Universities have been wasting electricity by keeping some parts of campuses at temperatures as low as 18 degrees Celsius, according to an environmental concern group.
Green Sense's investigations recorded that temperature in corridors in the Cheng Yu Tung Tower of the Centennial Campus of the University of Hong Kong.
The group also discovered many vacant classrooms in which the air conditioners had been left switched on, and ventilators keeping empty corridors cooled overnight.
Roy Tam Hoi-pong, president of Green Sense, described the situation as a waste of the public funds used for running the tertiary institutions.
Wan Tsz-ying, external vice-president of a green society at Chinese University, also expressed her concern.
"The air-con in some academic buildings is really cold … even if you only pass by the buildings without entering them, you can already feel the streams of cold air," she said.
Green Sense has advocated 23 degrees as the optimal temperature for air-conditioned spaces. The temperature recommended by the government is 25.5 degrees.
The group's findings were released ahead of its No Air Con Night on Thursday, an event in which more than 200 schools and 100 companies will call on their students and staff to refrain from using air conditioners from 7pm to 7am the next day.
Tam said the exercise should reduce the total electricity consumed by the whole of Hong Kong that night by about 1 per cent.
Although this was not a significant drop, Tam added, that it fitted with the campaign's goal of advocating energy conservation in the city.