A reactor is fundamentally a heat source
Strip away the complexity and a nuclear power plant does one thing at its core: it produces heat, exactly like a coal or gas plant, just from fission instead of combustion. That heat output is measured in megawatts thermal (MWth). But heat isn't electricity — a turbine and generator have to convert it, and that conversion is never 100% efficient. What actually reaches the grid is megawatts electric (MWe), and it's always smaller than the thermal figure.
Size a reactor's real output
Enter a thermal rating and efficiency to see electrical output, annual generation and homes powered.
The conversion
Nuclear plants convert roughly a third of their thermal power to electricity — a 3,000 MWth reactor typically delivers around 1,000 MWe. That two-thirds that doesn't become electricity isn't wasted maliciously; it's rejected as waste heat through cooling towers or a water source, an unavoidable consequence of how heat engines work.
Why efficiency sits around 33%
Turning heat into mechanical work through a steam turbine is bound by thermodynamics — specifically, efficiency depends heavily on how hot the steam gets before it hits the turbine. Nuclear plants run their reactors at lower temperatures than the most advanced fossil-fuel plants, for materials and safety margins, which caps their thermal efficiency around a third. It's a deliberate trade-off, not an engineering shortfall.
Capacity factor is the real story
A reactor's headline MWe number tells you its peak output, but what actually determines how many homes it powers over a year is its capacity factor — the share of the year it actually runs at full power. Nuclear plants have exceptionally high capacity factors, often around 90%, running continuously and pausing mainly for scheduled refuelling. Compare that to intermittent sources, and it's clear why a modest MWe rating times a high capacity factor adds up to billions of kilowatt-hours a year — enough for millions of homes.
That output depends on a steady supply of enriched fuel — see the uranium enrichment guide for how that's made — and on the same energy-density advantage covered in the energy density guide.
Frequently asked questions
What is the difference between MWth and MWe?
MWth is the heat a reactor produces; MWe is the electricity delivered to the grid after conversion. MWe = MWth × efficiency, typically ~33% for nuclear.
Why is nuclear thermal efficiency only about 33%?
It's bound by thermodynamics — nuclear plants run at lower steam temperatures than the most advanced fossil plants, for materials and safety reasons, which caps efficiency around a third.
Why does capacity factor matter so much for nuclear?
Nuclear plants run near-continuously, often around 90% of the year, pausing mainly for refuelling — which is why their annual output is so large relative to their nameplate size.