You want a PSU that is capable of running your miner comfortably. So if you miner requires 1000w to run then you want around 1200-1300 PSU.
You NEVER want to get a 1000w PSU for a 1000w Miner. Always allow yourself plenty of space. Most PSU reach peak efficiency (rated efficiency) at around 50-60% load.
My device is AntMiner S7 so how mutch W to run ?
And what is the PSU