In 2015, the United States added about 3,600 megawatts of wind power capacity in the first nine months. This addition is almost triple the amount from a year earlier and is attributed to private sector businesses and local governments moving to lineup their own supplies of electricity. The growth was driven largely by private companies who are now joining utilities which have traditionally been the largest developers of wind energy projects. Amazon, Hewlett Packard and the City of Washington D.C. all signed contracts in the third quarter of 2015 to purchase wind power. In addition, Microsoft and Wal-Mart have announced investments in two major wind products. The cost of producing energy through wind power has fallen by more than half over the past five years, and experts agree, it is the cheapest new source of renewable energy.