Yet despite literally paying people to take their electricity, wind power represents just 3.5 percent of all electricity generation in the United States. The big problem is not so much cost as reliability. Wind power is intermittent; it has a nasty habit of stopping, sometimes on a moment's notice. And since there is no commercially viable means of storing electricity, use of wind power requires the existence of back-up power plants (typically natural gas) that can be ramped up or down depending on which way the wind blows. This sort of redundancy is not only inefficient, but emissions levels are higher during the process of ramping a gas plant up and down, cancelling at least part of the environmental benefits of using wind. No amount of subsidies for generators will solve these problems, and, in fact, subsidies could serve to aggravate it by undercutting the profitability of back-up sources of power.