The Nuclear Energy Institute maintains a running comparison of US capacity factors by fuel type, and the chart puts nuclear at the top of the list. Capacity factor measures actual electricity produced against the maximum a unit could generate if it ran at full power for every hour of the year. NEI builds the series from US Energy Information Administration generation data.

Nuclear plants clear 90 percent on that measure, a level no other US generating source sustains. Coal and combined cycle natural gas units sit well below, because they are dispatched against fuel prices and demand rather than run flat out. Wind and solar fall lower still, since output tracks weather and daylight rather than operator decisions.

The gap comes from how the fleet is run. A US reactor operates continuously between refuelling outages, which come every 18 to 24 months and are scheduled in spring and autumn when regional demand dips. Between those outages the unit holds full power, with online maintenance handled through redundant systems.

That profile matters for grid planners sizing reserve margins. A gigawatt of nuclear capacity delivers close to a gigawatt of round the clock output, while a gigawatt of intermittent capacity delivers a fraction of it across a year. The difference is why capacity factor, rather than nameplate rating, is the figure resource planners compare when they weigh uprates at existing reactors against new capacity.

NEI publishes the underlying figures alongside its other operating statistics, including plant licence data, uprate history and yearly capacity additions across the operating fleet.

Source: Nuclear Energy Institute - https://www.nei.org/resources/statistics/us-capacity-factors-by-fuel-type