A Performance Calculator
 Grid-Connected PV Systems

Frequently Asked Questions
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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. How can I run PVWatts for a system outside the allowed system size range (0.49999 - 99999.0 kW)?
A. 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.

Q. What kind modules (solar panels) are assumed in PVWatts?.
A. PVWatts assumes crystalline silicon modules (see http://rredc.nrel.gov/solar/calculators/PVWATTS/interp.html, under Module choice.)

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 2013-01-29