Coal usage has fallen from 50% of the country’s energy mix in 2008 to only 37% in 2013.

In the past five years 21 gigawatts of power capacity has been shuttered, with plans to close a further 34GW in place.

Tens of thousands of gigawatts are considered “economically vulnerable” and could be considered for shuttering if market conditions remain weak.

Environmental regulations are a major factor in the transition away from coal.

As the 2015 deadline looms for miners to comply with the Environmental Protection Agency's mercury and air toxics standards, producers are weighing the costs of installing expensive pollution-control equipment on ageing plants against investing in cleaner alternatives.

Often the financials lead utilities towards the cleaner alternatives.

The Union of Concerned Scientists studied the 290GW of US coal that was not scheduled for retirement and concluded that required retrofits would render 71GW of it more expensive than the cost of building new wind capacity with the US23c per kilowatt-hour federal production tax credit included.

Even without the PTC, wind is a cheaper alternative to retrofitting 22GW of coal, according to UCS data.

Wind power’s biggest competition is undoubtedly gas.

With an abundance of the cheap fuel, it could easily replace all of the mothballed power stations without a need for alternative energy.

According to research firm SNL Energy, there is about 88GW of gas generation under development.

Keeping wind in the market, however, is the volatile nature of gas prices.

The polar vortex that hit North America this previous winter caused gas prices to soar, making wind generation a viable option.

It provided a real-world lesson in the drawbacks of relying too heavily on a single energy source.

A study from PJM Interconnection found that raising renewable energy from about 2% of the electricity supply at present to 30% by 2026 could cut carbon dioxide emissions by 40% and still maintain grid reliability.

UCS expects the EPA's carbon standards will have an impact on supply choices as policy makers and utilities look at how to reduce emissions while remaining cost-effective.