What’s the difference between single, two-stage, and variable speed compressors? Single-stage compressors have one output speed – high. Two-stage compressors operate most often at about 65% of total capacity, but full capacity during extreme temps. A variable-speed compressor has a fan motor that can move at different speeds and controls the amount of air dispersed in a home. A single-stage unit may offer the lowest initial investment, but it can cause high monthly energy bills. A two-stage unit is quieter and more efficient, while variable speed units are the most intelligent on the market and can save you big on energy costs over their lifetime.