People buying a window-type air conditioner should go for one that runs on an inverter if they want to save money on their electricity bills, the Consumer Council said on Monday.

The watchdog said it tested 13 window-type air conditioners and found that all of the inverter models performed better when it came to energy efficiency, using an average 27 percent less than the non-inverter types.

In an inverter model, the motor runs constantly at various speeds to ensure the desired room temperature, rather than switching on and off like a conventional model does. This makes the inverter type more energy efficient.

“If you are planning to buy a new [window-type] air conditioner, probably you have to consider the inverter type, based on the fact that it is getting more popular and more energy efficient [given that] electricity prices right now are more expensive,” said Gilly Wong, the council's chief executive.

Nora Tam, who chairs the council's research and testing committee, noted however that other things will also affect how much it costs to run an air conditioner.

“It's not only which model of the air conditioner you need to pay attention to, it's actually how long you run it and how often you run it, and whether you close all the windows and curtains and so on. All these affect the electricity bill and energy consumption," she pointed out.