There really is no direct relation between Steem Power and Reputation.
SteemPower is just the stake you have on Steem. The more stake you have, the more your votes and your account will be worth. Reputation on the other hand is "supposed" to be a visual indicator to see who are the trustworthy one's on this platform and who are the veterans. If you might have noticed, everytime someone upvotes your post your Reputation increases. The more upvotes you get the more high your reputation is.
Having said that, someone actually made a post showing how Reputation is directly linked to the number of received upvotes. In this post https://steemit.com/steemit/@holger80/reputation-can-be-directly-linked-to-the-amount-of-received-upvotes it is seen that the more high the reputation of an account is, the more upvotes he have already received and thus the more Steem and SP he had accumulated.
If we assume that an account never powers down then we can say that there really is somewhat a relation between Steem Power and Reputation. A higher rep account is most likely to have a big Steem Power.