Module 1 — Energy and civilisation
Lesson 1 of 4
Energy, Work and Power
Learning objectives
By the end of this lesson you should be able to:
- Define energy, work and power.
- Explain the difference between energy and power.
- Use joules, watts, watt-hours and kilowatt-hours correctly.
- Explain why confusing energy and power leads to poor engineering and public policy.
- Understand why both energy and power are essential when designing electricity systems.
Introduction
Every discussion about climate change, electricity markets, batteries, renewable energy and the power grid ultimately depends upon a small number of physical concepts. Unfortunately, these concepts are frequently misunderstood—even by politicians, journalists and occasionally engineers.
Perhaps the most common mistake is treating energy and power as though they are the same thing. They are not.
Energy tells us how much work can be done.
Power tells us how quickly that work can be done.
Understanding this distinction is the foundation upon which the remainder of this course is built.
What is energy?
Energy is the capacity to produce change.
Whenever something moves, heats up, emits light, charges a battery or performs mechanical work, energy is being transferred from one place to another.
Energy cannot be created or destroyed. It can only be transformed between different forms.
Examples include:
- Chemical energy stored in fuels
- Electrical energy transported through wires
- Gravitational potential energy in hydroelectric reservoirs
- Kinetic energy in moving objects
- Thermal energy stored as heat
- Nuclear energy within atomic nuclei
The First Law of Thermodynamics states that total energy is conserved.
Although energy continually changes form, the total quantity remains constant.
What is work?
Work is the transfer of energy.
In physics, work occurs whenever a force causes an object to move.
Examples include:
- Lifting a weight
- Compressing a spring
- Accelerating a vehicle
- Pumping water uphill
The SI unit of work is the joule (J).
One joule represents the work performed by applying a force of one newton through one metre.
Although this definition appears abstract, every activity in modern civilisation ultimately consists of transferring energy to perform useful work.
What is power?
Power describes the rate at which energy is transferred.
Power answers the question:
How quickly is the work being done?
The SI unit of power is the watt (W).
One watt equals one joule every second.
Power therefore measures flow rather than quantity.
An analogy is a water tank.
The volume of water stored represents energy.
The rate at which water leaves the pipe represents power.
You can have:
- a large tank emptied slowly,
- a small tank emptied rapidly,
- or any combination in between.
Exactly the same distinction exists between energy and power.
Energy versus power
A common misconception is that large power automatically means lots of energy.
This is false.
Imagine boiling a kettle.
The heating element might consume 3,000 watts.
If operated for one minute:
- High power
- Small amount of total energy
Now imagine a refrigerator.
It may only consume 100 watts while operating.
However, because it runs continuously throughout the day, its total energy consumption can exceed that of the kettle.
Power tells us how fast.
Energy tells us how much.
The two quantities are related through time.
Energy = Power × Time
Whenever one of these quantities changes, at least one of the others must also change.
Units used in energy systems
Joule (J)
The SI unit of energy.
Because electricity systems are so large, joules are rarely used directly.
Watt (W)
The SI unit of power.
One watt equals one joule per second.
Kilowatt (kW)
One thousand watts.
Typical examples:
- Hair dryer: 2 kW
- Electric kettle: 3 kW
- Domestic EV charger: 7 kW
- Rapid EV charger: 150–350 kW
Megawatt (MW)
One million watts.
Used for industrial equipment and power stations.
Examples:
- Wind turbine: 3–8 MW
- Gas turbine: several hundred MW
Gigawatt (GW)
One billion watts.
National electricity demand is usually measured in gigawatts.
For example, winter electricity demand in Great Britain often exceeds 40 GW.
Watt-hours
Because energy equals power multiplied by time, electricity bills are measured in watt-hours rather than watts.
A watt-hour is the energy consumed by using one watt continuously for one hour.
More commonly:
- 1 kilowatt-hour (kWh) = 1,000 Wh
- 1 megawatt-hour (MWh) = 1,000 kWh
- 1 gigawatt-hour (GWh) = 1,000 MWh
Household electricity bills are charged in kilowatt-hours.
Generators typically trade electricity in megawatt-hours.
National electricity production is often reported in gigawatt-hours or terawatt-hours.
A practical example
Consider a 2 kW electric heater.
If operated for three hours:
Power = 2 kW
Time = 3 hours
Energy consumed = 6 kWh
Notice that the heater never changes power.
What changes is the length of time it operates.
Longer operation increases energy consumption.
Higher power increases the rate of energy consumption.
Why this distinction matters
Much of the public debate around energy policy unintentionally confuses these concepts.
For example:
"We need more energy."
Sometimes this is true.
More often, the real problem is insufficient power during periods of high demand.
Likewise:
"A battery stores power."
This is incorrect.
A battery stores energy.
It also has a maximum power rating that determines how quickly that stored energy can be delivered.
Similarly:
"A wind farm generates energy."
More precisely:
A wind farm has a maximum power output, but the total energy it produces depends on how long the wind blows.
Throughout this course we will distinguish carefully between these concepts because electricity systems must satisfy both simultaneously.
They require sufficient energy over long periods and sufficient power at every instant.
Failure in either dimension results in an unreliable electricity system.
Key takeaways
- Energy is the capacity to perform work.
- Work is the transfer of energy.
- Power measures the rate of energy transfer.
- Energy and power are related but fundamentally different quantities.
- Electricity bills measure energy (kWh).
- Electricity networks must deliver both adequate energy and adequate power.
- Many errors in energy policy arise because these concepts are confused.
Looking ahead
Now that we understand the language of energy, we can ask a much larger question.
Why has access to abundant energy been the single greatest driver of human civilisation?
That is the subject of the next lesson.