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.