Solar PV - STC, Irradiation, Air Mass
Let's assume that a panel has an efficiency of 15%. What will be the output at STC, will it be - P = 15% * 1000 W/m2 = 150 W/m2 or will there be an adjustment for the air mass of 1.5? Basically, does the 1000 W/m2 already factor in the air mass?

Standard test conditions include the AM1.5 filtering of the insolation.
So at 25C, AM1.5, 1000 W/m^2, 15% efficiency  you'd get 150 Watts.
While it's 'obvious enough' in retrospect it's worth repeating that 1000 W/m^2 is termed &quot;1 sun&quot; but is not a measure of real world solar irradiance. ie at noon at any given location one may receive higher or lower levels of insolation.
A useful source of related information is http://www.gaisma.com
For example for my city of Auckland in New Zealand.
https://www.gaisma.com/en/location/auckland.html
The kWh/m^2 figures are average daily insolation by month at AM1.5, 25 degrees C.



So the 1.5 air mass is already factored into the 1,000 W/m2.....I wonder why they need to mention it at all then. Unnecessarily confusing!?

The radiation wavelength profile varies with amount of air traversed.
Shortwavelength (near UV and UV) are absorbed more by air - to the extent that for satellite use expensive triple junction cells are used to maximise energy obtained - but at ground level the wavelengths that make the GaN cells useful in space are very largely absent. So it's not just absolute energy but how it is distributed.
If you test solar cells  with incandescent &quot;heat lamps&quot; you get rated output at far below 1 kW per square metre apparent due to high IR output. A light meter intended for daylight use shows far lower light level than you'd expect.
These solar charged lights are being illuminated by tungsten-halogen lamps with a light level such that they produce the same electrical output as is achieved at 1000 w/m^2 with solar irradiation. A light meter intended for daylight use indicates that insolation is about 200 W/m^2.  The high IR level is converted by the PV panels but under-read by the light meter.
The black marks on the floor allow lights to be placed in the appropriate locations. Locations were determined using the metered light 3rd from left.
Photo: &quot;Sunwave Power ltd&quot;, Qingdao, China, June 2008.