<h2 id="help_about">About PVWatts</h2>

<p>The PVWatts<sup>&reg;</sup> Calculator estimates the electricity generation of a grid-connected photovoltaic (PV)
    system using hourly solar resource data for locations throughout the world. It allows homeowners, small building owners,
    installers, and manufacturers to develop preliminary production estimates of potential PV installations.</p>

<p>PVWatts is also available as part of NLR's desktop application <a href="//sam.nlr.gov">System Advisor Model (SAM)</a>,
    where it can be used with SAM's financial models. For software developers, it is available as a web API on the
    <a href="//developer.nlr.gov/docs/solar/pvwatts/">NLR Developer Network</a>, or as part of the
    SAM Simulation Core (SSC) <a href="//sam.nlr.gov/software-development-kit-sdk">Software Development Kit</a>,
    which includes the <a href="//nrel-pysam.readthedocs.io">PySAM</a> Python package.
</p>

<p><strong>
      Current Version: 8.7.4<br />
      Release Date: June, 2026<br />
      PVWatts API Version: 8.5<br />
      </strong></p>
<p><em>Information for Developers: The current version of PVWatts is based on the <a href="//developer.nlr.gov/docs/solar/pvwatts/">PVWatts V8</a> API,
        which runs Version 275 of the <a href="//github.com/nrel/ssc">SAM Simulation Core (SSC)</a> pvwattsv8 compute module with weather data from the
        PSM V3 TMY (tmy-2020), PSM V3 Himawari TMY (tmy-2020), Meteosat Prime Meridian V1.0.0 TMY  (PSM V4, tmy-2022) and Polar TMY (PSM V4, tmy-2023)
        datasets from the <a href="//developer.nlr.gov/docs/solar/nsrdb/">National Solar Radiation Database (NSRDB)</a>.</em></p>

<h3>Version History</h3>
<p>Feb 2026, Version 8.6.0. New arctic data and upgraded maps.</p>
<ul>
    <li>New grid cell weather data above 60 degrees latitude from the NLR NSRDB Polar TMY dataset, which uses data from the ERA5 reanalysis. This new data allows PVWatts to provide estimates for locations in Alaska and other high latitude locations that were not previously available.</li>
    <li>Map upgrades and improved user experience</li>
    <li>Latitude/longitude can now be entered for a location. For example:  39.77, -105.22</li>
    <li>Default weather data selection has been improved. If grid cell data is within 100km, it will be selected (which is the case for most locations in the world).
        If grid cell data is not available, the closest weather station within 100km will be selected (eg PSM V3 TMY).
        If not, you will be prompted to increase the search to 1,000km radius. If no weather data is within 1,000km of your location, results will not be generated.
    </li>
</ul>
<p>Sept 2025, Version 8.5.2. Removed battery survey banners.</p>
<p>Sept 2025, Version 8.5.1. Fix bug with array_type not effecting results.</p>
<p>Sept 2025, Version 8.5.0. Internal improvements, minor bug fixes and added battery survey banner.</p>

<p>Mar 2025, Version 8.4.2. Google map upgrade, limit international data about 60deg latitude.</p>
<p>Dec 2024, Version 8.4.1. Add new security and vulnerability messages.</p>
<p>Aug 2024, Version 8.4.0. Internal upgrades and improvements.</p>

<p>June 2024, Version 8.3.0. New weather data and model upgrade.</p>

<p>June 2023, Version 8.2.1. Refinements to the translations.</p>
<p>May 2023, Version 8.2.0. Added support for the Ukrainian language.</p>

<p>January 2023, Version 8.1. Fix column headings and remove total rows from hourly and monthly CSV export files. This update also includes updates to the website backend that do not affect PVWatts functionality.</p>

<hr />
<p><strong>November 2022, Version 8</strong> is a major update with improved models and several new features.

<ul>
    <li>Support for bifacial modules.</li>
    <li>New monthly irradiance loss inputs for for soiling and or snow.</li>
    <li>Optional input for albedo (ground reflectance).</li>
    <li>New solar resource data from National Solar Radiation Database (NSRDB) PSM V4 TMY tmy-2020 and PSM V4 Himawari TMY tmy-2020, updated from older NSRDB datasets from 2015.</li>
    <li>Improved internal models of the photovoltaic module, module thermal effects and inverter. These models are consistent with implementations in the
        <a href="//sam.nlr.gov">NLR System Advisor Model (SAM)</a> Detailed PV model.
    </li>
    <li>Update module type parameters to better reflect current market conditions.</li>
</ul>

<p>This version updates the SAM Simulation Core (SSC) compute module from pvwattsv5 to pvwattsv8, and the PVWatts API on the NLR Developer Network from PVWatts API V6 to PVWatts API V8.</p>

<hr />
<p>April 2022, Version 6.3.1. Added solar resource data for Southeast Asia and fixed problems with selecting solar resource data
    for some locations. We removed retail electricity rate input and monthly energy value results because PVWatts
    does not have enough information to account for important factors that affect the value
    of electricity generated by a photovoltaic system such as a building's electricity consumption, or electricity rate
    details such as fixed charges, time-of-use rates, or demand charges.</p>

<hr />
<p>May 2021, Version 6. Added Spanish translation of the user interface and documentation.</p>

