"interest" in steem power is quite a different beast, especially if you denote it in STEEM and not the internal unit VESTS. Let's just say it's a combination of "interest" payed to offset the dilution of STEEM minus the rewards payed to users of the platform. Technically, if you (as a SteemPower investor) don't participate on the platform and just let your SteemPower sit there, you would not make any profits (besides capital gains/losses) because all rewards are ultimately payed by the investors. The bigger an investor you are, the more you would need to participate in order to not lose more than you gain. Of course there is also the capital gains from speculation.

In the end, doing the actual numbers on SteemPower is quite not so easy.