Even starting out I just don't want to use something with a smaller hash rate, just like you wouldn't go NASCAR racing in a golf cart.
Thats not really how it works. Its about how much you pay $/gh. And how much you pay for your electricity.
So with your .10 per kWh on a S7, you would profit for 6 months, having paid 40% back of the unit + PSU, then spend as much as you earn for the next few, and then the halving halve your BTC return and you're left with units worth little and 600 USD loss per unit. 8% Diff raise, stable BTC price.
Well not really, because you sell your miner after. So you sell it for 1000$ maybe, there is 400 profit. and if btc price goes up, then you make more.
sell to whom?
it's is not going to be easy.
Clearly, no one who paid 7.9 BTC or even more for B1 will be able to earn this much BTC minus electricity cost.
almost the same machine (maybe minus some overclocking potential) will cost 2.54 BTC soon.
each B1 would not have made 5.4 btc+cost of electricity by the end of january.
however, if you calculate in $$, they might have some profit because they paid ~$1800 and already mined maybe around 3 BTC ($1370) minus ~$270 in cost.
So, only $700 remains to $$ profitability. So, if you sell B1 right now for 3 BTC, then exchange to $$, you'll have made $$ profit.