Thermal coal: Fluctuation in time
No weather swings, just slow shifts of policy and money
What this tab covers How availability changes over seconds, days, seasons, years and longer, how predictable it is, and what that means for the plants that use it.A coal stock does not fluctuate with weather. Its availability changes over decades through investment and policy, while plants burning it have seen falling capacity factors.
Variation by time scale
| Time scale | What happens | How large |
|---|---|---|
| Geological | Most hard coal formed in the Carboniferous; many younger lignites and sub-bituminous coals formed in the Cretaceous and later.2 | Carboniferous coal about 290 to 360 million years ago; Cretaceous deposits about 65 to 144 million years ago; coalification takes a few million years |
| Decades | Output kept rising in the 2010s and 2020s, driven by Asia.3 | 7.4 Gt in 2015 to 8.8 Gt in 2024 |
| Year to year | Demand shocks show up quickly in output.3 | Dip to 7.3 Gt in 2020 (pandemic), then 7.7 Gt in 2021 |
| Outlook to 2030 | IEA expects a plateau, then a gradual decline.6 | 9,111 Mt in 2025 to 8,641 Mt in 2030 |
| Daily and seasonal | The fuel can be stockpiled for weeks, so supply is steady. Plant output follows electricity demand and is increasingly pushed aside by wind and solar.7 | US coal plant capacity factor fell from 54.3% (2015) to 42.6% (2024) |
How predictable it is
High. Mined coal can be stockpiled, and output is limited by mine capacity, logistics and policy, not by weather. The main uncertainties are economic and political, such as demand in China and India and climate policy.
Capacity factors and utilisation
| Plant or measure | Value |
|---|---|
| US coal-fired plants, 2024 | 42.6%7 |
| US coal-fired plants, 2015 | 54.3%7 |
Unlike a flow source, coal in the ground does not vary from hour to hour. What changes is the rate at which people dig it up. That rate depends on mine investment, which takes years, and on demand, which depends on electricity growth, competition from gas and renewables, and policy. A coal plant can technically run near full output all year. In practice its capacity factor (actual output as a share of the maximum possible) is set by the market, and in the US it has fallen to about 43% as cheaper sources take its place.
Over the longer term, depletion happens mine by mine and basin by basin. Shallow, thick and high-grade seams are worked first, so later output needs deeper or thinner seams, longer haul distances or lower-rank coal.
Sources
- Coal formation Energy Education, University of Calgary, 2023
- Coal facts (world reserves from the Energy Institute Statistical Review, production from the IEA) Natural Resources Canada, 2026
- Coal 2025: Supply IEA, 2025
- Electric Power Annual, Table 4.08.A Capacity factors for utility scale generators primarily using fossil fuels US EIA, 2025