In response to feeding off of things, there are many things that one can "feed" from.
I have feed from/through: grounding, ambient energy, people, storms, the wind and several other things. But everything produces some type of energy that can be fed from. You just have to know how to manipulate it and control it to your advantage. If you choose to feed from several different things, then shielding is a must.
Hope this helps answer any lingering questions