Energy Explainer: Electrical load
Electrical load is the component in a circuit that draws or consumes power as opposed to providing it. If one was to imagine load in relation to an electrical power system, then the load is anything that consumes power. In a small-scale system, such as a torchlight, load can be the lightbulb; whereas in a bigger system, such as a household, load can be calculated as the cumulative power consumption of all electrical appliances in that house.
In a consumer’s home therefore, load is determined by multiple factors, such as the type of devices being used, the frequency of their usage, and the time of day or year. For example, on a given day, computers may be switched on for several hours, mobile phones may be charged overnight, while the refrigerator is running constantly. This results in varying level of electricity demand throughout the day. Additionally, demand on a winter’s day is generally higher than on a summer’s day, due to heaters and radiators requiring electricity for extended periods, and lights used more, resulting in a higher load.
There is a minimum level of electricity demand, referred to as the “baseload”, under which consumption typically doesn’t fall. The baseload runs for 24 hours a day as it is needed to power components that work continuously, e.g., traffic lights or transport systems, such as the underground. "Peak load” is the amount of electricity needed when demand is at its highest. This peak usually occurs over short periods. Demand is highly variable over the course of the day, and this is further impacted depending on the season.
Graph 1 below is a representative example of Great Britain’s load requirement of a summer’s day, and demonstrates how demand varies in relation to the time of day. Note that the curve does not fall below a certain amount of electricity demand - this amount is the baseload. Power demand peaks during times of increased collective electricity use, such as during office hours, when computers, printers and coffee machines are all in use, and in the evening, when people return home from work and turn on the kettle, TV and lights.
As already noted, demand varies according to the seasons. Graph 2 illustrates Britain’s electricity demand on a winter’s day. As can be seen from the charts, electricity demand is higher in winter than in summer.
Baseload- and peaking plants
The varying level of demand throughout a day results in varying load factor required on the grid. This load factor, which can be described as the ratio of total demand to peak demand, needs to be satisfied by sufficient capacity generated by power plants at any moment.
Each of these plants has a cost of generating an additional unit of electricity at any moment. This is known as the marginal cost. In order to reduce the cost of electricity, the cheapest generators are chosen to meet the demand. The available generation plants are ranked in ascending order from the cheapest to the most expensive. This is known as the merit order where the generators with the lowest marginal cost are selected.
As different energy technologies have inherently different mechanical features, the service they can provide for the electricity network is also highly variable. Different sources of energy will have different effects on the system.
Baseload plants are those with the technical capability to supply continual and stable electricity. Baseload plants are difficult to ramp up and down in a short period of time, making them unsustainable to provide energy reactively to rapid surges and falls in energy demand. Nuclear and coal plants are examples of traditional baseload plants. These plants have low operational costs than peaking plants.
Peaking plants are those that operate with features that can best respond to the surges in demand on the electricity system throughout the day. Peaking plants can be ramped up to produce higher quantities of energy quickly, thus are able to respond to suddenly higher load requirements. Peaking plants can also provide emergency back-up, if a baseload plant unexpectedly breaks down or becomes non-operational (also known as an outage). Peaking plants tend to have higher running costs than some baseload plants and due to the occasional nature of the power they generate they are able to command much higher prices per megawatt hour.
Comparing Load Sources
Graph 4 below categorises generation plants based on their load type. Some plants have the capability of providing both load types if the operating time to reach high load from low load is short (i.e., switching the plant on and off can happen quickly). For example, hydro power plants can be used for both, depending on the availability of water. Nonetheless, most types of generation plants are primarily suited to supply either baseload or peak load, due to their mechanical features and the features of their electricity generation.