For the solar and wind industries in the United States, it has been a long-held dream: to produce energy at a cost equal to conventional sources like coal and natural gas. That day appears to be dawning. In some markets renewable generation is now cheaper than coal or natural gas. Utility executives say the trend has accelerated this year, with several companies signing contracts, known as power purchase agreements, for solar or wind at prices below that of natural gas, especially in the Great Plains and Southwest, where wind and sunlight are abundant. Those prices were made possible by generous subsidies that could soon diminish or expire, but recent analyses show that even without those subsidies, alternative energies can often compete with traditional sources. According to a study by the investment banking firm Lazard, the cost of utility-scale solar energy is as low as 5.6 cents a kilowatt-hour, and wind is as low as 1.4 cents. In comparison, natural gas comes at 6.1 cents a kilowatt-hour on the low end and coal at 6.6 cents. Without subsidies, the firm's analysis shows, solar costs about 7.2 cents a kilowatt-hour at the low end, with wind at 3.7 cents. It is really quite notable, when compared to where we were just five years ago, to see the decline in the cost of these technologies," said Jonathan Mir, a managing director at Lazard, which has been comparing the economics of power generation technologies since 2008.