Can power plants be turned on and off on demand?

What determines whether a power plant can be turned on or off?

Can power plants be turned on and off on demand? The short answer: it depends on the type of plant. Not all plants operate in the same way; not all are equally flexible.

But what factors influence the flexibility of power plants?

Here are three key factors that determine whether a plant can be switched on or off:

1. Electricity prices: Power producers submit their bids to the day-ahead market—a type of auction that matches supply with forecasted demand for the following day based on price. The system operator then verifies that the match is technically viable and ensures that supply is guaranteed. 
2. Resource availability: No wind, sun, or water means no electricity generation.
3. Outages or maintenance: If there’s a technical issue or scheduled maintenance, the plant can’t operate.

Based on these factors, power plants can be classified from less to more flexible.

How flexible are power plants?

Plants powered by variable resources like wind or sunlight tend to be the least flexible, though each has its nuances. Other types of plants, though, allow for greater control over when and how they operate. 

Wind and solar photovoltaic (PV) 

• These plants only generate electricity when the wind blows or the sun shines.
• They can be turned off but not turned on unless the resource is available.
• If market prices drop below zero, plants may be left out of the market based on their submitted bids and may not operate during certain hours without an electricity generation schedule.

Run-of-river hydropower

These plants depend on the natural flow of a river, similar to solar and wind.

• Since water can’t be stored, electricity can only be produced when there’s enough flow.

Reservoir hydropower

These plants use stored water and a controlled flow through turbines to generate electricity. 

• They are highly flexible.
• Water can be stored and released on demand.
• However, water management is regulated by river basin organisations, which may require releases during high flow periods or restrict production during droughts.

Pumped-storage hydropower 

Pumped-storage hydroelectric power plants allow energy to be stored through a system of reservoirs located at different elevations. It's similar to having a water battery. 

• If there is a surplus of energy, water is pumped up to a higher tank. 
• Meanwhile, when energy is necessary, the water is dropped to generate power. 
• These systems are especially useful for balancing the electricty system.

Combined-cycle (natural gas)

Plants that run on fuels are generally more flexible to manage. Combined-cycle plants generate electricity by converting thermal energy from natural gas using a gas turbine and a steam turbine. 

• They’re flexible but not instantaneous.
• They typically require around 15 minutes to start up, depending on whether the system is cold, warm, or hot (based on how long it's been shut down).
• They can be turned on or off considering market conditions.

Nuclear power plants

Nuclear plants generate electricity from uranium fuel.

• They can adjust output from a technical minimum (60–70%) up to full capacity.
•  Even though they can be shut down, ramping back up is slow—it can take up to three days.
• After reaching full power, they typically need 3 to 10 days before they can reduce load or shut down again, depending on the specific plant.

Solar thermal

These plants use the sun’s heat to warm a fluid and generate electricity.

• Many include thermal storage systems (e.g., hot salts).
• This gives them some flexibility to operate even without sunlight.

Why does flexibility matter?

The more flexible a plant, the better it can adapt to market supply and demand, which improves profitability and helps stabilise the electricity system.
Plants are legally required to be available and to offer their energy. Otherwise, they must provide justification.

Spain’s system operator, Red Eléctrica, oversees the national transmission grid and supervises the generation schedule resulting from the electricity market. Moreover, it can also activate secondary markets to guarantee security of supply.