Choosing a category can be a little difficult sometimes. It depends on your foundation statement, thesis, or angle you're coming from. For example, your hub is about power outages. What caused the power outages?
Will your hub be about there being too many techno gadgets that drain energy causing brown outs, and rolling power outages? (Category Technology or the Environment?)
Or are you coming from an angle that the cost of energy is so high that some consumers can't afford it so, they have power outages? (Category Social or Economics?)
Or are you coming from an angle of power outages being caused by man-made accidents, storms, etc. (Category Business, Knowledge & Education?)
It could go on and on. It really depends on different things. I'm sorry if this didn't answer your question directly, but I hope it helps.