Compressor horsepower

I am looking at purchasing a new air compressor. I have a question as to how the horsepower is rated. I have found similar compressors with equal CFM air supply but one will state a HP and one will state a running HP. In this case the running HP is much lower than the HP stated. Is there a formula that can be used to compare the two for compressors in the 5 to 7 HP range?