<hr />
<p>August 2017, Version 6. Updates to solar resource data and several enhancements.</p>
<p>Version 6 uses the newest data from the NLR National Solar Radiation Database (NSRDB). The new NSRDB data covers
    the Americas including Hawaii between about 21 degrees South latitude (about 300 km North of Sao Paolo, Brazil)
    to about 60 degrees North (about 200 km south of Anchorage, Alaska), and the Indian subcontinent and parts of
    Central Asia as shown in this map:</p>
<p>You can still use solar resource data available in the older version of PVWatts by choosing a legacy data option,
    but we recommend using the new data because of its higher quality, better spatial resolution so you can use data
    for the exact location of your project, and because it covers a larger part of the world.</p>
<p>Behind the scenes, we have migrated PVWatts to the cloud. While you will not see the effect of this change,
    it will minimize website down times and give us more flexibility to add computing resources as more people use the site.</p>
<p>Other enhancements include:</p>
<ol>
    <li>Change the distance in miles shown on the Resource Data and Results pages to be the distance between the
        system location and the center of the NSRDB grid cell. In older versions, the distance was between the
        location and legacy weather station.</li>
    <li>Update the solar resource map to better reflect the latest solar resource data sources.</li>
    <li>Change default location to the NLR campus in Colorado from Latitude 0&deg; and Longitude 0&deg; in
        the Atlantic Ocean.</li>
    <li>Update Tilt to automatically change when you choose an array type. For fixed arrays, the default
        tilt angle is 20&deg; and for tracking arrays, it is 0&deg;. As always, you can type a value for
        Tilt to override the default value.</li>
    <li>Change the default value of DC to AC Size Ratio (On the System Info page under "Advanced Parameters")
        from 1.1 to 1.2 to reflect current trends in the photovoltaic market.</li>
    <li>Update the labels for economics inputs on the System Info page and results on the Results page to
        better reflect how the model works: The value of electricity generated by the system is based on
        a simple, flat electricity rate, not a cash flow model.</li>
    <li>Remove India-specific features and documentation. Now that weather data for India and many other
        countries is available from the NLR NSRDB, we can use the same version of PVWatts for any location
        in the world.</li>
    <li>Review and revise Help documentation to reflect the latest updates and enhancements.</li>
    <li>Update supporting supporting libraries (e.g. Google maps).</li>
    <li>Refactor data management system to correct minor bugs and make maintenance easier in the future.</li>
    <li>Add aSSL to comply with federal regulations.</li>
</ol>

<hr />
<p>October 2016, Version 5.3.8. Changed our source for India rupee exchange rates and a few bug fixes.</p>

<hr />
<p>July 2016, Version 5.3. Capacity Factor has been added to the displayed results page and both the monthly and hourly
    downloads.</p>

<hr />
<p>April 2016, Version 5.3. The release removed available incentives from the app. Our source of incentives
    was found not to be consistent enough to provide an accurate reflection of costs. If a consistently
    up-to-date source of incentives becomes available, we will add it back into the system.</p>

<hr />
<p>December 2015, Version 5. Provide additional information about the potential interannual variability in energy predictions in US
    locations. When calculating results, PVWatts makes several
    assumptions, include uncertainties in weather and cannot reflect all
    the variations between PV technologies and site-specific
    characteristics. PVWatts<sup>&reg;</sup> predictions are intended to represent
    typical behavior of a similar real-life PV system.</p>
<p>This information was based on our analysis of 30 years of historical solar irradiance and weather
    patterns for 239 locations in the US. The historical data was used to
    estimate PV system production ranges for two prototypical system
    configurations and create charts to show how your annual results may
    vary in a nearby location.</p>

<hr />
<p>July 2015, Version 5. PVWatts was updated to fix a problem that resulted in overprediction of system energy
    in fixed tilt systems with very high tilt angles and west or east facing azimuth orientations.
    The difference in predicted output for the vast majority of system configurations is well within
    the stated uncertainty of PVWatts (+/- 10%).</p>

<hr />
<p>September 2014, Version 5. This new version represents a major update to both the online web tool interface
    and the underlying algorithms. Compared with PVWatts Version 1, our new update will predict
    roughly 7-9% greater energy output for a fixed tilt system given similar assumptions.
    The new results much more closely match measured system performance data, and address
    concerns that PVWatts Version 1 tended to under-predict PV system performance given the
    default input assumptions.</p>
<ul>
    <li><strong>Losses</strong> We&apos;ve switched from derate factors to system loss percentages.
        The default value for system losses is 14%. Note that this value does not include the
        inverter performance.</li>
    <li><strong>Inverters</strong> Inverter efficiency is entered as a separate input. The
        default value is 96%. The underlying inverter model also has been revised to represent
        the performance of modern commercially available inverters.</li>
    <li><strong>Modules</strong> You can select between three different module types: Standard,
        Premium, and Thin-Film. These can help consider different module technology options in
        a generic way.</li>
    <li><strong>Trackers</strong> One axis tracking systems account for self-shading of one
        row to the next, or can be operated in a backtracking mode. Row spacing can be adjusted
        by setting the ground coverage ratio (GCR).</li>
    <li><strong>Sizing</strong> System sizing is explicitly handled by the new DC-to-AC Size
        Ratio input. This allows you to consider different system sizing strategies and operational
        effects such as clipping. The default value is 1.1.</li>
</ul>
<p>By default, PVWatts retains the NLR NSRDB 1961-1990 TMY2 dataset as the default option.
    However, you can select to use NLR NSRDB 1991-2005 TMY3 data or the satellite-based 10-km
    gridded SolarAnywhere data.</p>
<p>For more information on these changes, please consult the new online help system, where you
    will find a link to a detailed technical reference manual that explains all of the equations
    and assumptions used in the model.</p>

<hr />
<p>September 2025. This is PVWatts App v8.5.2. This version includes a number of internal upgrades.</p>