There will be the terahash capacity that the miner can be generating and it will be on the miner and the seller will let you know the details. Another one is the electricity it is consuming.
Both the terahash and the electricity are used to calculate how profitable a miner can be for you using the mining profitability calculator which you can get online.
Do not forget electricity usage cost.
A miner might have a high terahash but if its using a lot of power, it can eat up my profits quickly. I use mining profitability calculator like the one on asicprofit and helps me get an idea of how much i could earn after factoring in electricity cost