What is the rule of thumb to power an air compressor? I'm sure the concept is similar to the fluid rules, flow, pressure, time etc. but wanting to find an actual formula I can use. Electric or gas power.

 I have several compressor heads that I bought as store returns that are still in the box, and all the same brand and models. Thinking I may put two heads on one mount and one motor, but need to know about what size motor to find.

 Thanks.