Table of Contents
Understanding How Electricity Demand Changes Over Time
Electricity use is not constant. It rises and falls over the hours of a day, across days of the week, and between seasons. To design and operate energy systems and grids effectively, we need a clear way to describe how demand behaves over time. This is where the concepts of base load, peak load, and load profiles become essential.
In this chapter we focus on how demand varies, how it is described, and why these ideas matter for planning and operating power systems that increasingly include renewable energy sources.
What Is Electrical Load?
In the context of power systems, the word “load” usually means the total electrical power that consumers are using at a given instant. It is measured in watts, usually in kilowatts (kW), megawatts (MW), or gigawatts (GW) when speaking of whole cities or countries.
If you plot this power demand on a graph with time on the horizontal axis and power on the vertical axis, you get a picture of how the load changes. This picture is the starting point for understanding base load, peak load, and load profiles.
Base Load: The Always-Present Part Of Demand
Base load is the level of electricity demand that is present most or all of the time. Even late at night when many people are asleep, some electricity use continues. Street lights, refrigerators, data centers, industrial processes, and basic services keep running. The lower band of demand that rarely disappears is called the base load.
In a typical daily graph of demand, the base load looks like the “floor” of the curve. It does not mean the absolute minimum in a single unusual hour, but rather the relatively stable demand that appears every day. In many systems, base load is supplied by power plants that are designed to run for long periods with steady output, such as some large thermal or nuclear plants, or in the future, a combination of renewables plus storage and flexible resources.
Base load is described in terms of power, for example, a system may have a base load of 5 GW, meaning that its demand rarely drops below 5 GW.
Peak Load: The Highest Demand Periods
In contrast, peak load is the highest level of electricity demand that occurs over a defined period of time, such as a day, a season, or a year. If you imagine the graph of demand, the peak load is the top of the highest spike.
There are two related ideas that are often used:
The daily peak load is the maximum demand during a specific day.
The annual peak load is the maximum demand recorded over an entire year.
The annual peak load is especially important, because the system must be built to handle at least this value, otherwise there will be shortages or blackouts when demand is highest.
Utilities and grid operators pay a lot of attention to when peaks occur. In some countries, peak demand is in the early evening on winter days when people return home, turn on lights, heating, and appliances all at once. In other regions it may be on hot summer afternoons when air conditioners are used heavily.
Because peak load is short in duration compared to the rest of the year, meeting it can be expensive. Power plants or other resources that run only a few hours per year must still be built and maintained, contributing significantly to system costs.
Off-Peak And Shoulder Periods
Between base load and peak load, there are times of moderate demand. To describe these variations more precisely, two additional terms are common:
Off-peak periods are those with relatively low demand, often during the night or very early morning. Electricity prices are sometimes lower in these periods, and some equipment or loads are scheduled to run then.
Shoulder periods are the intermediate times between peak and off-peak, such as mid-morning or late evening, when demand is neither extremely high nor very low.
These distinctions help in planning tariffs, scheduling maintenance for power plants, and encouraging consumers to shift flexible use to less congested times.
Load Profiles: The Shape Of Demand Over Time
A load profile is a detailed description of how electricity demand changes with time. It is often shown as a line on a graph, sometimes called a load curve, where each point represents the load at a particular time.
Load profiles can be drawn for many different scales:
An individual appliance, such as an electric water heater.
A single building, such as a household, office, or factory.
A group of customers, such as all residential consumers in a city.
An entire region or country.
The basic idea is always the same. The load profile describes how demand climbs, falls, and fluctuates, and how long it stays at different levels.
Daily, Weekly, And Seasonal Load Profiles
Load profiles change over different time horizons.
A daily load profile shows how demand varies during a 24 hour period. It might reveal a morning increase as people wake up, a midday plateau, and an evening peak when many activities overlap. At night, the curve drops closer to the base load.
A weekly load profile shows how typical days within a week compare, such as higher demand on weekdays and lower demand on weekends, depending on work patterns and industry.
A seasonal or annual load profile reveals how demand changes with weather, daylight length, holidays, and economic activity. In some regions, electricity use is higher in winter due to heating, in others it is higher in summer due to cooling.
These patterns are characteristic of each system and are influenced by climate, lifestyle, industrial structure, and technology use. They are also not fixed forever. As societies change or as new technologies spread, load profiles evolve.
Load Duration Curves
A useful way to summarize the behavior of load over a longer period is the load duration curve. Instead of showing demand in chronological order, the load values are sorted from highest to lowest.
To build a load duration curve for one year, the demand for each hour of the year is taken, then the 8,760 hourly values are arranged so that the highest load is on the left and the lowest on the right. On the horizontal axis is the number of hours, and on the vertical axis is the load in MW or GW.
This curve shows at a glance how often the system operates at high, medium, or low loads. The area under the curve represents the total energy consumed in the period. The steepness of the curve indicates how variable demand is.
If the curve is fairly flat, demand does not vary much, and a large portion of generation can be from resources that run continuously. If the curve is steep, with high peaks and low troughs, more flexible or fast responding resources are needed to follow the changing load.
Base Load, Intermediate, And Peak Plants
Although generation technologies are discussed in other parts of the course, it is helpful here to note how different types of power plants relate to load characteristics.
Power plants that are relatively cheap to operate but expensive to build, and that prefer to run at constant output, are often used to meet base load. They are kept running many hours per year to spread their capital cost over a lot of generated energy.
Plants that can adjust output up and down and start or stop more easily are used to cover the so called intermediate load, the part of demand that is above base load but present for many hours.
Highly flexible and fast starting plants, or other resources that can quickly respond, are traditionally used to cover short duration peaks. These plants often have higher operating costs but are justified by their role in keeping the system reliable during rare high demand events.
