Calculating panel efficiency

General
#1

The stated efficiency according to LG, the manufacturer, is given as 18.3%.
Here are three examples using panel data and solar irradiance from a
Ambient Weather WS-2000 personal weather station.

At 10:00AM today my PV panels were receiving 416.4W/m^2
Each panel is 1.64m^2 using the gross dimensions of a panel
Total wattage of the array was 2597.84W
The average per panel was 2597.84/21 = 123.7W
Efficiency of PV panels = (100%* 123.7)/(1.64*416.4) = 18.1%

At 10:30AM today my PV panels were seeing 495.8W/m^2
Each panel is 1.64m^2
Total wattage of the array was 3106.13W
The average per panel was 3106.13/21 = 147.91W
Efficiency of PV panels = (100%* 147.91)/(1.64*495.8) = 18.2%

At 1:30PM today my PV panels were seeing 638.5W/m^2
Each panel is 1.64m^2
Total wattage of the array was 4115W
The average per panel was 4,115/21 = 196W
Efficiency of PV panels = (100%* 196)/(1.64*638.5) = 18.7%

Measuring daily insolation and efficiency
Measuring daily insolation and efficiency
#2

Interesting calculations. At a minimum, you are seeing the changing angle of incidence over time as it affects output. The times and gross power readings support this. Not to rain on your parade, but have you also tried including the temperature coefficient in the calculations?

#3

Thanks for the comments. No I haven’t. I was just happy to see that it agreed with the manufacturer’s value. If you think about it with constant sunshine which this day had the temp isn’t varying very much during the time period I am using. I have, however, extended my calculations to a time period of 10AM to 3PM in this graph.

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Here are the plot of area temperature (Red), (not temp of the panels but my weather station which is about 10’ higher and 40’ further south.)
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At least the atmospheric temp doesn’t seem to affect the results. On the other hand 16 of my panels face SW and the other 5 face NE. The latter see more sun before the others but both orientations see about the same total illumination between 10 and 3. Since this is an average efficiency the 16 panels will dominate after noon . About 3PM the 5 NE panels go into shadow as the Green kW curve indicates.