PVWatts™ Help

From the Viewer, users can access PVWatts™ Version 1 (site locations) and PVWatts™ Version 2 (40K Monthly Grid Cells) through an interactive map. The directions are the same for both formats. Once sent to the PVWatts calculator, the difference is that with site locations, users can download hourly values of 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 US 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.
Re-center the map and zoom in on a specific result.
A direct link to the Results Explained section of this help file.
Send to PVWatts
Once you've identified the location you are interested in, click the button shown below on the corresponding result to send the location information to the PVWatts Calculator.
Send the location information of a result to the PVWatts calculator.
PVWatts Calculator
A separate browser window will open (contents shown below) 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 (from 2008)
For additional help, see:
- Changing system parameters
- Calculating Overall Derate Factor
Review Calculated Results
After running the PVWatts Calculator, your calculated results will display (shown below) in the window. To output your calculation as text, select the "Output Results as Text" button.

Cautions for Interpreting Results

Results Explained

The results generated by querying a point (clicking or searching) fall into one of two primary categories:

Domestic (US 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.

Screenshot of the 40km Grid Cell toolbar

40km 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. For more information, see 'Grid Cell Explained' below.

Grid Cell Explained

Each grid cell displayed in the PVWatts Viewer is a 40km x 40km 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.

An image showing a 40km by 40km grid cell askew about 23 degrees that overlays the centroid and grid over New York and Pennsylvania, the images illustrates how a query point in Pennslyvania will be associated with the centroid and grid

40km Grid Cell
A sample grid cell (left) demonstrates the key elements used in the PVWatts calculation:

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 40km by 40km, grid cells frequently overlap portions of two or more states.

Additional Information

To learn more about the PVWatts Viewer, see:

PVWatts Contact

For questions or suggestions regarding the PVWatts Calculator or the PVWatts Viewer, please contact Webmaster.

Printable Version