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Green, grey and blue hydrogen: what the colours mean

Hydrogen itself is colourless. The colours are a shorthand for how it was made, and they matter because they decide how clean it really is.

Grey and blue

Most hydrogen made in the world today comes from natural gas through a process called steam methane reforming. Without any capture of the carbon dioxide produced, that is called grey hydrogen. If the carbon dioxide is captured and stored rather than released, it is called blue hydrogen. Blue hydrogen is cleaner than grey, though how much depends on how well the capture works and how much methane leaks upstream.

What it would keep running

  • Wind-charged green hydrogen
  • Micro-hydro charged green hydrogen
  • Night-rate grid charging, as clean as the grid
  • Electrolysis with no fossil feedstock
  • Fuel cell output with water as the by-product

Green hydrogen

Green hydrogen is made by electrolysis using renewable electricity, such as wind or hydro. No fossil fuel is involved, so the hydrogen carries a very low carbon footprint at the point it is made. There are other colour labels too, such as pink for nuclear-powered electrolysis, but grey, blue and green are the ones you will see most.

The label describes the source of energy, not the gas. A molecule of green hydrogen is identical to a molecule of grey. What differs is what happened to make it, which is why claims about colour deserve a look behind the label.

How it works →
The H2OME loop: an electrolyser splits water into hydrogen, the hydrogen is stored in a tank for weeks, a fuel cell turns it into electricity for your home, and the water returns to the loop.

What colour is home hydrogen?

A home system using electrolysis is only as green as the electricity feeding it. Charged from a wind turbine or micro-hydro, it is green hydrogen in the plain sense. Charged from off-peak grid power, its colour depends on what the grid is burning overnight. New Zealand's grid draws heavily on hydro and other renewables, while Australian grids vary a lot by state. We would rather explain that than slap a green label on everything.

Questions

Is all electrolysis hydrogen green?

No. Electrolysis powered by electricity from coal or gas generation carries that generation's emissions with it, plus the losses along the way. Only electrolysis powered by renewable electricity is properly called green, whatever the marketing says.

Is home hydrogen greener than diesel backup?

At the point of use, the fuel cell produces water while diesel produces exhaust. The full answer depends on the electricity used and the losses along the way, including round-trip efficiency of 30% or less. See our carbon footprint page for the honest version.

Can I make sure my hydrogen is green?

Charging from your own wind turbine or micro-hydro scheme gives you the clearest answer. Some power retailers also offer plans backed by renewable generation, which is worth asking about if you charge from the grid.

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