From the Document: "Policy options to increase the use of renewable energy sources for electricity generation have drawn congressional interest. Some renewable energy sources, such as wind and solar, have variable supply. Electricity generators using variable renewable energy (VRE) sources can only produce electricity when weather conditions are right (i.e., when the wind is blowing or the sun is shining). In contrast, conventional energy sources used to generate electricity are typically available when called upon. Frequently, the energy source can be stored on-site, or otherwise be directly accessed by generators. Examples include coal, natural gas, water stored in reservoirs, and nuclear fuel rods. Supplies of these energy sources are typically sufficient to generate electricity steadily for periods of days to months, although supplies are not guaranteed. Extreme weather events or disruptions in fuel transportation networks could potentially create supply shortages. Nonetheless, many consider these conventional energy sources to be more reliable than VRE. Policy, technology developments, and market forces are leading to an increased use of VRE sources for electricity generation. Members of Congress may be interested in potential reliability issues and possible solutions."
CRS In Focus, IF11257
Congressional Research Service: https://crsreports.congress.gov/