You have two quotes on the desk. Both say 50 HP. One says 240 CFM, the other says 250. One is eleven thousand dollars cheaper. Your purchasing department wants to know why you would not take the cheaper one.
Here is the answer, and it is not a matter of opinion. Ask both vendors for the CAGI data sheet on the exact model they quoted. Everything you need to make that decision is on one page, in the same format, from every manufacturer, verified by a third party. It is the single most useful document in this industry and most buyers have never been handed one.
What a CAGI data sheet is
The Compressed Air and Gas Institute runs a Performance Verification Program for rotary compressors and refrigerated dryers. Participating manufacturers publish performance on a standardized one page sheet, measured to ISO 1217 for compressors and ISO 7183 for dryers, and they contract an independent third party to verify the stated numbers.
Two things follow from that, and both matter to you.
First, the numbers are directly comparable. Everybody measures the same way and reports the same fields in the same order, so you can put two sheets side by side and the comparison is real.
Second, the numbers are checked. A manufacturer in the program has agreed to independent verification, and units get tested. That is a meaningfully different thing from a marketing spec sheet.
If a vendor cannot produce a CAGI sheet for the model they quoted, that is information too. Ask why.
The lines that matter, in order of how much they should move your decision
1. Package specific power
This is the one. Specific power is the input electrical power required to deliver 100 CFM at the rated discharge pressure, expressed in kilowatts per 100 CFM. Lower is better.
It is the whole efficiency argument in a single number, and it is the number that decides what the machine costs you over ten years. Everything else on the sheet is context for this line.
The word package is doing real work there. It means the number includes the whole machine as it sits on the floor, cooling fan and all, not just the airend. Vendors occasionally quote a bare compressor efficiency that looks better and does not describe anything you can buy.
Run the arithmetic before you sign. Take the difference in specific power between the two machines, multiply by your hours and your utility rate, and compare that to the difference in purchase price. On a machine running two shifts, a small specific power gap can easily be worth more than the eleven thousand dollars over the life of the unit.
2. Actual delivered flow, at the pressure you actually run
The sheet gives capacity at a stated discharge pressure, at stated inlet conditions. Two things to watch.
Check that both quotes are at the same pressure. A machine rated at 100 psig and one rated at 125 psig are not comparable, and the one rated lower will look better on both flow and specific power. If you run 110 psi, you want numbers at or near 110 psi from both vendors.
Then check the units and the reference conditions. CAGI sheets report actual volume flow at inlet conditions, which is not the same as the SCFM figure a marketing sheet might use. Our note on SCFM versus CFM versus ACFM explains why those three numbers disagree and by how much.
3. Total package input power
The kW the whole machine actually draws at full load. This is what shows on your bill and what your electrician needs. Compare it to the motor nameplate horsepower and you will often find the package draws more than the nameplate implies, because the nameplate describes the motor and not the package.
It also feeds directly into wire and breaker sizing, which is a real line item on an install.
4. Part load performance
Almost nobody runs a compressor at 100 percent all day. If the sheet includes part load data, and on variable speed machines it should, this is where the real world lives.
A variable speed machine and a fixed speed machine can look almost identical at full load and be wildly different at 50 percent. If your demand swings, the part load numbers matter more than the full load headline. Our breakdown of control modes covers what each type does when demand falls off.
5. Isentropic efficiency
Some rotary sheets now report this. It is a thermodynamic measure of how close the compression process gets to the theoretical ideal, and it is useful for comparing airend designs. For most buyers it is a secondary number. Package specific power is still the one that pays your bills.
6. Sound level
Reported in dBA at a stated distance under a stated method. Worth checking if the machine is going anywhere near people, because a difference of a few dBA is more noticeable than it sounds on paper. See our quiet compressor guide if noise is a constraint.
