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I have discovered that Steem Power grows by itself, although I don't know at what rate. This is most easily seen on this account which has a huge amount of SP, which I believe belongs to one of the founders:
https://steemit.com/@dantheman/transfers
If you refresh the page, you will notice that his Steem power increases with each refresh, without him collecting his rewards.

My calculations show an interest rate of around 1.95% per year, based on a 10 minute period of gains of this account.

But using it to vote doesn't reduce that, I believe.

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