Extending Daylight Saving Time by four weeks last year reduced U.S. energy consumption by 17 trillion British thermal units, or the equivalent of enough energy to power 100,000 households for a year. That's according to a report to Congress from the U.S. Department of Energy by researchers at Oak Ridge National Laboratory, Pacific Northwest National Laboratory and National Renewable Energy Laboratory. Researchers sought to quantify the savings resulting from the Energy Policy Act of 2005, which extended the duration of Daylight Saving Time. The extension went into effect in March 2007. The study found that electricity consumption in 2007 decreased by an average of 0.5 percent per day during the extra four weeks, which adds up to 1.3 billion kilowatt hours. Savings in northern regions were greater than in the south, which may be attributable to increased air-conditioning usage. The work is funded by DOE's Office of Energy Efficiency and Renewable Energy.