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NOTICE!
PVWatts® Legacy Calculators (Version 1 & 2) will no longer be supported after 2014.

Please use the most current version PVWatts®®.


Legacy Calculator Frequently Asked Questions and Survey Responses

We provide users the opportunity to send us questions and comments through our “Did you find what you needed?” survey (on most of our webpages). We’d like users to know that we do read the postings and will provide answers to the questions or comments that we see most often. If you have a question or feedback on PVWatts®, please send an e-mail to Webmaster.

General Questions About PVWatts® Legacy Tools

Question: Why is Mexico, Canada, Ecuador, Yeman, China (or some other countries) not included in PVWatts® Version 1 or Version 2?

Answer: When PVWatts® Version 1 International was developed, we did not have the right data for a lot of international locations. PWatts® needs TMY* files and when PVWatts® was originally created we were not able to find data of sufficient quality and period of record from which to create TMYs for all countries. If you don't see your area of interest in PVWatts®, it is most likely because we did not have the right data for that area.

*Typical Meteorological Year, PVWatts® version 2 or TMY2 files use both weather and solar data in its calculations. TMY files represent typical or average weather for a site; the TMY2s are a derivative of the 1961-1990 National Solar Radiation Data Base. TMY files are constructed by determining, for each month in a calendar year, which of each of the months best represent typical or average weather at a site for that month.

In response to users feedback, the developers are working on the international aspect of PVWatts®. The site will show a map of the country marking the locations of the nearest TMY dataset. While we might not have data for exactly your area of interest, if we have data close, you will be able to see a map of where the nearest TMY data is located. This feature should be released before the end of 2014.

Note: In the fall of 2013, NREL released the PVWatts® Calculator for India, the first country-specific PVWatts® Calculator for locations outside of the United States.

Question: Where do I enter the module efficiency into the calculator?

Answer: PVWatts® uses the module's nameplate DC Power Rating rather than module efficiency.

Question: My calculations don't give the same results as PVWatts®.
    I can't duplicate the numbers in your results table.
    How is the AC Energy (kWh) calculated?

Answer: PVWatts® performs and an hour-by-hour calculation with corrections for things such as the PV module temperature's impact on PV efficiency, reflection losses, and inverter efficiency as a function of load, in addition to the derate factors, so it's not just multiplying the monthly solar radiation by one factor. Consequently, PVWatts® results won't necessarily compare very well with results of simpler methods.

Question: How can I run PVWatt®s for a system outside the allowed system size range (0.49999 - 99999.0 kW)?

Answer: Note that the relation of system output to the system size is linear. To run run a PV system smaller than the allowed lower limit of PVWatts®, run PVWatts® using a system size that is some multiple larger than your desired system size; e.g., 10 or 100 times larger. Then divide the estimated electricty produced by this same multiplier.

Example: to run PVWatts® for a 310-W PV system (310 W = 0.31 kW) run PVWatts® for a 3.1-kW system (10 times the desired system size), then divide the reuslts by 10. Do the converse for a system size larger than the allowed upper limit of PVWatts®: divide your desired system size by some number (e.g., 100), then multiply the estimated electricity produced by this same number.

Example: to run PVWatts® for a 257-MW system (2573 MW = 2,573,000 kW), run PVWatts® for a 25.73-kW system, then multiply the results by 100.

Question: What kind modules (solar panels) are assumed in PVWatts®?

Answer: PVWatts® assumes crystalline silicon modules (see Module choice).

Questions About the Grid Data Calculator

Question: Why can't I get hourly output from the Grid Data (Version 2) calculator?

Answer: For the 40-km Grid version, performance is first calculated for the nearest TMY2 location and then translated to the desired 40-km grid cell location. The translation process uses grid cell monthly values of solar radiation and meteorological parameters. Because the translation process uses monthly values, hourly values of PV performance are not available for Grid Data calculator.

PVWatts® Contact

For questions or suggestions regarding PVWatts®, please contact Webmaster.