Green hydrogen is made by splitting water into hydrogen and oxygen using an electrolyzer powered entirely by renewable energy. The process is called electrolysis, and it requires roughly 50 to 58 kilowatt-hours of electricity to produce a single kilogram of hydrogen. That kilogram holds about 33.3 kWh of usable energy, which means a significant portion of the input electricity is lost as heat during the conversion. The “green” label comes from the energy source: if the electricity comes from solar panels or wind turbines instead of fossil fuels, the hydrogen produced is essentially carbon-free.
How Electrolysis Splits Water
At its core, electrolysis is straightforward chemistry. An electrolyzer contains two electrodes (an anode and a cathode) separated by an electrolyte, which can be a liquid solution or a solid membrane depending on the technology. When electricity flows through the system, water molecules break apart. Hydrogen gas forms at the cathode, and oxygen gas forms at the anode. The oxygen is typically vented to the atmosphere as a harmless byproduct.
The reaction requires purified water. Approximately 10 liters of water are needed to produce one kilogram of hydrogen through the electrochemical reaction alone. Another 10 to 20 liters per kilogram go toward cooling the system during operation. In regions where freshwater is scarce, facilities can use seawater run through a desalination unit before it enters the electrolyzer.
Three Main Electrolyzer Technologies
Not all electrolyzers work the same way. Three types dominate the market, each with different trade-offs in efficiency, cost, and operating conditions.
Alkaline Electrolyzers
Alkaline electrolysis is the oldest and most commercially proven technology. These systems use a liquid alkaline solution (typically potassium hydroxide) as the electrolyte and operate at temperatures between 70 and 90°C. Their efficiency sits around 70% on average, with a range of 62 to 82% depending on the system design. The main drawback is sluggish response times. Alkaline electrolyzers don’t ramp up and down quickly, which makes them less ideal for pairing directly with variable wind or solar power. On the plus side, they avoid expensive rare metals in their catalysts, keeping costs lower.
PEM Electrolyzers
PEM (polymer electrolyte membrane) electrolyzers use a solid polymer membrane instead of a liquid electrolyte. They operate at lower temperatures, typically 50 to 80°C, and achieve average efficiencies around 80%, with a range of 67 to 82%. Their biggest advantage is rapid response. PEM systems can ramp output up or down in seconds, making them well suited to track the fluctuating output of solar and wind farms. A U.S. Department of Energy analysis of current PEM systems puts average electricity consumption at 57.5 kWh per kilogram of hydrogen, including both the electrolyzer stack and the supporting equipment like pumps and power electronics.
The challenge with PEM technology is materials. The oxygen-producing side of the cell relies on iridium, one of the scarcest elements on Earth, as a catalyst. The hydrogen side uses platinum. Scaling PEM electrolysis to meet global hydrogen targets will require either dramatically reducing the amount of iridium per cell or building recycling infrastructure capable of recovering at least 90% of the catalyst at end of life. Both are active areas of development.
Solid Oxide Electrolyzers
Solid oxide electrolyzers (SOECs) operate at far higher temperatures, between 700 and 850°C. That extreme heat is both their strength and their limitation. Because high temperatures make the electrochemical reaction more favorable, SOECs can theoretically reach efficiencies up to 92%. In practice, current systems are less mature, and real-world efficiencies vary. These systems work best when paired with an industrial heat source, such as a nuclear plant or concentrated solar facility, which can supply some of the energy as heat rather than electricity. They’re not yet widely deployed at commercial scale.
Powering the System With Renewables
The electricity source is what separates green hydrogen from every other kind. “Grey” hydrogen comes from natural gas. “Blue” hydrogen adds carbon capture to that process. Green hydrogen uses only renewable electricity, most commonly solar photovoltaic panels, onshore or offshore wind turbines, or a combination of both.
The renewable capacity required is substantial. A detailed sizing study for a facility in Morocco targeting 100 tons of green hydrogen per day found that an optimized configuration needed roughly 562 megawatts of solar capacity and 456 megawatts of wind capacity to feed a 273-megawatt electrolyzer system. The combination of solar and wind helps smooth out production: solar generates during the day, wind often picks up at night, and together they keep the electrolyzer running at a higher utilization rate than either source alone.
Overbuilding renewable capacity is a common strategy. Installing more solar and wind than the electrolyzer can use at peak output ensures the system still produces hydrogen during cloudy or calm periods. The trade-off is excess electricity during ideal weather, which needs to be sold to the grid or stored.
What Happens After Production
Once hydrogen gas leaves the electrolyzer, it needs to be compressed, liquefied, or converted into another carrier before it can be stored or transported. Each option costs energy.
Compressing hydrogen gas to 350 bar (a common storage pressure) requires about 2 to 4 kWh per kilogram in practice, representing roughly 5 to 12% of the energy contained in the hydrogen itself. Compressing to 700 bar, the standard for hydrogen fuel cell vehicles, takes around 3 kWh per kilogram. These figures come from real-world demonstration stations tracked by the U.S. Department of Energy.
Liquefying hydrogen is far more energy-intensive. Cooling hydrogen to minus 253°C consumes 10 to 13 kWh per kilogram with current technology, which amounts to 30 to 40% of the hydrogen’s energy content. The theoretical minimum is about 3.9 kWh per kilogram, so there’s a wide gap between what physics allows and what today’s equipment achieves. Newer magnetic cooling methods may eventually bring that figure down to around 7 kWh per kilogram, but these aren’t yet commercially available.
For long-distance transport, some projects convert hydrogen into ammonia or liquid organic carriers, which are easier to ship but require additional energy for conversion and reconversion at the destination.
Putting the Energy Math Together
Understanding the full energy chain helps explain why efficiency matters so much for green hydrogen economics. Start with the electrolyzer consuming about 55 to 58 kWh of electricity per kilogram of hydrogen. Add 2 to 4 kWh for compression, or 10 to 13 kWh for liquefaction. The resulting kilogram of hydrogen contains 33.3 kWh of energy (measured by its lower heating value).
That means even before you use the hydrogen for anything, you’ve already consumed 1.7 to 2.1 times more electrical energy than the hydrogen contains, just in production and compression. With liquefaction, the ratio climbs further. This is why cheap, abundant renewable electricity is the single biggest factor in making green hydrogen viable. The cost of electricity typically accounts for 60 to 70% of the final hydrogen price.
Scale and Infrastructure Challenges
Building a green hydrogen facility involves more than buying an electrolyzer and connecting it to a solar farm. You need water treatment systems, hydrogen purification equipment, compression or liquefaction units, storage tanks, and either pipelines or truck loading facilities for distribution. The electrolyzer stack itself degrades over time. PEM stacks, for example, lose voltage gradually over their roughly 40,000-hour operating lifetime, which means electricity consumption per kilogram slowly increases from about 55 kWh at the start of life to around 60 kWh at the end.
Material constraints add another layer of complexity. Global iridium production is limited, and PEM electrolyzers currently need meaningful quantities of it. If the world scales PEM capacity to the levels envisioned in most net-zero scenarios, iridium supply could become a bottleneck unless catalyst loadings drop significantly or recycling rates exceed 90%. Alkaline and solid oxide technologies sidestep this problem by using more abundant materials, which is one reason many large-scale projects still favor alkaline systems despite their slower response times.
Green hydrogen production is scaling rapidly, with gigawatt-scale projects under construction across the Middle East, Australia, Europe, and the Americas. The fundamental process is well understood. The remaining challenge is economic: driving down the cost of renewable electricity, improving electrolyzer efficiency and durability, and building the supply chains to manufacture these systems at the scale the energy transition demands.