Since the start of the month, 11 of Southwark Council's largest sites and buildings - including John Harvard Library in Borough High Street - have been using green electricity.
This clean electricity does not pump carbon dioxide into the atmosphere.
Normal electricity (known as brown electricity) is generated at a fossil fuel burning plant which emits carbon dioxide gas – the main gas which causes global warning. Green electricity is generated from renewable energy sources which do not pollute the atmosphere.
Southwark Council say the switch has not only cut harmful emissions in the borough by 1,170 tonnes a year but will also save more than £22,000 on annual electricity costs.
The move follows a recent tender for electricity in which the sites were put forward for green supply offers. The contracts (awarded to London Electricity and Ecotricity) are for two years.
Cllr Nick Dolezal, Southwark's Cabinet Member for Transport and the Environment, said: "This is electrifying news! We win both ways – less CO2 and less money."