How can you work out the power of an Air Compressor?

The mfr always want to sell you a new one. Capacity of air compressors is a jealously guarded secret. Whatever they tell you, there is no way you can verify it, unless you run this simple test which few people know,

air_flow = tank_volume x (high_press - low_press) / atm_press / time_on_or_off

Run the test a few times with a good pressure gauge to get the hang of it. First when the compressor is off, with no external load, which will tell you what the losses are. Then, run it with the compressor running, which will tell you the difference between load and output. Add the two to obtain the compressor capacity.

Then, compare the obtained capacity with the stated load and pressure of the air tool.

As far as the hp, check the existing motor nameplate. If it has no motor, talk to a shop mechanic, they should be able to tell you. If it is single phase, it should be like 1hp, o about.

combined gas law, measure the pressure it can get in how much time, and there is even a factor for temperature, Boyle's law has no temperature factor.

basically there is 2 types of compressor, hvlp and hplv, hplv is most times a centrifugal type impeller like in a vacuum cleaner, the other type is most times a piston type pump.

hplv rely on a reserve tank to catch up to demand or have to be big pumps.

find pressure and cfm of your tools, calculate what % the time they are in use. that plus how long it takes the compressor to fill the tank to its max rated pressure will give you numbers top compare to see if its a workable situation