There would be 2 main ways to power it. One is through energy cells that are stored either on the back or on the hips of the armor and can power the armor effectively, but need to be replaced after about 100 hours of rigorous combat; or through kinetic energy that the armor then turns into electrical. So essentially; it takes a while and a lot of energy to get moving, but once you are moving there isn't going to be much that can stop you. I think Adepta Sororitas armor would most likely be kinetic-based due to their very zealous fighting style.

If you have any other thoughts on the matter or if there is official lore around power armor for people who aren't space marines; please let me know.