http://stackoverflow.com/questions/34641077/maximum-output-amperage-from-usb-smartphones
you can get technical here but it really depends on your battery, e.g it is always a constant 5 volt. but then you phone is powered in mA/h so say the m8 has 2600mAh looked it up, the that is the max you can pull to be fair i would say 5v @ 2000mA or 2 amps MAX and that is not even recomendend, that is just an edjucatded guess, leaving your phone after an hour of what ever is drawing 2amps with something 20% battery or maybe even closer to 10.
if you continuously do this you battery will die. you cant charge un charge charge uncharge it is not a good habit to have. (sorta like leaving a laptop plugged in 24/7) bat breaks..
now as far as storage goes there are some pretty nifty portable wireless solutions that would not have that effect.
key board and mouse if used often a wireless bluetooth devices are a serious option as well. much less energy ( if you want your phone and batt to last longer)
you can also stick a micro sd in there up to 128gigs for storage, that works too 0 power draw.
and with new marshmallow platform the sd card will just sort melt into you system
you also got 15 gigs of google storage which you can tell it to sync when plugged in and only using wifi if data is a concern of yours. as well as drop box and skydrive the later 2 not sure how much. ...
i hope it helps you out with starage and mouse and keybaodrd and all