Module 3 — Reliability and Security of Supply
Lesson 3 of 7
Capacity, Energy and Flexibility
Learning objectives
By the end of this lesson you should be able to:
- Distinguish between energy, capacity and flexibility.
- Understand why these represent different forms of system value.
- Explain why two resources producing the same amount of energy can have very different value to the electricity system.
- Recognise why modern electricity systems increasingly depend upon flexible resources.
- Appreciate why rewarding only energy production may fail to deliver reliable electricity systems.
Introduction
When discussing electricity generation, people often ask:
"How much electricity does it produce?"
This is an important question.
It is not, however, the only one.
A power station that produces large quantities of electricity may contribute relatively little during periods of system stress.
Conversely, another resource may produce comparatively little energy over a year yet become essential when demand is highest or when other generators fail.
To understand reliability, we must distinguish between three different concepts:
- Energy
- Capacity
- Flexibility
Although closely related, they describe different characteristics of electricity resources.
Understanding this distinction is one of the most important steps towards understanding modern electricity markets.
Energy
Energy is the total amount of electricity produced or consumed over a period of time.
It is measured in:
- kilowatt-hours (kWh)
- megawatt-hours (MWh)
- gigawatt-hours (GWh)
- terawatt-hours (TWh)
Energy answers the question:
How much electricity was produced?
For example:
A wind farm producing 500 MWh during one day has delivered 500 MWh of electrical energy.
Energy is the quantity that most electricity markets settle financially.
However, energy alone tells us very little about reliability.
Capacity
Capacity describes the maximum rate at which electricity can be produced or consumed.
It is measured in:
- kilowatts (kW)
- megawatts (MW)
- gigawatts (GW)
Capacity answers a different question:
How much power can this resource provide right now?
Imagine two generators.
Generator A:
- Maximum output: 500 MW
Generator B:
- Maximum output: 100 MW
Generator A clearly has greater capacity.
Capacity determines how much demand a resource can help meet during periods of peak system stress.
Capacity is not energy
Suppose a 100 MW generator operates continuously for ten hours.
It produces:
1,000 MWh of energy.
Now suppose a 500 MW generator operates for only two hours.
It also produces:
1,000 MWh of energy.
Both generators produce exactly the same amount of energy.
Yet their contribution to peak demand is very different.
One can supply five times more power when required.
This distinction lies at the heart of electricity planning.
Flexibility
Flexibility describes how easily a resource can change its output or consumption in response to changing system conditions.
It answers questions such as:
- Can output increase quickly?
- Can output decrease quickly?
- Can operation be shifted through time?
- Can consumption be delayed?
- Can the resource respond automatically?
Flexibility is increasingly valuable because electricity systems are becoming more variable.
Why flexibility matters
Electricity demand changes continuously.
Renewable generation also changes continuously.
The electricity system therefore needs resources capable of adapting to changing conditions.
Imagine a windy afternoon.
Wind generation is abundant.
Electricity prices are low.
Several hours later:
- Wind speeds fall.
- Demand increases.
- Sunset reduces solar generation.
The system must rapidly replace the lost renewable generation.
Flexible resources make this possible.
Examples of flexible resources
Different technologies provide flexibility in different ways.
Gas turbines
Can often increase output relatively quickly.
Batteries
Can respond within fractions of a second.
However, they have limited stored energy.
Pumped hydro
Can respond rapidly while providing longer-duration energy storage.
Flexible demand
Consumers may choose to:
- Charge electric vehicles later.
- Delay industrial processes.
- Reduce heating temporarily.
- Shift appliance use.
Rather than increasing generation, these resources reduce or move demand.
Interconnectors
Can import or export electricity depending upon conditions in neighbouring countries.
Inflexible resources
Some resources have limited flexibility.
For example:
Nuclear power stations are generally designed to operate continuously at high output.
Although technically capable of changing output, doing so may reduce efficiency or increase operating costs.
Similarly, wind and solar generation depend primarily upon weather conditions.
Their output cannot simply be increased whenever demand rises.
These resources may provide large quantities of low-cost energy while contributing relatively little operational flexibility.
Three generators
Imagine three generators.
Generator A
Produces:
5 TWh every year.
Cannot change output quickly.
Generator B
Produces:
2 TWh every year.
Can start within minutes.
Generator C
Produces:
Almost no energy during normal conditions.
Can reach full output within seconds whenever another generator fails.
Which is most valuable?
The answer depends entirely upon what the electricity system needs at that moment.
All three contribute value.
They simply contribute different kinds of value.
Why energy alone is not enough
Historically, many electricity markets rewarded generators primarily for producing energy.
This worked reasonably well when electricity systems consisted mainly of large conventional power stations.
Today's systems are different.
They increasingly contain:
- Wind farms.
- Solar farms.
- Batteries.
- Flexible demand.
- Distributed generation.
- Electric vehicles.
These technologies contribute value in many different ways.
Paying only for energy risks overlooking services that are essential for maintaining reliability.
The relationship between energy, capacity and flexibility
These concepts are connected but distinct.
Energy tells us:
How much electricity is produced.
Capacity tells us:
How much power can be delivered.
Flexibility tells us:
How quickly and how easily output or demand can change.
A reliable electricity system requires all three.
Removing any one of them creates problems.
An analogy
Imagine delivering parcels.
Energy is the total number of parcels delivered during the day.
Capacity is the size of the delivery vehicle.
Flexibility is how quickly the driver can change route when traffic conditions change.
A company needs all three.
A large vehicle is useful.
Delivering many parcels is useful.
Being able to adapt when roads close is also useful.
Electricity systems face the same challenge.
Why this matters for market design
Suppose two generators produce exactly the same amount of energy over a year.
One:
- Is available whenever demand peaks.
- Can increase output rapidly.
- Can respond during emergencies.
The other:
- Operates only when weather conditions are favourable.
- Cannot increase output when required.
If both receive identical payment simply because they produced the same amount of energy, an important part of their contribution has been ignored.
This observation has shaped electricity market design around the world.
Many systems now include payments not only for energy, but also for availability, capacity and operational services.
As we will see later in this module, energy is only one component of a generator's overall contribution to reliability.
A key insight
Perhaps the most important lesson is this:
Electricity resources do not create value in only one dimension.
Energy is valuable.
Capacity is valuable.
Flexibility is valuable.
Different technologies excel in different areas.
Understanding these different forms of value is essential for designing electricity markets that encourage both efficient operation today and reliable investment for the future.
Key takeaways
- Energy measures the total quantity of electricity produced over time.
- Capacity measures the maximum rate at which electricity can be produced or consumed.
- Flexibility measures how easily a resource can respond to changing system conditions.
- Two resources producing identical amounts of energy may contribute very different levels of reliability.
- Modern electricity systems increasingly depend upon flexible resources.
- Paying only for energy production may fail to reward services that are essential for reliable system operation.
- Different technologies provide different combinations of energy, capacity and flexibility.
- Understanding these distinctions is fundamental to modern electricity market design.
Looking ahead
Capacity alone does not guarantee reliability.
A generator must also be available when it is needed.
In the next lesson we examine the difference between capacity and availability, and why a generator's contribution depends not only on how much it can produce, but also on when it can produce it.