DC-to-AC derate factor from individual component derate factors. This offers more transparency of the loss factors used and permits loss factors to be changed to better match system-specific components or loss mechanisms.
The state average residential electric rates were updated for Version 1 to use 2003 data; electric rates for Version 2 were updated to use 2002 data.
The results page was revised to include monthly and yearly solar radiation values.
Functionality was added to allow Version 2 to be launched with NREL's internet map server Version 2 site, which provides a simpler user interface.
An option was added to Version 1 to enable the PVWatts calculator to output estimated values of AC energy produced by the PV system for each hour of the year.
The Version 2 calculator went online. Version 2 employs interactive mapping to allow users to select any location within the contiguous 48 states, Alaska, or Hawaii. Because Version 2 uses additional information to run its calculations, its results may vary slightly from those obtained using the Version 1.
PVWatts: A Performance Calculator for Grid-Connected PV Systems went online. The original version allowed users to choose a location from any of 239 options in the contiguous United States, Alaska, Hawaii, Puerto Rico, and Guam. User-selected inputs included AC rating, array type, array tilt, array azimuth, and electric rate.