The screenshots above come from the Bitcoin Wisdom mining calculator  ( https://bitcoinwisdom.com/bitcoin/calculator ) .  Having free electricity is a good starting point, but it doesn't guarantee that you'll even earn back your initial investment in hardware.  Lots of other people are also buying S7 at the moment, and the network difficulty has been jumping rapidly.  The faster the difficulty rate increases, the more quickly the earning potential of your miner disappears.  Right now, you might earn .7 to .8 BTC a month with that S7.  By the time you get the shipment from Bitmain in January, it might be .5 to .6 per month, and a few months later, it will be down to .3.  So it's a race to get the miner up and running as fast as possible, to reach a breakeven point before the network difficulty becomes too high for the S7 to matter.