The best way to size your air compressor is to measure your actual compressed air use for at least one week of typical operations and allow for reasonable estimates of expected changes.  Air Compressor Service can perform this service for a reasonable cost (free for our existing customers).  Industrial air compressors are too expensive to make a mistake by buying a machine too small for your application, or too big, which is the more common mistake.  One might logically assume that if a 15-hp compressor is good for your application, then a 30-hp would be better, but that is not the case, at least not for rotary machines.  Although a rotary machine (unless it is a variable speed unit) will run at a constant speed even with fluctuating air demand, if the demand is too low for a sufficient time, the machine will shut off.  Then when demand increases it will turn back on.  Frequent on/off cycling causes the machine to “burp” oil each time it shuts down which can cause a variety of problems.