The short answer
An inverter lets the compressor change speed. Instead of switching on and off, the system runs hard to reach the setting, then slows down to match what the room needs, holding a steadier temperature with less noise.
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Simpler air conditioners work like a basic heater thermostat: full power until the room reaches the setting, then off until it drifts away again. Most split systems made for homes behave differently, because of a board of electronics in the outdoor unit that most owners never see. It shapes how the system sounds, how steady the room feels and how well it copes on days when only a little cooling or heating is needed.
Fixed speed and variable speed
A fixed-speed compressor has two states: on and off. Its motor runs at a speed set by the mains supply, so the only way to control the room is to switch the compressor on and off, letting the temperature drift a little above and below your setting each time.
An inverter changes the compressor's speed instead. The system can run fast to cool or warm a room quickly, then slow right down and keep ticking over, delivering just enough to hold the temperature.
| Fixed speed | Inverter | |
|---|---|---|
| Compressor | Full speed or off | Speed varies with demand |
| Room temperature | Swings around the setting | Stays closer to the setting |
| Starting up | Brief surge of current | Ramps up gradually |
| Noise | Noticeable starts and stops | Quieter when running slowly |
| Found in | Many portable units | Most split systems |
How the electronics control the compressor
Mains electricity in the UK alternates at a fixed 50 hertz (Hz). An inverter first converts it to direct current, then uses fast electronic switches to build a new supply whose frequency it can vary. A higher frequency spins the compressor motor faster, pumping more refrigerant and moving more heat. A lower frequency slows it down.
That is why engineers often describe compressor speed in hertz. The phrase "DC inverter" on a specification usually means the compressor, and often the fans, use brushless DC motors driven this way.
The control board sets the speed from what it measures: the room temperature against your setting, the temperatures of the coils and the temperature outside. Fan speeds change at the same time, and the expansion valve opens or closes to keep the flow of refrigerant matched to the compressor. How those parts work together is explained in the refrigeration cycle, step by step.
What you notice day to day
- At switch-on, the compressor runs fast to bring the room to your setting. The outdoor unit is at its loudest then, and a boost or powerful mode, where there is one, pushes it harder for a limited time.
- As the room gets close, the compressor slows. The outdoor unit quietens, and the air from the indoor unit becomes less cold, or less warm when heating, because less heat is being moved.
- On mild days it may still stop and start. Every inverter has a minimum speed, and if the room needs less than the system's lowest output, it switches off and on to avoid overshooting.
That minimum speed is one reason a unit much bigger than the room needs is not an improvement: it spends more of its time at the bottom of its range, cycling on and off. See what size air conditioner you need.
Because an inverter system handles a steady setting well, there is less reason to keep switching it on and off through the day. Whether leaving it running uses more or less electricity overall depends on your home and how long rooms stay empty, as explained in leave it on or switch it off.
Efficiency at part load
Running slowly suits the refrigeration cycle. When the compressor moves less refrigerant, the coils have more surface for the amount of heat being moved, so the refrigerant doesn't need to be as cold in one coil or as hot in the other. The compressor does less work for each unit of heat, and much of the energy wasted when a system repeatedly stops and restarts is avoided.
This matters because for much of the year a room needs only part of a system's full output. The seasonal figures on the energy label, SEER for cooling and SCOP for heating, are worked out across a range of conditions that includes running at part load, so they reflect this kind of use rather than full power alone. The label is explained in energy labels: SEER and SCOP explained.
Efficiency at any moment still depends on the weather. When heating, it falls as the outdoor temperature drops, whatever the compressor speed.
What an inverter can't do
- Make a small system big enough. At full speed an inverter unit still has a maximum output.
- Slow down indefinitely. Below its minimum speed, it cycles like any other system.
- Avoid electronics faults. The inverter board is one more part that can fail, and if it does the system usually stops and shows an error code.
Related guides
- The refrigeration cycle, step by stepRefrigerant boils in a cold coil, taking in heat; a compressor squeezes the vapour, making it hot; it condenses in a second coil, giving the heat away; and an expansion valve drops its pressure so it can boil again.
- What size air conditioner do you need?Most bedrooms and smaller living rooms suit a unit of about 2.0 to 3.5 kW, and large open-plan spaces often need 5.0 kW or more. Sun, glazing, insulation and ceiling height can matter as much as floor area.
- Leave it on or switch it off?It depends on how long the room will be empty. For an hour or two the difference is small, so choose on comfort; for a day or longer, switching off or turning the setting back usually means less heat for the system to move.
- Energy labels: SEER and SCOP explainedSEER and SCOP are the seasonal efficiency ratings on an air conditioner's energy label, for cooling and heating. Each shows roughly how much heat the unit moves over a standard season for every unit of electricity it uses, and higher is better.