Baseload has two related meanings. Baseload demand is the lowest level of electricity use on the grid, the amount needed even in the middle of the night. Baseload power stations are plants designed to run almost continuously at a steady output to cover that demand.
Baseload, mid-merit and peaking
Grid planners have traditionally sorted power stations by how often they run:
| Type | How it runs | Typical examples |
|---|---|---|
| Baseload | Almost all the time at steady output | Nuclear, and formerly coal |
| Mid-merit | Ramps up and down with daily demand | Gas combined-cycle plants |
| Peaking | Only at the highest-demand times | Gas engines, pumped storage, batteries |
Baseload plants usually have high building costs and low running costs, so they are most economic when kept running. Nuclear reactors are also slow and costly to vary, which is why they run flat out between maintenance outages and have high capacity factors.
How the idea is changing in Great Britain
Coal used to provide much of Britain’s baseload. The last coal-fired power station, Ratcliffe-on-Soar, closed on 30 September 2024. Nuclear remains the main baseload source, alongside some biomass.
Wind and solar do not fit the old categories. Their running costs are close to zero, so they generate whenever the weather allows, and other sources fill the gaps around them. In a grid with a lot of wind and solar, the question shifts from “what runs all the time?” to “what can respond when the wind drops?” That flexible role is filled by gas plants, batteries, interconnectors, pumped storage and demand that can shift in time.
The government’s Clean Power 2030 Action Plan is built on this model: mostly wind and solar, with nuclear for steady output and flexible sources to balance the system, while gas is kept for back-up rather than everyday running.
Read more: The pros and cons of nuclear energy.