PVWatts is an online solar panel calculator for estimating the output of your solar energy system. It is developed and operated by the US-based National Renewable Energy Laboratory (NREL). One of the interesting features of PVWatts is the ability to analyze both the energy generation as well as cost savings of grid-connected photovoltaic systems.
Similar to our calculator, Sunmetrix Discover, users can choose the location of their project site and build a basic model of their system by specifying system size, slope of the panels, cost of electricity and other parameters. The outputs include monthly and annual energy production estimates.
There are two versions of PVWatts. Version 1 includes a Site Specific Data Calculator that allows users to select a location in the US from a map. Although some other countries around the world are also covered, there is no convenient map-based interface. Instead, users have to select from a predetermined list of cities. Once a location is selected, a simple form-based interface allows users to change the default system settings to match their specifications. Monthly electrical output estimates are displayed in a table format, and the users have the option of downloading a data file that contains detailed information about the expected performance of their photovoltaic system.
Version 2 of PVWatts has a very different look and feel. Also called the PVWatts Viewer, this version includes an interactive map and a well-designed user interface. Users can select their location using an address, postal code or geographical coordinates. However, once a location selected, the users are directed to the same calculation engine used in Version 1. Thus, rather than an upgraded version with different features, Version 2 seems to be more of a visual makeover for selecting the site of interest.
Our Verdict on PVWatts
We really like PVWatts and regularly use it to benchmark some of our own results. The technical competence of the researchers at NREL is beyond any doubt. However, the user experience is not very smooth. The existence of two parallel versions is a bit confusing, especially since both of them lead to the same calculation engine. Also, for the general public, the analysis results are not very accessible. For example, although there is an option to download a text file with detailed results, very few users can actually make sense of these data files and use them for further analysis.
Overall, PVWatts is a solid reference for homeowners who are interested in learning more about their potential. If time permits, comparing the results from multiple sources is always a good idea. Thus, we encourage our users to compare their Sunmetrix Discover results with the estimates of PVWatts.