A worked comparison
Two 50 HP fixed speed machines, both quoted at 125 psig, running 4,000 hours a year at 12 cents per kWh.
| Line | Machine A | Machine B |
|---|---|---|
| Rated capacity at 125 psig | 240 CFM | 250 CFM |
| Package input power | 42.0 kW | 40.5 kW |
| Package specific power | 17.5 kW per 100 CFM | 16.2 kW per 100 CFM |
| Purchase price | Lower by 11,000 | Higher |
| Annual energy at 4,000 hours | About 20,160 dollars | About 19,440 dollars |
That is roughly 720 dollars a year, which pays back the difference in about fifteen years. On those numbers, at that duty, the cheaper machine is genuinely the better buy and you should take it.
Now run the same two machines at 8,000 hours, which is a two shift operation, and the gap is about 1,440 dollars a year. Add a few cents to the utility rate and it closes faster. The point is not that efficiency always wins. The point is that the CAGI sheet lets you do the arithmetic instead of guessing, and the answer depends entirely on your hours and your rate.
These figures are illustrative. Use the real numbers off the real sheets for the models you were quoted.
How to actually use this when you are buying
- Ask for the CAGI data sheet for the specific model and pressure quoted. Not the family, not the nearest size. The exact model.
- Confirm both sheets are at the same discharge pressure. If not, get matching ones.
- Compare package specific power first. Everything else is secondary.
- Do the ten year math with your actual run hours and your actual utility rate. Energy usually dwarfs purchase price on a machine that runs, which is the argument our piece on sticker price versus ten year cost makes at length.
- Check part load if your demand varies. Full load numbers describe a condition you may rarely be in.
- Do not stop at the compressor. The dryer has its own CAGI sheet and its own energy cost, and a dryer that eats purge air is a real operating expense. See purge air loss.
- Remember the sheet does not cover everything. Service intervals, parts cost, local support and how quickly somebody can get to you at 6 a.m. are not on it, and they matter.
What the sheet will not tell you
Being straight about the limits, because we would rather you trust the rest of this.
The sheet reports performance at defined inlet conditions. Your compressor room is hotter than that, and if you are at elevation your machine is derated, which our note on altitude derating covers. The sheet is a level playing field for comparison, not a prediction of what you will see on the gauge.
It also says nothing about reliability, build quality, or what the airend costs to rebuild in year eight. See airend rebuild economics for that side of the decision.
Frequently Asked Questions
What is a CAGI data sheet?
A standardized one page performance sheet published by compressor manufacturers participating in the Compressed Air and Gas Institute Performance Verification Program. Performance is measured to ISO 1217 for compressors and ISO 7183 for dryers, reported in a common format, and verified by an independent third party, which makes two sheets directly comparable.
What is package specific power?
The input electrical power required to deliver 100 CFM at the rated discharge pressure, in kilowatts per 100 CFM. Lower is better. It is the single most important number for comparing two compressors, because it determines what the machine costs you to run. Package means the whole unit including cooling fans, not just the airend.
Why do two 50 HP compressors deliver different CFM?
Horsepower describes the motor, not the output. Airend design, rotor profile, discharge pressure and cooling all affect how much air you get per kilowatt in. That is exactly why specific power exists as a metric and why comparing nameplate horsepower tells you almost nothing.
Can I compare compressors rated at different pressures?
Not directly. A machine rated at 100 psig will show better flow and better specific power than the same machine at 125 psig. Get both sheets at the pressure you actually intend to run, or the comparison is meaningless.
What if a vendor will not provide a CAGI data sheet?
Ask why. Not every manufacturer participates in the program, and not participating is not automatically disqualifying, but it does mean you have no verified, comparable performance data for that machine. At that point you are comparing a verified number to a marketing claim, and you should price that uncertainty into the decision.
The short version
Get the sheet, match the pressure, compare package specific power, and multiply the difference by your hours and your rate. That is the entire method, and it turns a purchase decision that usually comes down to who quoted lowest into one you can defend with arithmetic.
If you want help running that math on quotes you already have, send them over. Or browse our rotary screw air compressors and ask us for the data sheet on anything you are considering. We will send it without being asked twice.
