A new law went into effect on Jan. 1 in Washington, D.C. requiring building owners to begin measuring the energy use of new and existing commercial buildings using the Environmental Protection Agency’s Energy Star Portfolio Manager tool.

The law, approved and signed in 2008, is the first in the nation to require building owners to publicly disclose their energy ratings to prospective tenants and buyers. Building owners must begin revealing their energy ratings beginning on Jan. 1, 2012.

You can’t manage what you don’t measure,” says Cliff Majersik, executive director of the Institute for Market Transformation, a Washington, D.C.-based non-profit focused on increasing energy efficiency in buildings. “We applaud Mayor Fenty and the city council for their leadership to slash energy waste and reduce energy costs for building owners and consumers.

Other states and cities have adopted similar measures, including California and Seattle. Most recently, New York City on Dec. 9 approved a package of four bills geared toward improving energy efficiency of existing buildings. The package includes a similar benchmarking provision. But the District of Columbia is the first to require that the ratings be disclosed to potential tenants and buyers.

You want to start measuring energy use now so you have time to improve your ratings before you have to disclose them publicly,” says Jack Beuttell, Global Sustainability Manager for Hines, a global commercial real estate firm with 10 buildings in Washington, D.C., that are measured using Energy Star.