Written by IGCSEPhysics Content Team · Physics subject adviser: K. S. Tan, 15+ years teaching IGCSE Physics · Checked against the Cambridge IGCSE Physics (0625) 2026 to 2028 syllabus
Electricity demand changes throughout the day and across seasons. The national grid must match supply to demand at all times.
Demand patterns
- Base load: the minimum demand that exists 24 hours a day. Met by power stations that run continuously (nuclear, large coal/gas).
- Peak demand: times of highest electricity use (typically early evening, cold winter days). Met by power stations that can start up quickly (gas turbines, hydroelectric, pumped storage).
- Night time: demand drops. Some power stations reduce output; pumped storage uses excess electricity to pump water uphill for use during peaks.
Why some sources suit base load
| Source | Start-up time | Best for |
|---|---|---|
| Nuclear | Very slow (days) | Base load |
| Coal | Slow (hours) | Base load |
| Gas turbine | Fast (minutes) | Peak demand |
| Hydroelectric | Very fast (seconds) | Peak demand |
| Pumped storage | Very fast (seconds) | Peak demand |
Challenges of renewable sources
Wind and solar are intermittent (they depend on weather conditions). They cannot be turned on to meet demand. This means:
- Backup generation (often gas) must be available.
- Energy storage (batteries, pumped storage) helps balance supply.
- A diverse mix of sources provides more reliable supply.
Common errors and how to correct them
- Thinking nuclear power stations can be turned on and off quickly. They run best continuously.
- Assuming wind farms always produce their rated power. Actual output depends on wind speed.
How to apply this in an exam
Explain why the source suits either base load or peak demand. Mention start-up time, reliability and ability to adjust output.
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