Free Tools for Estimating PV System Output


Thinking of installing a photovoltaic (PV) system but want to know how much energy and at what time of year a system would produce the most energy? You could contact some vendors and get some proposals to compare. But if you want to become an educated consumer and have a better idea of your site’s solar potential before you start soliciting vendors (or to verify what they told you), these free online tools can estimate your energy production and utility savings. They can be easy to use by relying on default assumptions, but the more custom data you enter, the more accurate your results can be.

PVWatts version 1, PVWatts version 2, and the new kid on the block—IMBY (“In My Backyard”)—are courtesy of the U.S. Department of Energy’s National Renewable Energy Laboratory, the federal government’s research and development center for renewable energy and energy efficiency. The three tools are interrelated and are basically three ways to answer the same questions.

PVWatts Version 1

Site-Specific Data Calculator

This version was launched in 1999, and allows users to choose the closest of 239 locations in the United States and its territories (and more than 360 additional sites around the world). For example, Washington, DC, residents choose the nearby Sterling, Virginia, option. A “site-specific data calculator,” Version 1 uses hourly solar radiation data derived from 30 years of data collection (1961–1990) from the National Solar Radiation Data Base. This data is used by the calculator to estimate monthly and annual energy production and financial savings. The tool calculates production based on characteristics of a grid-tied, crystalline-module PV system.

Many parameters can be adjusted to refine the system details, such the array’s DC rating; the DC-to-AC derate factor; whether it is fixed tilt or tracked; and, if it is a fixed array, the tilt and azimuth. Version 1 also gives you a default electricity cost, but it’s based on generalized 2004 data. It’s best to enter your current electricity cost for more accurate results.

You can also customize the DC-to-AC derate factor, which is the percentage of available AC energy compared to the DC rating of the array (see “Derating a System” sidebar). The yielded values are accurate to ±30% for monthly calculations and ±10% for the annual value.

The biggest limitation of Version 1 is that you can only choose from four or five locations in each state, so regional climatic variations aren’t factored in. However, it can yield hourly data over the year, which is useful to evaluate the benefits of time-of-use (TOU) metering available with some utilities—you can see if your proposed PV system’s output should be peaking (or not) during the same time of high utility rates.

PVWatts Version 2, 40 km Grid

Grid Data Calculator

A more refined version of PVWatts— 40 km Grid (aka Version 2) entered the picture in 2001. It allows U.S. users to zero in on their location by ZIP code, by street address, or by latitude and longitude coordinates. One gets most of the same information as with PVWatts version 1. While you can see where your house is on the satellite image, the data is intended for the entire 40-kilometer (25-mile) square that includes your street address. The downside is that hourly system output is not calculated. 

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