Some leading corporations—Google, Siemens AG, HSBC, and Citigroup among them—are discovering new profits by retrofitting office buildings to be more energy efficient. Savings due to lowered energy costs are yielding profits added to the bottom line—and those savings can be big.

Citigroup projects savings of almost $100 million annually on its 92 million square feet of real estate by turning off escalators, changing thermostat settings, switching off unneeded lights and installing timed lighting, using solar heat, and adopting other power-cutting methods.

Siemens AG is measuring emissions from its 800 buildings in the U.S. and is considering a requirement for green design in leased properties.

HSBC has produced a green prototype design for its 400 bank branches.

Google has installed a 1.6 million megawatt solar rooftop at its headquarters, large enough to power 1,000 homes.

Power use by all buildings worldwide accounts for nearly 20% of all greenhouse gas emissions. One more time, the lesson becomes clear: more efficient energy use can help the environment while producing healthy profits.