Central AC System GuideInverter and variable-capacity air conditioners change how the compressor runs. Instead of switching fully on and fully off, an inverter drive varies the compressor motor speed so the system produces only as much cooling as the space needs at any moment. This guide explains how variable-capacity operation works, why it saves energy across a cooling season, and how the Department of Energy measures that seasonal efficiency with the SEER2 rating.
How inverter compressors differ from fixed-speed units
Energy Star describes a conventional single-speed compressor as working like an on/off switch, running at maximum capacity when the thermostat calls for cooling and shutting off once the set temperature is reached. An inverter or variable-speed compressor instead lets the speed of the compressor motor and fan fluctuate in response to the cooling needs of the space.
By ramping down when conditions are moderate and ramping up when it is hot outside, the system avoids the large swings of a fixed-speed unit. Energy Star notes that this lets a variable-speed unit maintain temperature with greater energy efficiency.
Why steady low-speed running saves energy and lowers noise
Because an inverter system can hold a low, consistent output, it avoids the energy-intensive spikes that come each time a fixed-speed compressor restarts. Energy Star notes that on most days keeping a variable-speed unit running can be more efficient than turning it on and off throughout the day, because it operates consistently at a lower, less energy-intensive speed without big jumps in cooling demand.
Quieter operation is a direct result of the same behavior. Energy Star states that the technology allows a variable-speed unit to operate more quietly than a single-speed unit when there is less of a cooling load.
Measuring seasonal efficiency with SEER2
Variable-capacity performance is captured in the seasonal efficiency rating the Department of Energy uses for central systems. The Department of Energy states that effective January 1, 2023, it changed how HVAC systems are tested and updated the national standards, introducing SEER2 to denote cooling efficiency in British thermal units per watt-hour.
Because SEER2 measures output across an entire cooling season rather than a single operating point, it rewards equipment that can run efficiently at partial load. The Department of Energy also directs federal buyers to purchase Energy Star certified or FEMP-designated central air conditioners.