I've read lots of posts saying roi is impossible. I pay .13usd kwh and think it's possible.
Well, 7,000 GH/s at 3,550W is a decent ~0.51 J/Gh ...and judging by your numbers, it's essentially 6 AntMiner S5's.
With that in mind, at $0.13 per kWh, you're looking at $11.08 in electricity costs per day. So long as 7 TH/s generates more than that in the same period, you'll be good.
Here's what to think about instead: At what point will your setup
no longer generate a profit (expenses equal revenue)? This largely depends on two factors; price per Bitcoin, and the network difficulty. There are other factors for larger setups, but we'll keep it simple for this one.
If a Bitcoin is valued at $230 (current price off of Coinbase as I write this), and at the current difficulty of 41,272,873,895, your revenue should come in around
BTC0.08529572, or $19.62, per day (ignoring variance).
This means you'll be able to maintain profitability until the price per Bitcoin drops to about $129.90, with difficulty staying constant (though unlikely given many more miners would drop out of the game at that price).
Alternatively, if the price stayed constant instead, you could maintain profitability until the difficulty rises to roughly 73,076,881,507...you can be the judge on how long it takes to get there (1, 3, 6, 12 months, etc).
Obviously neither of these variables are constant, so I'd advise you do to your own calculations. As the price per Bitcoin rises, your difficulty ceiling rises by a similar proportion as well.