Imagine a blog post but a lot longer.  Blog posts tend to be somewhere around 500-600 words but with a power page, it is a page specifically built to be informative by having more information than normal.  Think of a blog post that goes in-depth to break down even the smallest bits of information.  We tend to think of a power page as being somewhere in the ballpark of 1400-2000 words, dedicated to one topic.