Solar Panel size & efficiency calculations

[svetz]

From the LG 340 spec sheet, we know the panel's collection area must be smaller than 1.645 m^2, 340W, and 19.8% efficient.
The collection area is somewhat of a guess based on the panel's physical dimensions ... but should be smaller than 1.645.

Do the numbers work out?
At STC's 1000W/m^2 the panel "sees" 1000x1.645 = ~1645 W due to its size. The calculation is total watts collected * eff = watts panel generates.
Since its rated 340 watts and 19.8% efficient the actual collection surface must be 340 / 19.8% = 1717 m^2. Eh? Bigger can't be right.
Going the other way, the efficiency would be 340/1645 = ~20%. Something isn't adding up. Is LG understating their efficiency? If you know why please explain.

What about Real Life?
One of my panels is currently reporting lastReportWatts: 167 W.
A nearby weather station reports current solar radiation at 521.8 watts/m². So, how does real life compare (keep in mind this is NOT at STC, instead 78 °F and 77% humidity)? (The local weather station was a wunderground station see @jasonhc73 's post How much "Sun" do you actually get? )

167 W * 1000 / 1645 / 521.8 = 19.46% efficiency at real-life conditions. Wow! Not shabby at all for 10:17 AM.
It would be interesting to do this adding in the Pmax temperature coefficient.

If you have panels, what type of panels do you have and what real-world efficiency do you see?

 

[BiduleOhm]

There's 60 cells and each cell is 0.026147 m² (0.1617 * 0.1617) so the total collection area is 1.569 m².

So at STC that's 1569 W and with a 19.8 % efficiency we have 310.7 W. But, there's a power tolerance of 0 to +3 % so if we use 19.8 + 3 we have 357.7 W so 340 W is realistic as it's between the two (21.7 %).

My guess is the 19.8 % figure is the minimum efficiency guaranteed but not the typical one. It would help if they used the classic min/typ/max pattern of a lot of datasheets for electronic components for example, it's a lot clearer and you know what to expect worst/typical/best case.

 

[tictag]

Most PV panels I've seen have the inter-connecting strips on top of the cell, which would effectively decrease the collection area. Not by much, I guess, but might play into detailed calculations.

There's 60 cells and each cell is 0.026147 m² (0.1617 * 0.1617) so the total collection area is 1.569 m².

How did you find the exact cell area? I've never seen that specified on a PV panel before.

 

[BiduleOhm]

They give the exact size: 161.7 mm so square it and you have the area (that's what I put inbetween the parenthesis BTW)