Free Tools for Estimating PV System Output: Page 2 of 2


Portal to Both PVWatts

The new PVWatts Viewer is now the interactive, map-based gateway to either version of PVWatts (it queries both simultaneously and selects the one to give you the most accurate results). You can tell which one you are in by looking at the top band across the browser window. Either you specify a location (U.S. ZIP code, full street address, or latitude and longitude) or you can click on the map and zoom around to the spot you want either in map or satellite views. Once you specify your location, you are taken to a PVWatts calculator where you can enter additional information:

  • DC Rating. The total rated kW for the array. For example, if you have twenty 225 W modules in your array, the array’s DC rating is 4.5 kW (20 × 225 W).
  • DC to AC Derate Factor. The default value for this is 0.77, but can be adjusted based on the type of inverter used, module power tolerance, DC and AC wiring losses, etc. (see “System Derating Calculations” sidebar).
  • Array Type. Choose from fixed or tracked: one-axis (east-west) or two-axis (east-west and up-down).
  • Array Tilt. This value defaults to equal the site’s latitude but should be adjusted, especially for roof-mounted systems where the array is likely to be mounted parallel to the roof plane.
  • Array Azimuth. The default value is 180° or true south, but you’ll need to adjust this according to your array’s orientation.
  • Cents per kWh. The default value relies on your state’s 2004 generalized data, so for more accuracy you should enter your own utility rate, which can be found on your monthly electric bill.

PVWatts calculates the solar radiation (kWh per m2 per day), AC energy (kWh), and energy value ($) by month with annual totals. If the Viewer uses Version 1 you can also get an output of hourly performance data. If that button is dimmed, you are using 40-kilometer gridded data and cannot access the hourly data. In either case, you can print the monthly and total output results to a new browser page, in spreadsheet-friendly CSV output.

In My Backyard (IMBY)

IMBY is a positive spin on the derogatory phrase “not in my backyard—NIMBY” (which refers to people who oppose the siting of nuclear power plants, toxic waste dumps, etc. near them—but not anywhere else). This tool estimates the energy that can be produced with either a residential-sized PV or wind system (see “IMBY and the Wind” sidebar) at a specified location.

IMBY can pinpoint locations in 49 states (sorry, Alaska) and part of northern Mexico. Enter your address information, click Find, and the satellite image goes to your location. Using a drawing tool, you can delineate the precise area for a PV array. IMBY then offers an estimate of PV array size in DC kW.

Its summary provides customizable Payback, System Inputs, and Electric Rate categories, allowing users to more precisely calculate initial cost and payback (in years), factor in rebates and tax credits, and adjust the initial cost per watt of the system. Just like PVWatts, you can enter your per kWh electricity cost, and modify system parameters, such as the tilt, azimuth, and derate amount. IMBY calculates your gross system cost with a default setting of $8 per watt. Fortunately, you can change it, as actual prices have been decreasing. Of course, you won’t know your per-watt cost until you get some bids, so this feature is not useful if you haven’t started shopping. IMBY then estimates a simple payback in years—an unsophisticated financial analysis technique. Return on investment and net present value would be more helpful.

The simulation also provides monthly and annual production estimates, and assigns a dollar value to the energy produced. IMBY will graph your projected production, as well as your demand. Do so by either by choosing one of the 15 sample profiles or uploading a comma-separated file of your electrical demand during the 8,760 hours of one year— but one is unlikely to have such information on hand.

While a great concept and a beautifully styled tool, IMBY still has some big bugs to work out. Using several different browsers on both Mac and Windows platforms, IMBY proved unstable, with tendencies to be slow and to lock up. It often won’t do the same thing twice or upchucks (a technical computer term) when you try to enter some data, or jumps to an extreme satellite close-up in Colorado (not kidding). You may need to switch browsers to see how it holds up. With Chrome it consistently crashed. But using Firefox on the same computer, it ran without a hiccup.

The ability to be able to zoom in on your property or rooftop and precisely draw even an irregularly shaped PV array is a terrific function. We tested IMBY by delineating several rooftops that already had PV arrays. For rooftop A, IMBY estimated a 10 kW array. In reality, due to required setbacks and the specifics of the chosen module size, about 9.7 kW of modules will fit. After delineating the existing array on rooftop B, IMBY came back with very close to its actual 3  kW nameplate rating. The drawing tool is precise, so going short or long by a pixel will change the results. For rooftop C, IMBY’s results were 40% below its actual DC nameplate rating of 7.1 kW.

IMBY only calculates fixed-tilt PV arrays. It uses high-resolution satellite-derived solar radiation data, rather than historical data. While IMBY lets you change the overall DC-to-AC derate factor, it doesn’t allow you to modify the individual derate variables as does PVWatts. For this, you’ll need to calculate the derate factor in PVWatts and enter this new value in IMBY.

Which Tool is Best for You?

  • If you live in the United States, PVWatts 40 km Grid will likely give you the best results.
  • If you don’t live in the United States, use PVWatts version 1 if you are near one of the more than 360 international data locations.
  • If you want to compare generation and consumption information, use IMBY.
  • If you want to measure the area available for your PV array and get a rough estimate of possible system size, use IMBY.
  • Whether IMBY works correctly or works with your computer operating system and chosen browser is another matter.


Andy Kerr consults and writes about public lands, wildlife, and energy topics. He splits his time between Ashland, Oregon, and Washington, DC. Over the years, he’s owned five solar hot water and four photovoltaic systems.

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