Air density of I.2 is an average value at ground level. Of course the air density varies with altitude and temperature.
As full rated power wind speed, 10 m/s is an often used value. Other values can be chosen. http://www.energykitesystems.net/FAA/FAAfromMakani.pdf Makani provides a rather low value of 9 m/s.
Because the conversion system (the turbines aloft for a flygen like Makani, but also the reel-out phase for yoyo mode with the generator at ground) should slow down the device for 1/3 in a standardized calculation.
“Verification: 33 x 1.2/2 x 70.666666 x70.666666 x 70.666666 x 0.0857575 = roughly 600 kW.”
If I remember correctly I used the classic wind energy formula, multiplying the full area of the secondary turbines (33 m²) with air density/2, and with cubed apparent wind speed.
I added a coefficient of 0.0857575 to obtain 600 kW, because the turbines are very large in regard to the expected efficiency in regular flight, that due to vertical takeoff requirement. Indeed 33 m² of secondary turbines is a huge value compared to the wing area.
Some precision; I do not know what are all features of Makani M600, so I complete with hypothetical numbers in order to obtain 600 kW. If we consider the 9 m/s wind speed value as specified by Makani (instead of 10 m/s), some other variables should have a higher value.