Every time you charge your phone, stream a video, or save a file to the cloud, you are using electricity. But where does that electricity come from? The answer varies wildly depending on where you live, and it has a massive impact on your carbon footprint.
Here is a breakdown of every major electricity source, how it works, and what it means for the environment.
Coal
Coal-fired power plants burn coal to heat water, creating steam that spins a turbine connected to a generator. It is one of the oldest methods of generating electricity and still one of the most widely used globally.
Carbon intensity: 900-1,100 gCO2/kWh.
Coal is the most carbon-intensive mainstream energy source by a wide margin. Beyond CO2, burning coal releases particulate matter, sulphur dioxide, and mercury. Countries like Poland, India, and parts of China still depend heavily on coal. It is cheap and reliable, which makes it hard to phase out quickly despite its environmental cost.
Natural gas
Gas plants work similarly to coal plants but burn natural gas instead. Modern combined-cycle gas plants are significantly more efficient, using both the gas combustion and the resulting hot exhaust to generate power.
Carbon intensity: 400-500 gCO2/kWh.
Gas produces roughly half the CO2 of coal per kWh. Many countries (including the Netherlands) use it as a "bridge fuel" while transitioning to renewables. The downside is methane leakage during extraction and transport. Methane is a greenhouse gas over 80 times more potent than CO2 in the short term.
Nuclear
Nuclear plants generate electricity through nuclear fission: splitting uranium atoms to release enormous amounts of heat, which produces steam to drive turbines.
Carbon intensity: 5-15 gCO2/kWh.
Nuclear is one of the lowest-carbon sources available. France gets about 70% of its electricity from nuclear, which is why its grid is so clean. It provides reliable baseload power regardless of weather. The downsides are high construction costs, long build times, radioactive waste management, and public safety concerns. Despite this, many climate scientists argue nuclear is essential for decarbonisation.
Wind
Wind turbines capture the energy of moving air and convert it into electricity. Wind pushes against large blades mounted on a tower, causing them to spin. That spinning motion drives a generator inside the turbine, which produces electrical current. It is the same principle as a bicycle dynamo, just on a much larger scale. Onshore turbines are placed on land; offshore turbines sit in the sea where winds are stronger and more consistent.
Carbon intensity: 7-15 gCO2/kWh (lifecycle, including manufacturing).
Wind is one of the cleanest energy sources. The emissions come almost entirely from manufacturing and installing the turbines, not from operation. Onshore wind is among the cheapest new energy sources to build today. The main limitation is that wind is inconsistent: it does not blow at the same strength all the time, so backup power or energy storage is needed to fill the gaps.
Solar
Solar panels are made of special materials (called silicon cells) that generate electricity when sunlight hits them. The photons in sunlight knock electrons loose in the silicon, creating an electrical current. No moving parts, no combustion, just light turning into power. A second type, concentrated solar power, uses mirrors to focus sunlight into intense heat, which boils water to spin a turbine and generator.
Carbon intensity: 20-50 gCO2/kWh (lifecycle).
Solar has dropped dramatically in cost over the past decade and is now the cheapest source of new electricity in many regions. Like wind, solar output is inconsistent. It peaks at midday and drops to zero at night. Battery storage is increasingly used to bridge the gap, but grid-scale storage remains expensive.
Hydropower
Hydroelectric plants use flowing or falling water to spin turbines. This can be a large dam, a river run-of-river system, or pumped storage that moves water between reservoirs.
Carbon intensity: 10-30 gCO2/kWh.
Hydro is clean, reliable, and can ramp up quickly to meet demand. Countries like Norway and Sweden rely heavily on it. The environmental trade-offs include disrupting river ecosystems, displacing communities for dam construction, and methane emissions from flooded vegetation in reservoirs (particularly in tropical regions).
Geothermal
Geothermal plants tap heat from below the Earth's surface. Hot water or steam from underground reservoirs drives turbines to generate electricity.
Carbon intensity: 15-55 gCO2/kWh.
Geothermal is clean and provides constant baseload power regardless of weather or time of day. The catch is geography: it only works well in areas with significant underground heat, such as Iceland, parts of the US, and East Africa. Iceland generates nearly 100% of its electricity from geothermal and hydro combined.
Biomass
Biomass plants burn organic material (wood chips, agricultural waste, or dedicated energy crops) to generate steam and electricity.
Carbon intensity: 50-100 gCO2/kWh (varies significantly).
Biomass is considered "carbon neutral" in some frameworks because the CO2 released was recently absorbed by the plants during growth. In practice, the picture is more complex. Transporting biomass uses energy, and burning forests for fuel can take decades to offset through regrowth. It remains controversial among environmental scientists.
The full picture
The cleanest grids in the world combine reliable low-carbon baseload (nuclear, hydro, geothermal) with growing shares of wind and solar. The most carbon-intensive grids still rely on coal and gas for most of their supply.
This matters for your digital footprint because data centres and your personal devices all run on grid electricity. If the grid powering a data centre is clean, your digital activities have a lower carbon footprint. If it runs on coal, every email, every video, every search carries a heavier environmental cost.
This is why the same Google search produces 10 times more carbon in India than in Sweden. The search is identical. The energy source is not.