A Performance Calculator for GridConnected PV Systems 



For more information regarding the algorithms employed by PVWatts please see
David F. Mennicucci, "Photovoltaic Array Simulation Models", Solar
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.
Last updated 20130129
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 hourbyhour 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.
Cells, 18(1986) pp. 383392.
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.
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 40km 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.
Please send questions and
comments to webmaster
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