Thunderbug is right, you can use the APU for electrical requirements upto any altitude. However you will only get 100% output upto 30,000 ft, then it's output ability decreases progressivley upto ceiling to around 80% give or take, isa conditions (i can't remember the exact figure!). Regarding the packs, its 2 packs upto 15,000ft and one pack to 20,000ft. Hope this helps