Load in plain terms
In electrical engineering, “load” means anything that uses electricity, and also the amount of power being used. A refrigerator is a load. So is a factory, and so is the total of everything connected to a city’s network. When someone says “the load is high right now,” they usually mean the total electricity demand across a system is high.
Load is measured in units of power, such as kilowatts and megawatts. The distinction between power (a rate) and energy (an amount over time) matters here, and is part of understanding energy units. Load is a rate: how much is being drawn at this moment. A chart of load over time is called a load curve or load profile, and it is one of the most useful pictures in the industry.
Why demand is never flat
Millions of separate decisions add up to the total. People wake up, cook, run appliances, heat or cool buildings, and charge devices. Businesses open and close. Streetlights switch on at dusk. Because many people follow similar routines, the combined load has recognizable shapes, even though no one controls it centrally.
A typical day
Exact shapes vary by region, season and mix of customers, but the following pattern is common in many places.
- Overnight. Demand is usually at its lowest, when most people are asleep and businesses are closed. Some demand continues, from refrigeration, always-on equipment and industrial processes.
- Morning rise. As people wake and businesses start up, load climbs, with heating, lighting, cooking and commercial activity.
- Daytime. Commercial and industrial use tends to keep demand relatively high on weekdays. In hot regions, air conditioning can push the afternoon higher.
- Evening peak. In many residential areas, demand reaches its highest point in the late afternoon or evening, when people return home, cook, use entertainment devices and, depending on the season, heating or cooling, while lighting is needed after dark.
- Late evening decline. Demand falls again as people go to bed.
Weekends and holidays usually look different from weekdays, with lower commercial demand and a shifted morning rise. Very large events, like a widely watched broadcast, can occasionally cause a noticeable short-term change.
Seasonal patterns
Seasons shift the whole picture. In places with cold winters, heating drives high demand, especially on the coldest mornings and evenings. Where heating is by electricity, such as with electric resistance or heat pumps, this can be substantial. In hot regions or summers, air conditioning raises demand on hot afternoons. Many grids are therefore either winter-peaking or summer-peaking, depending on climate and how buildings are heated and cooled.
Weather is the biggest driver of short-term differences. A heat wave or a cold snap can push demand far above normal, and forecasts of temperature are a central input to forecasting demand.
Base load, peak load and the shape between
Some terms are used often in this area.
- Base load is the minimum level of demand that is present most of the time.
- Peak load is the highest demand reached over a given period, such as a day or a year.
- Load factor compares average demand to peak demand. A high load factor means the system runs close to its peak more steadily; a low one means demand is spiky.
Historically, some power plants were run almost continuously to cover base load, others were turned on as demand rose, and “peaker” plants ran for only a few hours a year to meet the highest peaks. As more wind and solar are added, the framing is shifting, since planners increasingly look at the demand left over after subtracting variable generation, often called net load.
Why this shapes how the system is run
The grid has to be built and operated for the peak, not the average. Generators, lines and transformers must be large enough to handle the highest demand, even though that level may occur only a few hours a year. This is costly, because equipment built for rare peaks sits partly unused the rest of the time.
It also explains why grid operators spend so much effort forecasting. They schedule resources hour by hour to match the expected load curve, and keep reserves available in case reality differs from the forecast. Demand forecasts rely on weather, calendar effects and historical data.
Ways of reshaping demand
Because peaks are expensive, there is ongoing interest in flattening them, or moving some use to other times. Common ideas include:
- Pricing that varies by hour, such as time-of-use tariffs, which reward shifting flexible use to cheaper periods.
- Programs where customers allow limited, short adjustments to devices such as thermostats or water heaters.
- Smart charging for electric vehicles, so that charging happens when the system has spare capacity.
- Energy efficiency measures that reduce demand overall, such as better insulation and efficient appliances.
- Energy storage, which can charge when demand is low and discharge at peak times.
Effectiveness varies. Results depend on how customers respond, on program design and on local conditions, so it is wise to treat claims about large savings with some caution.
Growing and changing demand
Demand is not static over the years. Efficiency improvements can reduce it, while electrified heating, vehicles and new data centers can increase it. Planners must forecast these trends under uncertainty, and projections differ.
This article is general information about how demand works, not a forecast or advice for a particular system.