Load following
Adjusting a power plant’s output over minutes to hours to track changes in electricity demand or in variable renewable generation.
Load following sits between fast frequency regulation (seconds) and unit commitment (day ahead). Plants are characterised by ramp rate, minimum stable load and start-up time:
- OCGT / gas engines: ramp 10–25% of capacity per minute, minimum load 20–50%, start 5–30 minutes.
- CCGT: 3–8% per minute, minimum load 30–50%, hot start under an hour.
- Hard coal: 1.5–6% per minute, minimum 25–40%; lignite: 1–4% per minute, minimum 35–50%.
- Nuclear (PWR/BWR): 1–5% per minute between 50% and 100% power, constrained late in the fuel cycle by xenon-135 transients.
- Hydro with reservoir and batteries: near-instant, full range.
The need for flexibility grows with variable renewables. California's evening ramp on the duck curve exceeds 20 GW over three hours. France's nuclear fleet routinely load-follows, with some units cycling daily between about 50% and 100%. Frequent cycling raises wear, maintenance cost and heat rate: a coal plant at 50% load can lose 2–4 percentage points of efficiency. Storage, demand response and interconnectors increasingly supply this service.
Related terms
All terms →- Dispatchable generation
Generation whose output can be scheduled, raised or lowered on request by the operator, independently of weather.
- Duck curve
The shape of net load over a day in solar-rich grids: a deep midday dip as PV output peaks, followed by a steep evening ramp.
- Combined-cycle gas turbine (CCGT)
A power plant that runs a gas turbine and uses its hot exhaust to raise steam for a second, steam turbine, reaching electrical efficiencies of 55–64%.
- Variable renewable energy (VRE)
Renewable generation whose output varies with weather and time of day, mainly wind and solar PV, and cannot be fully dispatched.
- Demand response
Changes in electricity consumption by end users, in response to price signals or operator requests, to balance the grid.
- Baseload
The minimum level of electricity demand on a grid over a day, season or year, historically served by plants that run continuously at high output.