I have no source but based on the numbers I crunched, it seems to be about 1% per year. Maybe a tiny bit more? @freedom does not appear to be very active. In theory you could record the SP number and check it again exactly 24 hours later. As long as the wallet had had no other activity over that 24 hour period, you'd be able to calculate a daily interest rate.
RE: 💰💲💸 No, just holding STEEM POWER won't make you a MILLIONAIRE so easily!💸💲💰