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Where do our emissions numbers come from?

Dec 09, 2021

We show emissions from electricity generation with the unit gCO2eq/kWh in various places on this website. But what does it mean, and where does it come from?

What does gCO2eq/kWh mean?

The generation of electricity produces greenhouse gases which contribute to global warming. Different energy types produce different amounts of greenhouse gases. To be able to compare greenhouse gas emissions from different sources of electricity, we measure the amount of greenhouse gases emitted per unit of electricity generated:

Greenhouse Gases / Energy

The amount of energy can be measured in units such as kWh (kilowatt hour) or MWh (megawatt hour). To measure emissions from electricity generation we follow the Intergovernmental Panel on Climate Change (IPCC) standard which is kWh. So far, our metric is:

Greenhouse Gases / kWh

Electricity generation can produce different types of greenhouse gases. The most common is carbon dioxide (CO2), but there are also other gases such as methane (CH4). Each greenhouse gas stays in the atmosphere for a different length of time. Also, the warming effect of each gas is different. To account for these differences, the IPCC, as well as this website, uses the CO2eq metric. CO2eq is an expression of the equivalent amount of CO2 which would have the same warming effect on the Earth as the combination of gases that are emitted. Emitting one gCO2eq is the equivalent of emitting one gram of CO2, in terms of the warming effect that is caused. This explains our main metric:

gCO2eq/kWh

Sometimes a similar metric is used – kg CO2eq per MWh. A kg is 1000 grams, and a MWh is 1000 kWh so this metric is describing exactly the same thing. You may also see tonnes of CO2eq per GWh, which is also measuring the same thing:

1 gCO2eq/kWh = 1 kg CO2eq / MWh = 1 ton of CO2eq per GWh

What are direct greenhouse gas emissions?

Direct greenhouse gas emissions are gases that are released at the power plant, as a direct consequence of the process which is generating the electricity. For example, when fossil fuels are burnt, the carbon atoms combine with oxygen from the air to create carbon dioxide (CO2). Here are the numbers that the IPCC use (https://www.ipcc.ch/site/assets/uploads/2018/02/ipcc_wg3_ar5_annex-iii.pdf#page=7):

Minimum Average Maximum
Coal 670 gCO2eq/kWh 760 gCO2eq/kWh 870 gCO2eq/kWh
Gas 350 gCO2eq/kWh 370 gCO2eq/kWh 490 gCO2eq/kWh
Biomass * * *
Geothermal 0 0 0
Hydropower 0 0 0
Nuclear 0 0 0
Solar 0 0 0
Wind 0 0 0

From this table, it’s clear that electricity generation from fossil fuels – coal and gas – directly causes the emission of greenhouse gases. It’s also clear that the other common sources of electricity have no direct emissions of greenhouse gases. Biomass is a special case. Here is what the IPCC says:

* Direct emissions from biomass combustion at the power plant are positive and significant, but should be seen in connection with the CO2 absorbed by growing plants.

In other words, the burning of biomass does emit greenhouse gases, but the growing of the biomass itself absorbs CO2. It would therefore be misleading to measure only the emissions of the burning of the biomass without taking the opposite effect into account. This is the cause of some controversy since the overall greenhouse effect of biomass is difficult to measure.

What are lifecycle greenhouse gas emissions?

Direct greenhouse emissions arise from the burning of fuel to generate electricity. But greenhouse gas emissions can arise from many other processes that are required to generate power. This includes the construction of power plants, the mining of fuel and the management of waste. To account for all these processes, the IPCC as well as this website use the lifecycle greenhouse gas emissions metric. Here are the numbers that the IPCC use:

Minimum Average Maximum
Coal 740 gCO2eq/kWh 820 gCO2eq/kWh 910 gCO2eq/kWh
Gas 410 gCO2eq/kWh 490 gCO2eq/kWh 650 gCO2eq/kWh
Biomass (dedicated) 130 gCO2eq/kWh 230 gCO2eq/kWh 420 gCO2eq/kWh
Geothermal 6.0 gCO2eq/kWh 38 gCO2eq/kWh 79 gCO2eq/kWh
Hydropower 1.0 gCO2eq/kWh 24 gCO2eq/kWh 2200 gCO2eq/kWh
Nuclear 3.7 gCO2eq/kWh 12 gCO2eq/kWh 110 gCO2eq/kWh
Solar (PV Utility) 18 gCO2eq/kWh 48 gCO2eq/kWh 180 gCO2eq/kWh
Wind (onshore/offshore) 7.0 gCO2eq/kWh 11.5 gCO2eq/kWh 56 gCO2eq/kWh

Fossil fuels – coal and gas – are clearly the highest emitters of greenhouse gases. Geothermal, hydropower, nuclear, solar and wind all have more than 10 times lower lifecycle greenhouse gas emissions than any fossil fuel. The maximum value for hydropower is very high, which illustrates that some hydropower dams emit a lot of greenhouse gases. Still, the average is very low. Biomass at an average of 230 gCO2eq/kWh emits considerably more than other low-carbon alternatives but still much less than fossil fuels. For wind, the IPCC list slightly different values for on and offshore – we have chosen the lowest and highest value of either, as well as the average of the two for the middle value.

The middle column – the average lifecycle emissions in units of gCO2eq/kWh are the numbers that we use on this website to estimate greenhouse gas emissions.

What about oil?

The IPCC does not include oil in its report. Globally, oil is used to generate less than 2% of our electricity. To include oil, we use the average lifecycle greenhouse gas emissions value used by

OurWorldinData (https://ourworldindata.org/safest-sources-of-energy), which is:

720 gCO2eq/kWh

In summary, the emissions values that we use on this website are as follows:

Average lifecycle emissions
Coal 820 gCO2eq/kWh
Oil 720 gCO2eq/kWh
Gas 490 gCO2eq/kWh
Biomass 230 gCO2eq/kWh
Solar 48 gCO2eq/kWh
Geothermal 38 gCO2eq/kWh
Hydropower 24 gCO2eq/kWh
Nuclear 12 gCO2eq/kWh
Wind 11.5 gCO2eq/kWh

Questions and feedback are much appreciated. Feel free to reach out to [email protected]

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