Capacity factor is the single most important operational performance metric for a nuclear power plant, directly determining revenue generation and the economic viability of the investment. Calculated by dividing actual energy produced by the theoretical maximum output over a given period, capacity factor accounts for all sources of downtime: planned refueling outages, unplanned maintenance, regulatory shutdowns, and load-following reductions. The U.S. commercial nuclear fleet has achieved industry-leading capacity factors averaging over 93% in recent years, meaning plants operate at or near full power for roughly 340 days per year, a performance level unmatched by any other generation technology.
For SMR developers, projecting high capacity factors is essential to closing the LCOE gap with competing technologies. Longer refueling intervals are a key design goal: while conventional PWRs typically refuel every 18-24 months during planned outages of 20-30 days, several advanced SMR designs target significantly longer fuel cycles. TerraPower's Natrium aims for refueling intervals that minimize downtime, while some microreactor designs like Westinghouse's eVinci and Radiant's Kaleidos are designed for multi-year or even full-core-life operation without refueling. NuScale's multi-module VOYGR architecture allows individual modules to be refueled while others continue operating, maintaining high plant-level capacity factors even during maintenance campaigns.
The capacity factor advantage of nuclear over intermittent renewables is a central argument in the technology's value proposition, particularly for data center customers requiring 24/7 power reliability. Solar photovoltaic systems typically achieve capacity factors of 20-30%, onshore wind reaches 30-45%, and even the best offshore wind sites achieve 45-55%. When data center operators like Google, Amazon, and Meta sign PPAs for nuclear power, they are buying capacity factors above 90%, meaning the contracted megawatts deliver reliable energy around the clock without the need for backup storage or fossil fuel peaking plants. This reliability premium is reflected in the willingness of hyperscalers to sign long-term nuclear PPAs at prices above wholesale market rates, as seen in Microsoft's 20-year agreement with Constellation Energy for the 835 MW Three Mile Island Unit 1 restart.