Steem power converts to Steem, so that would be your basis.
@arcange's SteemSQL database has the witness quotes of the value of Steem, which are updated very often. You can find spot values for Steem there.
I'm not sure how to consider Crypto for taxes yet. I know some is property and some is capital gains upon conversion in the US. I'm sure there will be more information on the interwebs for me before tax time is over here in the US.