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PVWatts® Frequently Asked Questions

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

General Questions About PVWatts


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 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 (or some other countries) not included in PVWatts Version 1?

Answer:

When PVWatts Version 1 International was developed, we did not have the right data for Mexico. PWatts needs TMY* files and when PVWatts was created we were not able to find data of sufficient quality and period of record from which to create TMYs for Mexico. The same goes for a large number of other international locations. If you don't see your area of interest in Version 1, 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.


Question:

Why are my results different between PVWatts versions?

Answer:

The new version of PVWatts does not include the 40km-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 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. This matches exactly the PVwatts Version 1 results. You can then change to the closest TMY3 weather file (on the Resource page) or the closest 10k gridded dataset (also called SolarAnywhere Data by 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.