The next factor to consider is tank size.
Tank sizes are generally stated in US gallons. For example, 30 gallon is a common tank size. So the question remains, how large a tank do I need? First of all, don't confuse a large tank with more run time for your air tools. If you use your impact intermittently, a large tank is fine. However if you have need for continuous use, you will need a small tank with big enough pump and motor. If the pump and motor are powerful enough, you shouldn't run out of air. You can save some dollars by purchasing an air compressor with a large tank and smaller motor for intermittent use. If you need to run a 1" impact wrench (about 20 CFM) intermittently, and have a small compressor with a large tank, you might have enough air stored in the tank to do the job. However, if you are constantly running your air tool, you will need to invest in a more powerful air compressor to do the job.