London-based companies will throw away £79m this summer alone, through wasted energy, say campaigners.
Businesses produce much of London's carbon emissions
The Carbon Trust's drive to make firms more energy-efficient will point out both the environmental, and financial benefits of going green.
It says that, even in the summer months, 20% of energy bills are from wasted energy.
A £4m advertising campaign begins on Thursday to demonstrate how the trust can help firms save energy and money.
The Carbon Trust, set up by the government, carries out energy surveys of businesses and offers interest-free loans up to £100,000 to improve efficiency.
It highlights the case of Regis House, near London Bridge, where it says Land Securities saved more than £44,000 a year through following some of its recommendations.
Spokesman Dr Garry Felgate said: "Businesses in greater London are wasting millions on energy, hitting the environment and their bottom line.
"Today's campaign will empower companies in greater London to make business sense of climate change by taking cost effective action - to cut carbon and make significant savings on their bottom line."