As a thought experiment an Antminer S3 would run on an S8's electrical system and would generate 1p an hour to offset your fuel costs
The 980ti would do slightly better at 5p an hour, but would require a carpc installation to work so adds complexity.
Using a 110hp TDI engine (since I have lots of those) to drive a 40KW generator would power 25 Antminer S9s and generate £9 an hour. Running it on kerosene (cos it's heating the house, right?) would be about £7 an hour in fuel costs, based on off-the-interwebs guestimates of diesel genset fuel consumption.
What you really need is, say, a 40 foot water wheel in your back garden powering an alternator, and run some miners from that. Making money from gravity
