Hi Fred,A two stage is better for the following reasons: 1. Can operate easily to a higher pressure - e.g. 175 psig as compared with 125 or 135 psig. That means more cubic feet of air can be stored in the same sized tank and gives a longer time period between compressor starts for a similar air consumption. 2. Cooler discharge temperatures. A two-stage discharging at 175 psig operates at a lower compression ratio per stage than a single stage operating at 125. For example the first stage discharge would be about 40 psig. The air would then typically be passed through an intercooler device (finned tubing) before being introduced into the second stage where it would be compressed and discharged at 175 psig. The compression ratios per stage would be about the same at 3.6:1. By comparison, the single stage, when discharging at 125 psig, has a compression ratio of about 9.5:1. The higher compression ratio with no opportunity for any interstage cooling results in higher discharge temperatures. This, in turn causes everything to run hotter with less moisture dropout in the storage tank. This is particularly noticeable when a single stage is of marginal capacity for the usage and runs for lengthy periods with little down time for cooling off. Most two-stage units are designed and rated for continuous operation without overheating. I question if most single stage ones are. 3. More efficient regarding power required for the same capacity. The first two above are the more important considerations for a home shop unit as the power difference is small in the size units generally used in such instances. If you were talking 50 HP (or more) units, the power savings of a two-stage could be a key decision factor. In general, the two-stage would be expected to cost more for a similar capacity.  Rod
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