Energy Institute

Do the power plants that provide our electricity operate all the time?

Power plants operate as needed and come on and off line in response to demand. However, electricity has to be produced all the time – 24 hours a day and 365 days a year to maintain the integrity of the system.

Our electricity needs (demand) change seasonally and daily. We use more electricity in winter than in summer and by day we use most electricity in the early evening and least in the very early morning (Fig. 1). Electricity supply and demand have to be in balance at all times; generators stop, start and adjust their output to meet our changing needs.

Figure 1. Electricity demand in May 2016 showing hourly and daily variation


  • Demand (MW)

Generators ramp up and down according to rules applied by the System Operator (responsible for technical operation) and to a schedule determined by the Market Operator (responsible for market and competition). The schedule is designed to maintain the quality of electricity supply and to achieve the lowest wholesale price for electricity every half hour 24 hours a day 1.

Electricity generators like coal and peat-fired plants that run for most (over 75%) of the time are called “base load” plants.  Others, such as gas or oil-fired “peaking plants” operate more flexibly and some may only run for very short periods (less than 5% of the time) to meet peak demand. Combined cycle gas turbine (CCGT) plants, the most efficient plants on the Irish system, will run from 20% to 60% of the time 2.

The demand for electricity, or the amount of electricity we use at any given time, while broadly predictable can change quickly. A surge in demand has to be met by an increase in supply in real time; so fast-responding generators are needed to pick up the load and generate power to maintain a stable supply of electricity. These fast-response plants are expensive to operate so the System Operator (SO) will bring on slower responding, but less costly to operate, plants to replace them as demand stabilises.

One of the ways that the System Operator (SO) prepares for surges in demand is by having some plants in a state of readiness called “spinning reserve”. This is plant that is ready and is immediately available to deal with surges in demand, and also to cover for faults that put other plants out of action.

Demand can surge in any part of the country; as a large factory stops and starts, as a quarry begins to crush stone or as a cold front pushes in over Ireland the demand for electricity for heating will change; the available supply has to increase by bringing additional plant on line.

The choice of what plant is “dispatched” to meet the additional demand depends on where it is located and the price of its electricity. Generators that are located at some distance from the centre of demand can only be dispatched if the grid has the capacity to carry the electricity across the country to where it is needed. If the dispatch of the additional plant would lead to congestion (technically a constraint) the SO may have to dispatch a more expensive plant closer by.

Peaking plants have exceptionally fast response times and can produce electricity within minutes. They are expensive to operate, partly because they are used so little, but the benefit is a reliable electricity supply. For example, if there is a sudden spike in demand in the early evening when most electricity is being used, these plants can be brought online to meet that demand.

The SO needs a lot of information to be able to operate the system safely and reliably at lowest cost. That information includes the cost of electricity from each generator, the location of the generator in relation to the capacity of the grid and the technical capacity of the generator to ramp up and follow the changing demand. Many of the calculations needed to reach the best technical decision at any time are automated and with experience the SO will have worked out many effective responses to events that repeat 3.

In general, the stronger the grid, the greater the freedom there is for the SO to dispatch the lowest-cost plant as determined by the Market Operator (MO) and by doing so achieve the lowest wholesale price. Where the grid is congested higher-cost plant may have to be scheduled regardless. Thus the grid is the backbone of the electricity system and of the electricity market as well. No grid means no functioning market and higher prices result.