Generation, Networks and Demand

Learning objectives

By the end of this lesson you should be able to:

Describe the three main physical parts of an electricity system.
Explain how electricity moves from generators to consumers.
Distinguish between transmission and distribution networks.
Understand the difference between electricity demand and energy consumption.
Recognise why generation, networks and demand must be coordinated as one system.
Explain why electricity cannot be governed as though supply and demand were physically independent.

Introduction

An electricity system can be understood through three broad physical components:

Generation
Networks
Demand

Generators produce electrical power.

Networks transport it.

Demand consumes it.

That description sounds simple, but the components do not operate independently.

Every generator, cable, transformer and consumer is connected through the same physical system.

A change in one part of the network can affect conditions elsewhere almost immediately.

This is one of the defining features of electricity.

It is not enough for a buyer and seller to agree that a quantity of electricity has been traded. The physical system must also be capable of generating, transporting and consuming that electricity safely at the required time and location.

Generation

Generation is the process of converting another form of energy into electrical energy.

Electricity is not normally found in nature in a form that can be collected and delivered directly to consumers. It must be produced using a generator or another conversion technology.

Common sources include:

Natural gas
Coal
Nuclear energy
Wind
Solar energy
Hydroelectric power
Biomass
Geothermal energy

Different technologies convert energy in different ways.

A gas turbine burns fuel to produce hot gases that turn a turbine.

A nuclear power station uses heat from nuclear reactions to produce steam and drive a turbine.

A wind turbine converts the movement of air into electrical power.

A solar photovoltaic panel converts sunlight directly into electricity.

Dispatchable and variable generation

Generators differ not only in cost and environmental impact, but also in how controllable they are.

Dispatchable generation

Dispatchable generators can usually increase or decrease their output in response to instructions, subject to their technical limits.

Examples may include:

Gas turbines
Hydroelectric plants with stored water
Biomass plants
Some nuclear units, although these may be less flexible

Dispatchable generation is valuable because its output can be adjusted when demand changes.

Variable generation

Variable generators depend upon weather or other external conditions.

Examples include:

Wind turbines
Solar panels
Run-of-river hydroelectric generation

Their output can often be reduced, but it cannot always be increased on demand because the underlying energy source may not be available.

A wind farm cannot produce full power when there is insufficient wind.

A solar farm cannot produce at night.

This does not make variable generation unusable. It means the wider electricity system must account for uncertainty and variability.

Generator capacity and output

It is important to distinguish between a generator's capacity and its actual output.

A wind farm may have a capacity of 500 MW.

This means it can produce up to 500 MW under suitable conditions.

It does not mean that it produces 500 MW continuously.

Actual output depends upon:

Weather
Fuel availability
Maintenance
Technical constraints
Market schedules
Network conditions

Capacity describes the maximum possible rate of production.

Output describes what the generator is producing at a particular moment.

Electricity networks

Generators and consumers are rarely located beside one another.

Electricity networks connect them.

The network consists primarily of:

Overhead lines
Underground cables
Substations
Transformers
Protection systems
Control and communication equipment

These assets allow electricity to move across cities, regions and countries.

The network is not merely a delivery service added after electricity has been traded.

It is an active physical system with limits that determine which transactions can actually be delivered.

Transmission networks

Transmission networks carry large amounts of electricity over long distances.

They connect:

Large generators
Major substations
Population centres
Industrial regions
Interconnectors between countries

Transmission systems operate at very high voltages.

Using high voltage allows a given amount of power to be transferred with lower current, reducing resistive losses and allowing electricity to be moved more efficiently.

Transmission networks can be thought of as the motorways of the electricity system.

They move large volumes of power between major parts of the country.

Distribution networks

Distribution networks carry electricity from the transmission system to individual consumers.

They serve:

Homes
Shops
Offices
Schools
Hospitals
Factories
Electric vehicle chargers
Local generation and storage

Distribution networks operate at progressively lower voltages as electricity approaches the point of use.

Transformers reduce the voltage in stages until electricity can be safely supplied to buildings.

Distribution networks can be thought of as the regional roads and local streets of the electricity system.

A changing distribution system

Historically, distribution networks were designed mainly for one-way power flows.

Electricity moved from large central power stations through the transmission network and then down through distribution networks to passive consumers.

That model is changing.

Modern distribution networks increasingly include:

Rooftop solar panels
Batteries
Electric vehicles
Heat pumps
Community energy systems
Flexible industrial loads
Small-scale generators

Consumers can now import electricity at one moment and export it at another.

The distribution network is therefore becoming a much more active and complex part of the electricity system.

Demand

Demand is the rate at which consumers are using electrical power at a particular moment.

It is measured in watts, kilowatts, megawatts or gigawatts.

Demand changes continuously.

It varies with:

Time of day
Day of the week
Weather
Industrial activity
Human behaviour
Electricity prices
Technology

Demand is usually lower overnight and higher during periods when households and businesses are active.

Cold weather can increase heating demand.

Hot weather can increase air-conditioning demand.

Electric vehicles and heat pumps can create new demand patterns.

Demand is not the same as energy consumption

Demand describes the rate of electricity use at a particular moment.

Energy consumption describes the total amount used over a period of time.

A factory drawing 10 MW has a demand of 10 MW.

If it operates at that level for three hours, it consumes 30 MWh of energy.

This distinction matters because networks must be sized to accommodate power demand, not merely total energy use.

Two consumers may use the same amount of energy in one day but place very different demands on the network.

