The most energy a fusion reactor has ever produced in a single experiment is 69 megajoules, enough to power roughly 30 homes for an hour. That record was set in October 2023 at the Joint European Torus (JET) in the UK during a plasma burst lasting just 5.2 seconds. Future reactors currently under construction aim to produce far more, with ITER targeting 500 megawatts of sustained thermal power. But understanding these numbers requires some context about where fusion energy stands today, what’s being built next, and how those figures translate into usable electricity.
What the Current Record Looks Like
JET’s final record-breaking experiment squeezed 69 megajoules of fusion energy from just 0.2 milligrams of fuel, a mixture of two hydrogen variants called deuterium and tritium. That’s a staggering energy-to-fuel ratio. For comparison, burning 0.2 milligrams of coal would barely produce enough heat to feel on your fingertip. The same mass of fusion fuel released enough energy to boil several hundred liters of water.
Still, 69 megajoules over 5.2 seconds works out to about 13 megawatts of power, roughly what a few large wind turbines generate. JET beat its own previous record of 59 megajoules set in 2021, but after four decades of operation, the facility shut down at the end of 2023. Its purpose was never to generate grid-ready electricity. It was a proof of concept, and on that front, it delivered.
Why Fusion Fuel Is So Energy-Dense
When a single deuterium nucleus fuses with a tritium nucleus, the reaction releases 17.6 million electron volts of energy. That number is small on its own, but fusion reactions happen billions of times per second inside a reactor plasma. Gram for gram, fusion fuel releases roughly 10 million times more energy than burning fossil fuels. This is why such a tiny amount of fuel (0.2 milligrams in JET’s case) can produce meaningful energy output.
The challenge has never been the energy content of the fuel. It’s been creating and sustaining the extreme conditions needed for fusion to happen: temperatures exceeding 100 million degrees Celsius, powerful magnetic fields to contain the plasma, and systems to keep the reaction stable long enough to extract useful energy. Most of the energy produced by today’s experimental reactors gets consumed by the heating systems needed to start and maintain the reaction in the first place.
ITER: The Next Major Step
ITER, the massive international fusion project under construction in southern France, is designed to produce 500 megawatts of thermal fusion power from just 50 megawatts of input heating. That tenfold energy return, expressed as a Q factor of 10, would be the first time a fusion device produces significantly more energy than it consumes. JET, by contrast, never achieved a Q greater than about 0.67. It always required more energy to heat the plasma than the fusion reactions gave back.
ITER’s 500 megawatts of thermal output would be sustained for 400 to 600 seconds per pulse, a massive leap from JET’s 5-second bursts. To put 500 thermal megawatts in perspective, that’s comparable to the heat output of a medium-sized coal plant. It would be enough to generate roughly 150 to 200 megawatts of electricity if connected to conventional steam turbines, sufficient to power a small city of around 200,000 people.
ITER is not designed to actually send electricity to the grid, though. Like JET, it’s an experimental facility. Its job is to prove that sustained, net-positive fusion power is physically and engineeringly possible at scale.
How Fusion Heat Becomes Electricity
A fusion reactor doesn’t produce electricity directly. It produces intense heat, which then needs to be converted using essentially the same technology that coal and nuclear fission plants use: steam turbines. The fusion reaction heats a surrounding structure called a blanket, that heat transfers to water or another working fluid, the fluid drives a turbine, and the turbine spins a generator.
This conversion process is where a significant portion of the thermal energy gets lost. According to projections from the U.S. Department of Energy, fusion power plants using conventional steam systems would achieve roughly the same thermal efficiency as today’s power plants, typically around 33 to 40 percent. So a fusion reactor producing 500 megawatts of heat would deliver somewhere between 165 and 200 megawatts of electricity to the grid. Advanced conversion systems could push that efficiency higher, but those technologies are still in development.
What Commercial Fusion Plants Would Produce
Most designs for first-generation commercial fusion plants target somewhere between 200 and 500 megawatts of electrical output, putting them in the same range as mid-sized fission nuclear plants. A single plant at the higher end of that range could power 400,000 to 500,000 homes. Several private companies, including Commonwealth Fusion Systems (a spinoff from MIT), are designing compact fusion reactors that aim to reach these output levels.
The fuel requirements would be remarkably small. A 500-megawatt fusion plant would consume only a few hundred kilograms of deuterium-tritium fuel per year. Deuterium is abundant in seawater, essentially limitless. Tritium is rarer and would need to be bred inside the reactor itself using lithium, but the quantities involved are tiny compared to the fuel needs of any fossil fuel plant. One estimate puts tritium losses at roughly 1 gram per year for every 500 megawatts of generation.
How Fusion Compares to Other Power Sources
- Coal plant (large): A typical coal plant produces 600 to 1,000 megawatts of electricity but burns millions of tons of coal annually and emits massive amounts of CO₂.
- Nuclear fission plant: A modern fission reactor produces 1,000 to 1,400 megawatts of electricity. Fusion aims for similar output without long-lived radioactive waste or meltdown risk.
- Solar farm (utility scale): A large solar installation produces 200 to 400 megawatts at peak, but output varies with weather and time of day. Fusion would run around the clock.
- Wind farm (offshore): Large offshore wind farms produce 500 to 1,200 megawatts of nameplate capacity, but actual output averages 35 to 50 percent of that due to variable wind.
Fusion’s real advantage isn’t necessarily producing more power per plant than these alternatives. It’s the combination of near-limitless fuel, zero carbon emissions during operation, no risk of meltdown, and relatively small amounts of short-lived radioactive waste. The main barrier remains engineering: no one has yet built a reactor that produces net electricity, let alone one that does so economically. The earliest commercial fusion plants are optimistically projected for the late 2030s to 2040s, though timelines in this field have historically slipped.