Sorry Floridabear, your math is off. At $0.15 kw/h power cost, a Single is still profitable to a worthwhile degree even at 900 million difficulty. It's going to be a bit of time before we see 900 million difficulty, from 19 million now.
profitable it mean, that it will make roi x2 at least, x 3 if you want to race with the difficulty, so you can buy the next device