STEEM POWER is like shares in Steemit - they grow over time. STEEM is the basic crypto currency and STEEM DOLLARS are yet another currency pegged to 1 USD.
I think this is the most complicated part about Steemit. I wouldn't worry about it personally as the designers of Steemit have worked out the economics. If all you ever want to do is convert to btc then you probably want to convert your earnings into STEEM and withdraw to an exchange like Bittrex
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