Steem Power basically doesn't exist, it is only a way of presenting Vests. Vests are the real unit of stakeholding behind the scenes. Every new block, the amount of Steem Power which a Vest represents increases very slightly.

Vests are also how we measure if you're a minnow, dolphin, orca or whale. Minnows are below 10 million Vests. Dolphins are above that and below 100 million Vests. Anyone with at least a billion Vests is a whale.