Here is an ongoing test that I am starting tonight showing the impact of STEEM Power interest while Powering Down.
This was before I started the "Power Down".
This was 5 minutes after.
And after 1 hour and 15 minutes after I have earned .250 SP
Conclusion
So for now it's obvious that if you Power Down you still earn STEEM Power interest, and I'm curious to see what happens after a week when I receive the first "weekly" payout.
But I will stop it before I get to 700 SP, because this is just a test and I am wanting to grow my STEEM Power..