A Performance Calculator
for
 Grid-Connected PV Systems

Frequently Asked Questions
* *
General Questions About PVWatts

Q. Where do I enter the module efficiency into the calculator?
A. PVWatts uses the module's nameplate DC Power Rating rather than module efficiency.
Q. 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?
A. 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.

For more information regarding the algorithms employed by PVWatts please see

        David F. Mennicucci, "Photovoltaic Array Simulation Models", Solar
        Cells, 18(1986) pp. 383-392.

Q. I am not able to see the results of the PVWatts calculations.
A. The PVWatts calculations open in a new window, so if you do not have your pop-up blocker set correctly, you will not be able to see the calculations. You will need to allow pop-ups from rredc.nrel.gov.
Questions About PVWatts: 40 km Grid (formerly Version 2)

Q. Why can't I get hourly output from PVWatts: 40km Grid ?
A. For the 40km 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 PVWatts: 40 km Grid.
Other Questions or Concerns About PVWatts

Q. I don't see my question/concern covered here.
A. See Help in the Tools menu   (     )   of the PVWatts Viewer.
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Last updated 2011-04-21