Source: U.S. Energy Information Administration,

Note: Only utility-scale projects (those greater than one megawatt and connected to the grid) are measured. U.S. Energy Information Administration, Preliminary Monthly Electric Generator Inventory Only utility-scale projects (those greater than one megawatt and connected to the grid) are measured.

New electricity generating capacity is brought online throughout the year, but December has historically been the most significant month for both onshore wind and solar photovoltaic (PV) capacity additions in the United States. Over the past nine years (2010–2018), 43% of all onshore wind capacity and 33% of all solar PV capacity have been added in December. U.S. wind and solar capacities receive a production tax credit (PTC) and an investment tax credit (ITC), respectively, which may provide incentives to come online at the end of the year.

In the United States from 2010 through 2018, December wind capacity additions ranged from 29% to 60% of a given year’s total wind capacity additions. In 2018, 58% of wind capacity additions were installed in December, the highest share since 2013, when 60% of wind capacity additions were installed in December.

Over the past nine years, U.S. solar PV capacity additions in December ranged from 18% to 46% of a given year’s total solar PV capacity additions. December’s share of annual solar PV capacity additions in 2018 (31%) was close to its average over the past nine years (33%). The highest monthly share of annual U.S. solar PV capacity additions occurred in 2015, when 46% of the annual additions came online in December.

Although a relatively large amount of U.S. natural gas-fired generating capacity comes online in December as well, over 45% of all natural gas-fired capacity additions between 2010 and 2018 occurred in the second quarter of the year (April, May, and June). In 2018, second-quarter U.S. natural gas-fired generating capacity additions accounted for almost 50% of the annual additions. Natural gas-fired capacity added during the second quarter help ensure that adequate capacity is available to meet peak summer electricity demands.

Capacity additions for other technologies in the United States (all utility-scale electricity generation from sources besides wind, solar PV, and natural gas) are more evenly distributed throughout the year.

Source: U.S. Energy Information Administration,

Note: Only utility-scale projects (those greater than one megawatt and connected to the grid) are measured. U.S. Energy Information Administration, Preliminary Monthly Electric Generator Inventory Only utility-scale projects (those greater than one megawatt and connected to the grid) are measured.

Historically, 2012 had the most onshore wind capacity additions with 13.1 gigawatts (GW) installed in total, 5.1 GW of which were installed in December. The PTC—which provides a tax incentive per megawatthour generated by a qualifying technology in the United States for 10 years—initially expired at the end of 2012, leading developers to accelerate projects to bring them online before the end of the year. Although the PTC was renewed as part of the American Taxpayer Relief Act of 2012 signed on January 2, 2013, accelerated wind capacity additions in 2012 led to fewer additions in 2013. Still, most wind additions in 2013 occurred in December.

Solar PV experienced its largest single month of U.S. capacity additions in December 2016 with 3 GW added, almost twice as much as the next largest month for capacity additions (December 2017, with 1.6 GW installed). Similar to the PTC, the investment tax credit (ITC) was initially set to expire at the end of 2016. The Consolidated and Further Continuing Appropriations Act, passed on December 16, 2015, extended full ITC eligibility for utility-scale solar PV projects into the 2020s, and it decreased the ITC to 10% indefinitely.

Principal contributors: Richard Bowers, Kenneth Dubin