Lawmakers' filibustering is a major reason why the Legislative Council has used 40 per cent more electricity than projected five years ago, auditors say.
By comparison, the government offices next door sharing the Admiralty complex have used 3 per cent less power than expected.
But the Tamar site is expected to take as many as 176 years for its energy savings to offset the cost of installing environmentally friendly equipment - far exceeding the expected nine years.
The findings were part of a report by the Audit Commission looking into design problems at the complex.
"The original estimates were based on the number and duration of meetings and activities held in the former Legco building [in Central]," the Architectural Services Department said in the audit report that was released yesterday.
The surge in electricity usage was due to a "substantial increase in the number and duration of meetings" and system rectification and testing works after office hours and at weekends, the auditors said, citing the Legco Secretariat.
These activities were not foreseen when the department drew up the estimates in 2009.
The government offices and legislature moved into their new home in 2011.
According to the projections, the Tamar complex was supposed to use 37 million kilowatt-hours of electricity a year, including 7.5 million kWh in the Legco building.
But Legco used almost 10.7 million kWh in the 13 months to August, 43 per cent more than estimated.
In the past year, lawmakers employed the delaying tactic of filibustering at least three times during debates on a higher old-age allowance, formation of a culture bureau and the pro-establishment camp's anti-filibuster efforts.
The government also came under fire from the auditors for insufficient promotion of its energy-saving measures, despite its HK$109 million investment, and for the long payback period of some green installations.
For example, it will take 176½ years to recover the cost of installing dimming controls for a footbridge linking the complex with the Admiralty Centre. The HK$300,000 lighting system saves only HK$5 a day, while it will take 87 years to recover the cost of the complex's HK$30.8 million LED lighting system.
Edwin Lau Che-feng, of Friends of the Earth, said the complex was not a commercial company but a landmark in which the government demonstrated its determination to create a green environment. "It's not a big sin. But the report sends a message to lawmakers that they should work out a basket of measures to save more energy."