A pre-CRC energy investigation by Cardiff Council found that the electricity bill for the city’s County Hall was £500,000.
And some staff were bringing in their own heaters because the building was so energy inefficient.
Cardiff carried out to survey to find out how prepared it was for the CRC, which has its deadline day for registration on September 30.
The council has set itself a target of reducing CO2 emissions by 60% by 2018.
Councillor Simon Wakefield said all council buildings would have to be looked at to target energy savings.
The Carbon Trust believes County Hall needs new lighting, cooling equipment, and an improvement in its perimeter heating systems.
Raoul Bhambral, co-ordinator for Friends of the Earth Cardiff, said: “The council will definitely have to do everything it can to reduce emissions not only in its own buildings, but across Cardiff as a whole.”