If you are debating whether or not you need an air conditioner, chances are the answer is: YES YOU DO. A hot, humid grow room is a bad grow room.
An AC's cooling ability is rated either by BTU or tonnage. 12,000 BTU's = 1 ton.
A general rule of thumb is that for every 1 watt of HPS light, the end user will need 3 BTU's of cooling power. Personally, I always go 20% over this to amount to account for environmental fluxes.
Thus a grower running 4x 1000 Watt HPS lights will need a minimum of 14,400 BTU's of cooling.
So The formula is: (3 BTU's) X (1.2) X (total watts of HPS light) = total BTU's of AC needed.
If you are right at the edge, always size up rather than down.