Earth's heat: Fluctuation in time
Steady around the clock, but reservoirs can tire
Steady around the clock, but reservoirs can tire 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.Geothermal output is steady day and night and across seasons, so plants run as baseload with high capacity factors. The main change happens over decades, as reservoirs lose pressure or cool if not managed.
Variation by time scale
| Time scale | What happens | How large |
|---|---|---|
| Seconds to days | Constant; no dependence on weather or daylight4 | Plants can run continuously at rated output |
| Seasonal | Electricity steady; direct heat use peaks in winter because demand does4 | Worldwide direct-use capacity factor about 0.27; heat pumps 0.19 in heating mode |
| Decades (reservoir behaviour) | Pressure and steam output fall if extraction outpaces recharge; reinjection of water slows the decline7 | At The Geysers, California, severe steam pressure decreases by 1989; about 20 million gallons a day of treated wastewater now injected |
| Decades (land surface) | Fluid withdrawal compacts rock and the ground sinks8 | Wairakei, New Zealand: 15 m of subsidence after 50 years of extraction |
| Geological | Heat flow declines slowly as radioactive elements decay and primordial heat escapes3 | Effectively constant on human time scales |
How predictable it is
Very high: output depends on reservoir physics, not weather. Uncertainty lies mainly in exploration (whether a well finds enough hot fluid) and in long-term reservoir decline.
Capacity factors and utilisation
| Plant or measure | Value |
|---|---|
| Geothermal power generation, typical (WEC) | about 95%4 |
| New Zealand, 2023 | 85%6 |
| Turkiye | about 75%6 |
| United States, 2023 (down from 74.6% in 2018) | 69.3%6 |
Unlike most renewables, geothermal does not vary with the hour or the season. Reported capacity factors range from around 70% to 95%; the lower figures largely reflect ageing fields whose output has fallen below nameplate capacity, rather than any natural fluctuation.
The important clock is the reservoir's. The Geysers in California showed that a high-temperature reservoir can be over-produced within a couple of decades, and that injecting wastewater can partly restore it.
Sources
- Earth's internal heat budget Wikipedia, 2025
- World Energy Resources 2013: Geothermal World Energy Council, 2013
- Renewables 2025 Global Status Report: Geothermal REN21, 2025
- The Geysers: Water (steam field recharge) Calpine / Geysers.com, 2024
- Update on subsidence at the Wairakei-Tauhara geothermal system, New Zealand USGS Publications Warehouse, 2009