Err..?
But why?
You can farm with simple devices like the antminer U3 ~70 times faster than current gen computers, why would you want to downsize for more electricity consumption to mine bitcoin. You will only stuff yourself over in the long run, there are reasons for devices being created solely for mining SHA-256d (Bitcoin's algo) Computers are way too slow to do anything since early 2014. No profit will be made, electricity will be wasted. Don't try it
No but OP doesn't seem to know anything. If he mean to mine altcoins, its feasible. Then the payout can be in BTC, hence earning BTC.
@OP If you tell us what kind of hardware you have and what you pay for electricity it may be possible to point you to something worthwhile. But with merely old computers, without any decent GPU, that would be unlikely. And just to make it doubly clear. No you can't mine strait BTC without current, up to date dedicated BTC-mining machine "ASIC".
