PVWatts® Legacy Calculators (Version 1 & 2) will no longer be supported after 2014.
Please use the most current version PVWatts®.
Legacy Calculator Help
From the Grid Data Calculator (Version 2), users can also access the Site Specific Data Calculator (Version 1) through an interactive map. The directions are the same for both formats. Once your parameters are entered and calculated, the difference is that with site locations, users can download hourly values of photovoltaic (PV) performance. Hourly downloads are not available for grid cells.
- Select Location
There are a variety of ways you can select your location:
- PVWatts® Tool - Enter a zip code, specific address, or coordinates.
- Map Interface - While the PVWatts® tool is active, you can pan, click and zoom to find your location.
- Locate Tool - (Deprecated) The locate tool also allows you to enter a zip code, specific address, or coordinates.
- Viewing Results
Depending on where you click, you may see results from more than one data set. For example, clicking within the United States will often produce a site location result as well as a grid cell result. Click the arrows or drag the scroll bar slider on the right of the results panel to view additional results.
- Send to PVWatts®
Once you've identified the location you are interested in, click the Send to PVWatts® button.
- PVWatts® Calculator
A separate browser window will open indicating your system parameters. The default values will automatically be populated. Enter new system parameters, if desired, and press the Calculate button when you are ready to run the PVWatts® Calculator.
- Site Location - Cell ID, State, Latitude, Longitude
- PV System Specifications - DC Rating, DC to AC derate factor, array type and Tracking system (array tilt or array azimuth)
- Energy Data - Cost of electricity
- Review Calculated Results
After running the PVWatts® Calculator, your calculated results will display in the window. To output your calculation as text, select the "Output Results as Text" button. Cautions for Interpreting Results
The results generated by querying a point (clicking or searching) fall into one of two primary categories:
United States (TMY2) or International Hourly Site
Displays the latitude, longitude, state, region, location and/or country of the hourly station located closest to the point queried. Additional information such as a WBAN number or Station ID may be provided. This information is sent to PVWatts® for use in the calculator.
40-Kilometer (km) Grid Cell
Displays the latitude and longitude of the grid cell centroid of the located point. The state shown in the results is the state in which the centroid of the grid cell resides.
Note: The state returned by the grid cell may not match the state at the exact location of the queried point if the grid cell spans more than one state. This has no impact on the PVWatts® calculation results.
Each grid cell displayed in the Grid Data Calculator (Version 2) is a 40-km x 40-km area of interpolated solar resource data assembled using the Climatological Solar Radiation (CSR) model. Grid cell resolution is driven by the input data base components used to run the model.
- Query Point - The point found by clicking, entering coordinates or searching by address or zip code. Querying this point will return the centroid of the grid cell in which the point is located.
Note: Results may vary when searching by zip code as the point returned is the centroid of the zip code and zip code boundaries are subject to change.
- CENTROID - The exact center of the grid cell. The latitude and longitude of this point are returned in the point query results.
- STATE BOUNDARY - Depicted by a solid black line. At 40-km by 40-km, grid cells frequently overlap portions of two or more states.
To learn more about the PVWatts® legacy calculators, see:
- Legacy Calculator Frequently Asked Questions
- Site Specific Data Calculator (Version 1) Calculator
- Grid Data Calculator (Version 2) Calculator
- How to Change Parameters in Legacy Calculators
- How to Interpret Results in Legacy Calculators
- Legacy Calculator Revision History.
For questions or suggestions regarding PVWatts®, please contact Webmaster.