The US Energy Information Administration reports that natural gas-fired power plants generated 2% of the electricity produced in Pennsylvania in 2001. Over the following 20 years, that 2% proportion increased rapidly, reaching 52% in 2021. Natural gas displaced most coal-fired generation, which fell from 57% in 2001 to 12% in 2021.

Natural gas production in the state has also risen significantly over the past two decades, from 0.1 trillion cubic feet (Tcf) in 2001 to 7.6 Tcf in 2021, making the states output second only to that of Texas. Pennsylvania sits on top of the Marcellus shale – the portions of the Marcellus under Pennsylvania and West Virginia constitute the largest natural gas field in the USA. Although natural gas has been produced in the Marcellus for a long time, production became much more economical after fracking and horizontal drilling were developed. Th first shale well using these techniques was drilled in Pennsylvania in 2004. At the same time that natural gas production in Pennsylvania was increasing, coal production was declining, falling 40% from 74.1 million tons in 2001 to 42.5 million tons in 2021.

Increased production in Pennsylvania and elsewhere in the United States made natural gas abundant and relatively cheap. So utilities and power plant operators began to close aging coal-fired power plants in Pennsylvania, many of which were built in the 1970s and 1980s, and to replace them with new natural gas-fired combined-cycle plants.

Although coal’s generation share started declining in Pennsylvania in 2007, it remained the largest source of in-state electricity generation until 2015, when nuclear power overtook coal. Pennsylvania is home to eight nuclear reactors at four nuclear power plants.

In 2019, the remaining reactor at Pennsylvania’s Three Mile Island NPP closed, , and as a result, along with significant investment in new combined-cycle natural gas plants, natural gas consumption overtook nuclear as Pennsylvania’s largest source of in-state electricity generation.