For example:

Consumer A uses 24 kWh evenly across 24 hours.
Consumer B uses 24 kWh during a single one-hour period.

Both consume the same energy.

However, Consumer B creates a much higher peak demand and requires significantly more network capacity.

Peak demand

Peak demand is the highest level of electricity demand experienced during a given period.

The electricity system must be capable of meeting this peak even if it occurs for only a small number of hours each year.

Peak demand influences how much:

Generation capacity is required
Network capacity must be built
Reserve capacity must be available
Flexibility the system needs

This creates an important economic challenge.

Some infrastructure may be used intensively for only a few hours but must still be built, maintained and financed throughout the year.

Flexible and inflexible demand

Not all demand must occur at a precise moment.

Inflexible demand

Some electricity uses cannot easily be delayed.

Examples include:

Hospital equipment
Lighting currently in use
Industrial safety systems
Critical communications
A cooking appliance while a meal is being prepared

Flexible demand

Other uses can be shifted within limits.

Examples include:

Charging an electric vehicle overnight
Heating water within a permitted time window
Operating some industrial processes
Charging a battery
Pre-heating or pre-cooling a building

Flexible demand can help the system use available generation and network capacity more efficiently.

However, flexibility does not mean that demand can simply disappear.

The underlying energy service still has to be delivered within the consumer's constraints.

The electricity system as a chain

A simplified electricity system can be represented as:

Primary energy source

↓

Generator

↓

Transmission network

↓

Distribution network

↓

Consumer demand

At first glance, this looks like a one-way supply chain.

In reality, the system is becoming more interconnected.

A household may consume electricity from the grid, produce electricity from solar panels, charge a battery and export electricity later.

An electric vehicle may eventually act as both a consumer and a source of power.

The boundary between generation and demand is therefore becoming less clear.

Why location matters

Electricity demand is not simply a national total.

It occurs at specific places within the network.

Similarly, generation is connected at particular locations.

A country may have enough generation overall but still experience local problems if the network cannot transport electricity from where it is produced to where it is needed.

For example:

A region may have abundant wind generation.
A distant city may have high demand.
The transmission lines connecting them may already be operating near their limits.

In that situation, additional wind output may need to be reduced even though consumers elsewhere still require electricity.

This is known as network congestion.

It demonstrates why national supply and demand totals are not enough to describe the physical condition of the system.

Why timing matters

Electricity also has to be delivered at the correct time.

Producing 1 MWh at midday does not automatically satisfy a demand for 1 MWh during the evening.

The quantities may be equal, but they occur at different times.

Storage or flexible demand may help shift energy between periods, but this requires physical assets with their own limits and losses.

Electricity therefore has both a spatial and temporal dimension.

It must be available:

At the right location
At the right time
At the required power level
Within the technical limits of the network

Networks do not create unlimited transport capacity

It is sometimes assumed that once electricity enters the grid, it can be delivered anywhere.

That is not correct.

Every part of the network has physical limits.

Lines and transformers can carry only a certain amount of power.

Voltages must remain within acceptable ranges.

The system must remain stable following faults or sudden changes.

Electricity cannot simply be routed like a digital message through whichever path a market participant prefers.

Its physical flow is governed by the electrical properties of the interconnected network.

Why generation, networks and demand must be coordinated

Generation cannot be planned without considering demand.

Demand cannot be served without considering the network.

The network cannot be operated safely without knowing where generation and demand are connected.

These are therefore not three independent systems.

They are three parts of one continuously interacting physical system.

A market may arrange for a generator to sell electricity to a consumer, but the system operator must still determine whether:

The generator can produce it.
The network can transport it.
The consumer is using it at the same time.
The wider system remains secure.

This distinction between commercial agreement and physical delivery will become increasingly important throughout the course.

An illustrative example

Imagine that a wind farm in the north agrees to sell 100 MW to a city in the south.

Financially, the transaction is straightforward.

The wind farm is the seller.

The city supplier is the buyer.

Physically, however, several additional conditions must be satisfied.

The wind farm must actually have enough wind to generate 100 MW.

The transmission network must have sufficient capacity.

Other generators and consumers connected to the system will affect the resulting power flows.

Voltage and stability limits must remain secure.

Demand must exist at the same moment the electricity is generated.

The contract describes the commercial relationship.

It does not override the physical conditions required for delivery.

Why this matters for electricity markets

Ordinary commodity markets can often treat production, transport and consumption as partly separable activities.

A tonne of grain can be stored in a warehouse and delivered later.

A barrel of oil can be shipped along a chosen route.

Electricity is different.

Its production, transport and consumption occur through one synchronised network.

Markets therefore cannot be designed independently of engineering constraints.

A financially valid trade may still be physically undeliverable.

Electricity markets must coordinate economic decisions within the feasible operating limits of the power system.

Key takeaways

Electricity systems consist broadly of generation, networks and demand.
Generators convert other forms of energy into electrical power.
Transmission networks move large quantities of electricity over long distances.
Distribution networks deliver electricity locally and increasingly connect distributed generation, storage and flexible demand.
Demand is the rate at which electricity is being used, while energy is the total consumed over time.
The location and timing of generation and demand both matter.
Networks have finite physical limits.
Generation, networks and demand must be coordinated as one system.
Financial transactions describe commercial relationships, but they do not guarantee physical delivery.

Looking ahead

Electricity networks do not normally operate using a steady, one-directional flow.

They use alternating current, in which voltage and current repeatedly change direction.

In the next lesson, we examine alternating current and frequency, and explain why system frequency provides a real-time indication of whether generation and demand are in balance.