Picture an area as large as the city of Philadelphia filled with solar panels.
Now imagine the same for Pittsburgh. Put them together, and that’s how much Pennsylvania land could be converted into solar energy plants by 2030 if the commonwealth joins the Regional Greenhouse Gas Initiative.
All that land would produce less than 10% of the state’s total electricity, according to the Pennsylvania Department of Environmental Protection. 
The sheer size of solar farms compared to conventionally fueled power plants is beginning to get attention as advocates press for expanding their use.
“Solar Power’s Land Grab Hits a Snag: Environmentalists,” read a recent headline in The Wall Street Journal. “These large projects are increasingly drawing opposition from environmental activists and local residents who say they are ardent supporters of clean energy. Their objections range from a desire to keep the land unspoiled to protection for endangered species to concerns that their views would no longer be as beautiful. 
In one part of Nevada, the Sierra Club is opposed to a 7,100-acre solar plant. In another, residents are fighting the construction of a plant that would cover 14 square miles with solar panels up to 20 feet tall. At peak performance, it would produce 850 megawatts of electricity.
By comparison, the coal-fired Bruce Mansfield plant in Beaver County that shut down in 2019 could produce three times more power (2,490 MW) on less than one square mile of land. And unlike a solar farm dependent on weather and time of day, the coal plant could produce maximum power 24 hours a day if it had fuel to burn.
Consider the proposed project near Gettysburg examined by the news outlet Postindustrial. NextEra Energy Resources has leased 1,000 acres of land near the battlefield to install solar panels with a maximum production of 75 megawatts.
Compare that to the Cheswick power plant scheduled to close in September, which can produce eight times more power on less than 100 acres.
“This proposed project is mind-boggling. I mean, what are we doing?” asked farmer Thomas Newhart, who lives next to the proposed NextEra solar plant in Mount Joy Township. 
Pennsylvania can expect to see much more of that if the Wolf administration moves forward with its plan to join RGGI and use the fees collected for producing carbon emissions to stimulate investment in solar energy. The DEP’s RGGI point person said last year that the administration expects utility-scale solar projects in the state to go from virtually nothing to roughly 9,300 megawatts of capacity by 2030. 
How much land would that take? The proposed Mount Joy project would have a capacity of .075 megawatts per acre, slightly less than the Nevada project’s .093 megawatts per acre.
Nevada’s solar patterns have the highest potential for energy in the U.S., according to data from the National Renewable Energy Laboratory, while Pennsylvania is in the bottom third of all states.  The Solar Energy Industries Association, a trade group, estimates that the megawatts per acre for utility-scale solar is .1 to .2, depending on the technology. 
To provide 9,300 megawatts of solar capacity at .075 megawatts per acre would require 124,000 acres, or 194 square miles of land. That’s as large as the land area in Philadelphia (134 square miles) plus Pittsburgh (58). Using the more accommodating estimate from the SEIA (.1 megawatts per acre) equals 145 square miles, still larger than Philadelphia.
Even so, the solar panels would be able to supply less than 10% of the state’s current electricity production. U.S. Energy Information Administration data shows solar panels, on average, produce less than 25% of their maximum capacity over the course of a year.  While that number, known as “capacity factor,” increased sharply in the early part of that last decade, it plateaued several years ago and recently began to drop. Researchers from Lawrence Berkley National Laboratory said the expansion of solar plants into less sunny areas has offset improvements in technology, leading to the decline. 
Below is a printable version of the research brief.