I haven't noticed any meaningful drop in battery from this feature. Also, if you have Power Nap turned on while on battery power, and the battery level reaches 30 percent of capacity, Power Nap will be suspended.

However, Power Nap only works on recent Apple notebooks with flash storage and cool microprocessors (like your MacBook Air).

http://arstechnica.com/apple/2012/07/os-x-10-8/18/#power-man...

http://www.macworld.com/article/1167970/up_close_with_mounta...