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PVWatts® 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. On this page, you will find the answers to questions often asked of our PVWatts® Webmaster. If you have a question that is not addressed below, you can also visit the Help section on NREL's PVWatts® Calculator.

General Responses or Questions About PVWatts®


Survey Response:

Users ask why they see HTTP Error 503 or Service Not Available errors.

Answer:

The PVWatts® V2 calculator server has been experiencing problems which we are not able to solve. These problems are causing the server to crash just about every day which generate HTTP Error 503 or Service Not Available errors. With the development of the new PVWatts® site, we will not be moving the PVWatts® V2 application to a new server or repair the problem, we will however continue to restart the application when we notice it is down. We recommend users switch to the newest PVWatts® site. We apologize for the inconvenience.


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:

PVWatt®s performs an hour-by-hour calculation with corrections for things, such as the photovoltaic (PV) module's temperature impacts on efficiency, reflection losses, and inverter efficiency as a function of load, in addition to the derate factors. PVWatts® does not multiply the monthly solar radiation by one factor and the results won't necessarily compare very well with results of simpler methods.

For more information regarding the algorithms employed by PVWatts®, please see:

Dobos, A. P. (2013). PVWatts® Technical Reference. 11 pp.; NREL Report No. TP-6A20-60272.

Question:

How can I run PVWatts® 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 electricty 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 How to Interpret Results in Legacy Calculators, under Module Choice).


Question:

I don't see any information about the PV panels needing to be cleaned nor how often.

Answer:

In the latest version of PVWatts®, the help document addresses the question on dirt as:

Soiling - The derate factor for soiling accounts for dirt, snow, and other foreign matter on the surface of the PV module that prevent solar radiation from reaching the cells. Soiling is location- and weather-dependent. There are greater soiling losses in high-traffic, high-pollution areas with infrequent rain. For northern locations, snow reduces the energy produced, depending on the amount of snow and how long it remains on the PV modules. NREL continues to work on improving the modeling of soiling and snow and is working to include historical datasets as available.

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 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 user 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 comparable data relatively near, 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:

Why are my results different between the current version and legacy versions of PVWatts®?

Answer:

The new version of PVWatts® does not include the 40-km resolution gridded data used in PVWatts® version 2 which is the likely cause of the variation you are seeing. It does contain two different site-specific data sets and one gridded data set (and perhaps more options in the future). The 40-km resolution gridded data, while a good dataset, is not deemed to be of as high quality as the other datasets. Solar resource assessment has progressed since that dataset was built.

The tool currently defaults to using the closest TMY2 to the desired location. This matches exactly the PVWatts® Legacy Version 1 results. You can then change to the closest TMY3 weather file (on the Resource page) or the closest 10-km resolution gridded dataset (also called SolarAnywhere® from Clean Power Research®). We will be modifying the default to use the closest TMY3 file. Our consensus is that this will provide the most accurate estimates for performance based on the available datasets.


PVWatts® Contact

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