As long as you have free electricity, getting a miner that is the best $/Th is what you are looking for. The things you look at when buying a miner is the cost of the miner and the price you need to pay to run the miner. If you remove the price you pay to use the miner (electricity) then you are looking at the price to acquire the miner.

960kwh would be about half of a S9, in reality 960kwh would be half of almost any machine out there right now. So that should be taken into account when figuring out what you want to get, especially if you only get 960kwh for free, this calculation of half a miner doesn't taken into any account of what your home uses.