What does zero carbon mean?

You can see here why coal is so bad, it produces over 50% more carbon compared to mains gas. In 2010, electricity produced the most carbon per unit, but over the last decade we have halved the carbon content by shutting down coal power stations and installing lots of renewable energy – mainly offshore wind turbines in the North Sea. By the 2030s our electricity will be almost zero carbon.

But back to the point. Here are the main ways of defining zero carbon:

Net-zero (annual, “location-based”, without offsetting)

We use more fuel for heating in the winter and the mix of fuels generating our electricity varies by the hour. It’s impractical to calculate carbon emissions on an hourly basis, so we tend to use average annual carbon factors for fuels and electricity.

So your carbon footprint will be your annual fuel consumption multiplied by the carbon factor for that fuel. If you have any renewable energy, like solar panels, you multiply the generation by the negative of the electricity carbon factor. The logic behind this is that you are “replacing” fossil fuel usage on the grid by generating your own electricity.

So for example:

Gas usage: 10,000 kWh x carbon factor 0.2 = 2,000 kgCO₂

Electricity usage: 3,000 kWh x carbon factor 0.23 = 690 kgCO₂

Solar panel electricity generation: 4,000 kWh x carbon factor -0.23 = -920 kgCO₂

Total: 1,770 kgCO₂ (1.7 tonnes)

Clearly this example doesn’t achieve zero carbon, but you can see that if you reduced your gas and electricity usage enough, the solar panels would negate the carbon so your house would be “net” zero carbon.

There’s a problem with this though, I wonder if you can see it. I mentioned above that carbon from our electricity will be almost zero carbon by the 2030s. This means that electricity from solar panels will be (negative) almost zero carbon. But gas usage will still produce carbon emissions.

So you can get to zero carbon under this definition now, but you won’t be able to in the future unless you get off gas altogether or as a country we decarbonise our gas supply. Decarbonising our gas supply is possible from sewage and food waste but we can’t produce enough to meet our current gas demand.

Net-zero (annual, “market-based”, without offsetting)

Electricity suppliers also have their own mix of fuels, with some offering 100% renewable tariffs. So on this “market-based” definition, you can use a carbon factor specific to your tariff.

If you sign up to a 100% renewable tariff, the carbon factor is zero. I think you can quickly see that this creates the same problem as above for carbon emissions from gas.

Net-zero (with offsetting)

This is the same as either of the other definitions, but for the remaining carbon emissions you buy “carbon credits” aka “carbon offsets”. Some energy suppliers buy them for you and then say that you have zero carbon gas.

A carbon offset is linked to a carbon reducing project somewhere else, usually in the “developing” world. For example an organisation gives or sells LED lights and solar panels to people using kerosene (oil) lamps. This saves carbon and for every tonne, they get a “credit” which they can sell.

These projects are a good thing in general, although there has been some abuse of the system, but offsets should only really be used where it’s impossible or extremely expensive to replace or avoid fossil fuel use, e.g. on essential air travel. Because ultimately the whole world has to get to zero carbon, and at that point there won’t be anything left to offset against.