In systems with more renewable energy, storage, demand response, and other options increasingly contribute alongside traditional plants, but the basic idea of matching different types of supply to different parts of the load profile remains relevant.
Peak Load, Capacity, And Reserve Margins
To avoid blackouts, the total available capacity of a power system must be greater than the expected peak load. In practice, extra capacity is needed, because some plants may be unavailable at times due to maintenance or unexpected failures, and forecasts can be wrong.
The difference between the installed capacity of the system and the expected peak load is often expressed through the concept of a reserve margin.
If the installed capacity is $C$ and the expected peak load is $P_{\\text{peak}}$, then the reserve margin $R$ as a percentage is
$$R = \\frac{C - P_{\\text{peak}}}{P_{\\text{peak}}} \\times 100\\%.$$
The system must maintain sufficient capacity above the expected peak load to ensure reliability. The reserve margin quantifies this extra capacity as a percentage of peak demand:
$$R = \\frac{C - P_{\\text{peak}}}{P_{\\text{peak}}} \\times 100\\%.$$
Insufficient reserve margin increases the risk of supply shortages during high demand or unexpected outages.
The appropriate reserve margin depends on many factors, including the reliability of plants, variability of demand, and the characteristics of renewable generation, which are discussed in other chapters. However, the connection to peak load is direct. The higher the peak relative to average demand, the more capacity is needed to keep the system secure.
Load Factor And Utilization Of Capacity
To describe how efficiently installed capacity is used, the concept of load factor is introduced. It uses the load profile over a period and compares the average demand to the peak demand in that period.
If $E$ is the total energy consumed in a given time, $P_{\\text{peak}}$ is the maximum load in that time, and $T$ is the duration of the period, the load factor $LF$ is defined as
$$LF = \\frac{E}{P_{\\text{peak}} \\times T}.$$
Equivalently, it can be written as
$$LF = \\frac{P_{\\text{average}}}{P_{\\text{peak}}},$$
where $P_{\\text{average}}$ is the average load over the period.
The load factor measures how steadily the system uses its maximum capacity:
$$LF = \\frac{P_{\\text{average}}}{P_{\\text{peak}}}.$$
A higher load factor means demand is relatively stable. A lower load factor means sharp peaks and low utilization of capacity.
If the load factor is close to 1, the system operates near its peak load for much of the time. If it is low, for example 0.4, the system has large peaks relative to average demand, which can signal that much of the system capacity is underused much of the time.
Improving load factor by smoothing peaks and filling valleys can help reduce the need for new capacity and can lower costs.
Customer And Sector-Specific Load Profiles
Not all consumers use electricity in the same way. Different categories of users have characteristic load profiles.
A typical residential household may have low demand overnight, a rise in the morning, lower use during working hours if people are away, and a strong evening peak. The shape changes with climate and habits, but the concentration of use in certain hours is common.
Commercial buildings, such as offices or shops, often show higher demand during business hours and lower demand in evenings and at night. Their peaks may occur in the middle of the day, especially where lighting and cooling loads are significant.
Industrial facilities can have very different profiles depending on the process. Some plants run continuously and contribute to base load. Others follow shifts or operate only during certain hours, creating more variable patterns.
Understanding these sector specific load profiles helps in designing tariffs, energy efficiency programs, and demand response, and in planning where and how to integrate renewable generation and storage close to customers.
The Role Of Load Profiles In Planning And Operation
Accurate knowledge of base load, peaks, and full load profiles plays a central role in many aspects of energy system planning and grid operation.
In long term planning, load forecasts and expected load profiles guide decisions about how much capacity to build, what types of generation technologies to invest in, and where to strengthen the grid. Planners must consider how changes in population, economy, technology, and policy will affect future demand patterns.
In short term operation, grid operators monitor the load profile in real time and compare it to forecasts. They schedule which plants run when, in order to match supply to demand at least cost while respecting technical and environmental constraints. They also plan for reserves to handle sudden changes.
Tariff design often uses information about load profiles. Time of use pricing, where electricity prices vary by time of day, can incentivize consumers to shift some demand away from peak periods. This can reduce strain on the system and potentially delay or avoid costly investments in new capacity.
For integration of variable renewable energy, such as solar and wind, load profiles are considered together with generation profiles. The times when the sun shines or the wind blows may or may not match the times when demand is highest. Storage, demand response, and other flexibility options are then used to bridge gaps between supply and demand over time.
Changing Load Profiles In A Modern Energy System
Load profiles are not fixed. They evolve as technologies and behaviors change. For example, widespread adoption of air conditioning has increased summer peaks in many regions. Similarly, large scale use of electric heating can increase winter peaks.
Digital technologies, more efficient appliances, and automation can also reshape demand. In the future, electric vehicles, heat pumps, and smart devices that can shift their operation in response to prices or signals from the grid may significantly alter load profiles.
In some settings, self generation with rooftop solar changes not only how much electricity is purchased from the grid but also when it is used. The shape of net load, which is the grid demand after subtracting local generation, can become very different from the original demand profile.
As the energy transition advances, understanding and actively shaping load profiles will become increasingly important. Instead of viewing demand as fixed and inflexible, many systems are beginning to treat it as a resource that can adjust within limits to help balance the grid and make better use of renewable generation.
Summary Of Key Ideas
Base load is the portion of demand that is present most of the time. Peak load is the maximum demand within a given period. The complete time varying pattern of demand is described by the load profile, which can be examined over days, weeks, seasons, and years.
Tools like load duration curves and indicators like load factor provide compact ways to understand how much demand varies and how efficiently capacity is used. These concepts are fundamental for planning generation capacity, designing tariffs, integrating renewables, and ensuring that power systems remain reliable and affordable as they evolve.