You size a compressor for the most demanding tool you own, not for the average one. A 40 A plasma cutter needs about 150 to 180 litres per minute at 5 bar, a 60 A cutter about 200 to 250, a 100 A cutter and above reaches 400 litres per minute. An air angle grinder takes as much as 400 to 600 litres per minute, which is more than most plasma cutters.
Watch how output is quoted. Makers often give displacement rather than actual free air delivery. On a piston compressor the delivery is usually 60 to 75 percent of the displacement. We quote the measured tank filling time so that units can be compared.
A piston compressor is cheaper and enough for intermittent work. It has a low duty cycle, usually 50 to 60 percent, and it is loud. A screw compressor runs continuously, is quieter and gives dry air at a steady pressure, but it costs more and needs regular oil and separator changes.
For plasma cutting and for spraying you must add a filter with a dryer. Moisture and oil destroy plasma nozzles within a dozen or so minutes and ruin paint finishes. The tank alone does not dry the air well enough.
Tank capacity smooths out demand. For a 40 A plasma cutter count on 100 litres as a minimum, for a 100 A cutter rather 270 litres and more. On tanks we check the condition of the shell and the inspection paperwork, and we state what we found in the product listing.