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How Do Base and Variable Loads Affect Energy Systems?

  • Starter
  • Article
  • Ages 10+
  • 5 min read
An electrical substation with storage containers, transmission lines and a power station near houses in daylight.

Base load is the electricity needed all day, and variable load is the demand above it that rises and falls with the hour, day and season. An energy system must serve both: steady sources cover the base, flexible sources and storage cover the ups and downs. Supply must always match demand.

Contents
The flat band is the base load that is always there; the raised region is the variable load.
  • Base load: the minimum at all times
  • Variable load: the part that rises and falls
  • Boundary between the two parts

What you will learn

Before you start

No prior knowledge is needed. You only need to know that electricity is made at power plants and travels along wires to where it is used.

What is base load?

Base load is the minimum electricity demand the grid must meet at all times. The US Energy Information Administration's page on generation capacity says base-load service normally supplies all or part of the minimum demand on a system. Energy Education describes baseload power as the minimum electric power that must be supplied to the grid at any given time.

Because it is always needed, base-load generating units tend to run nearly continuously, according to the EIA page.

How does variable load rise and fall?

The rest of the demand does not stand still. Energy Education says demand for electricity fluctuates widely through a day, so baseload power is not necessarily enough. According to the US Department of Energy, peak power use often occurs on summer afternoons and evenings, when solar generation is falling.

The EIA adds that peaking units can serve relatively short demand periods, often a few hours a day, to support air-conditioning loads.

Base-load sources run steadily, load-following sources rise and fall, peaking sources switch on only at the highest demand.
  • Base load: runs steadily
  • Load-following: rises and falls with demand
  • Peaking: short and tall

How does the energy system serve the two kinds of load?

Each part of the load uses a different kind of source. The EIA groups plants by how they operate. Nuclear plants generally run as base-load service because of their low fuel costs and technical restrictions on load-responsive operation. Intermediate-load service follows the load; many gas-fired combined-cycle units can do this because they ramp up and down quickly. Peak-load service helps meet demand when it is at its highest.

The reason lies in balance. According to the EIA page on delivery to consumers, for the grid to remain stable, the amount of electricity supplied must match demand. If supply and demand fall out of balance, local or even widespread blackouts can happen.

When supply equals demand the grid is stable; when they diverge, blackouts can happen.
  • Supply matches demand: balanced
  • Supply and demand diverge: tilted
  • Risk of blackout

How does storage help with the ups and downs?

Storage lets electricity be used at a different time from when it was made. According to the Department of Energy, without storage electricity must be generated and consumed at the same time. Storage can be charged when generation is high and consumption is low, then dispensed when the load or demand is high.

The battery fills while supply is plentiful, then feeds the house when demand is high.
  • When generation is high: sunshine
  • Storage: charges then discharges
  • When demand is high: a house using power

Real-world example

Picture an illustrative town. Every home's refrigerator and other always-on devices create a base level of demand. On a hot afternoon every home switches on air conditioning at once, so demand shoots up and then falls as the evening cools. Steady sources hold the base, while flexible sources or storage cover the climb. This is a model, not data from a real town.

Common misunderstandings

Base load means low demand. Base load is the minimum that must be met at all times, and it still needs sources running almost continuously.

Base-load sources alone are enough. Demand fluctuates widely during a day, so baseload power is not necessarily enough and the grid needs load-following capacity too.

Electricity can simply be set aside without any technology. Without storage, electricity must be generated and consumed at the same time.

Frequently asked questions

What is base load?

The minimum electricity demand the grid must meet at all times.

How does variable load differ from base load?

Variable load is the part of demand that rises and falls with the hour, day and season; base load is the nearly constant minimum.

Why must supply match demand?

For the grid to remain stable, the electricity supplied must match demand; a mismatch can cause blackouts.

What does energy storage do for the grid?

It charges when generation is high and consumption is low and discharges when demand is high.

Summary

  • Base load is the minimum needed at all times; the part above it is variable load.
  • Steady sources cover the base; load-following and peaking sources cover the ups and downs.
  • Supply must match demand, and storage helps shift electricity to when it is needed.

Terms in this article

English and Vietnamese terms, with the meanings used here. Hover, tap or Tab to a term with a dotted underline for an explanation as you read.

baseloadtải nền
The minimum, nearly constant level of electricity demand that the grid must meet at all times.
variable loadtải biến thiên
The part of electricity demand above the base that rises and falls with the hour, day and season.
peak loadtải đỉnh
The highest level of electricity demand, often lasting a few hours a day.
gridlưới điện
The network that carries electricity from where it is made to where it is used.
storagelưu trữ năng lượng
Technology that captures electricity, keeps it as another form of energy and releases it when needed.
peaking plantnhà máy điện đỉnh
A plant that runs mainly when demand is high, covering the rising and falling part of the load.

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Sources

These sources are written for older readers. If they feel hard, read them with an adult.