Single-Stage: A single-stage unit can either be off or on at full capacity. It will operate at full capacity regardless of how much cooling is required. Because air conditioning systems are sized to provide adequate cooling on the hottest day of the year, this means single-stage systems are often providing more cooling than is required. This reduces the efficiency of the system and can lead to uncomfortable conditions.
Two-stage: Two-stage compressors help address this mismatch of cooling needs and system output. A unit equipped with a two-stage compressor will have two distinct levels of cooling output, typically called the low and high stages. The high stage will have sufficient capacity to maintain comfortable temperatures in extreme weather, while the low stage will allow for efficient operation during mild conditions.
Variable Speed: Variable speed compressors are the full-out extension of two-stage compressors. Rather than having specific output levels at which to operate, a variable speed unit can produce any output within a range. This range will vary from model to model and will enable the unit to match the required cooling or heating load in real-time and maximize comfort and efficiency.
Two-stage and variable-speed units often have a higher rated efficiency than single-stage models, but even if all three of these units had the same rated efficiency, the longer cycles of a two-stage unit would save approximately 3 percent over a single-stage model, and a variable-speed model would offer savings of approximately 7 percent.