heavy.
edit: wait, this is in the rules forum and not the homebrew forum. have paizo made power aromor available?!
http://paizo.com/pathfinderRPG/prd/technologyGuide/artifacts.html
Technology Guide has stats for a Powered Armor!
I asked because I could see arguments for every type...Heavy obviously because it's powered armor.
Medium because of ACP and max dex bonus (seems similar to mithral full plate).
Light because it has no speed penalty stated, in fact it increases the wearer's speed.
I can't see the problem, honestly:
Technological Artifact
Slot armor
Type Heavy; Weight 50 lbs.; AC +10; Max Dex +4; Penalty -2; Spell Failure 60%; Speed (30 ft.) see text; Speed (20 ft.) see text; Capacity 100; Usage see text
It's an heavy armor, as clearly stated in the Type entry. So, no doubt, it's heavy, even if it has some feature that could remember some lighter armors.
In addition, we're talking about technological artifacts, here:
In the case of technological items, artifact status refers to an item that is beyond the means of anyone within the campaign setting to create or replicate, as well as beyond conventional measures of value. Unlike magical artifacts, technological artifacts can be destroyed just as other items could, and indeed may be quite fragile.
The Powered Armor is something that goes over and beyond a normal armor. It's the non plus ultra of customized heavy armors. It's like getting the BFG in Doom 2, or the Shrink Ray in Duke Nukem 3d (yes, I'm an old school nostalgic :P), so it has to be really over any kind of nonmagical heavy armor you've ever seen, and for sure it's even beyond most of magical heavy armor you've ever seen. So, what's the problem?