Why Electricity Follows Physics, Not Contracts

Learning objectives

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

Explain the difference between commercial transactions and physical electricity flows.
Understand why electricity follows the laws of physics rather than contractual arrangements.
Recognise why market schedules and physical operation are fundamentally different problems.
Explain the role of the System Operator in reconciling commercial intentions with physical reality.
Understand why engineering constraints define what markets can achieve.
Appreciate why modern electricity markets are fundamentally cyber-physical systems rather than purely financial markets.

Introduction

Throughout this module we have explored how electricity systems physically operate.

We have seen that:

Electricity must be generated and consumed at almost the same instant.
Supply and demand must remain balanced continuously.
Networks have thermal, voltage and stability limits.
Electricity naturally distributes itself across interconnected networks according to physical laws.

These ideas lead to one of the most important concepts in modern electricity systems:

Electricity follows physics, not contracts.

This statement may sound obvious.

Yet it is perhaps the single most misunderstood aspect of electricity markets.

Many people imagine that when a generator sells electricity to a consumer, the electricity physically travels from one to the other.

That is not how power systems work.

What does a contract actually do?

Suppose a wind farm signs a contract to sell 100 MWh of electricity to a supplier.

The contract specifies:

The buyer.
The seller.
The quantity.
The delivery period.
The price.

These are commercial commitments.

The contract creates legal rights and financial obligations.

It determines who pays whom.

What it does not determine is the physical path that electricity takes through the network.

What happens physically?

Imagine that the wind farm begins generating electricity.

Does every electron produced by the wind farm travel directly to the supplier's customers?

No.

Instead, electrical power spreads across the interconnected network according to the physical characteristics of that network.

Every generator contributes to supplying the entire synchronised system.

Every consumer draws power from that same interconnected system.

The commercial relationship exists only in the accounting records.

The physical system behaves according to electrical laws.

An analogy

Imagine pouring water into one side of a large interconnected lake.

Water immediately redistributes throughout the lake.

It does not travel in a narrow pipe directly to one particular person drawing water elsewhere.

Electricity behaves similarly.

Once power is injected into the interconnected network, its distribution depends upon the physical properties of the system rather than the commercial intentions of market participants.

Although this analogy is not perfect, it captures an important idea:

Markets determine ownership.

Physics determines movement.

Electricity follows the path of least impedance

Unlike road transport, electricity cannot be instructed to follow a particular route.

Suppose two transmission lines connect the same locations.

If one line has lower electrical impedance than the other, a greater proportion of power naturally flows through it.

Operators cannot simply command individual electrons to use a different line because a contract says so.

Power flows are determined by:

Network topology.
Electrical impedance.
Voltage.
Generator outputs.
Consumer demand.

These physical relationships exist regardless of how electricity has been bought and sold.

A simple example

Imagine two generators.

Generator A is located in the north.

Generator B is located in the south.

A supplier signs a contract purchasing electricity only from Generator A.

Does that mean the supplier's customers receive electricity solely from Generator A?

No.

Consumers receive electricity from the entire interconnected system.

Generator A contributes to maintaining overall system balance.

Generator B also contributes.

The supplier's contract determines how financial settlement occurs.

It does not isolate physical electricity flows.

Why markets still matter

At this point it might seem that contracts are unimportant.

That would be the wrong conclusion.

Markets perform essential functions.

They determine:

Which generators are expected to operate.
Which consumers purchase electricity.
How investment is rewarded.
How operating costs are recovered.
How commercial risks are allocated.

Markets coordinate economic activity.

The physical system then implements those decisions within the limits imposed by engineering.

Markets therefore remain essential.

They simply do not replace physics.

Scheduling versus operation

Electricity systems involve two distinct processes.

Commercial scheduling

Market participants decide:

Who intends to generate.
Who intends to consume.
How much energy will be traded.
The agreed financial arrangements.

This produces a planned schedule.

Physical operation

System operators then determine:

Whether those plans are physically feasible.
Whether network limits are respected.
Whether frequency remains stable.
Whether voltages remain acceptable.
Whether sufficient reserves exist.

Physical operation therefore transforms commercial intentions into secure system operation.

The two activities are related but fundamentally different.

What happens when reality differs from the plan?

Suppose tomorrow's market schedule assumes:

Strong wind generation.
Moderate demand.
One gas station operating.

Tomorrow arrives.

Instead:

Wind speeds are much lower than forecast.
Demand is higher.
The gas station unexpectedly trips.

The market schedule has not changed.

The physical system has.

Operators must now:

Increase other generators.
Discharge batteries.
Increase imports.
Reduce flexible demand.
Activate reserve resources.

Commercial schedules provide a starting point.

Real-time operation deals with reality.

The role of the System Operator

The System Operator exists because commercial markets alone cannot ensure physical security.

The System Operator continually monitors:

Generation.
Demand.
Frequency.
Voltage.
Network loading.
Equipment availability.
Weather.
Reserve margins.

Using this information, operators issue instructions that maintain safe operation.

Their responsibility is not to maximise profit.

It is to ensure the electricity system remains secure.

Why balancing markets exist

Many electricity markets include balancing mechanisms.

These exist because commercial schedules prepared hours earlier will never perfectly match real conditions.

Balancing mechanisms allow the System Operator to:

Increase generation.
Reduce generation.
Increase demand.
Reduce demand.
Procure reserves.
Resolve unexpected imbalances.

Notice the sequence.

Markets produce plans.

Balancing corrects deviations from those plans.

The need for balancing arises because physical reality changes continuously.

Why location matters

Suppose a wind farm in northern Scotland produces more electricity than expected.

Consumers in southern England require additional electricity.

Can the extra generation simply be delivered?

Not necessarily.

The transmission network connecting these locations may already be close to its thermal limits.

Although sufficient generation exists nationally, the required transfer may not be physically possible.

Operators may therefore:

Redispatch generators.
Curtail wind generation.
Increase generation closer to demand.
Use storage.
Modify interconnector flows.

Again, the limitation comes from physics rather than contracts.

Why time matters

Electricity also follows physics through time.

Suppose a generator promises to produce:

100 MWh tomorrow.

Instead it generates:

20 MWh during the morning.
80 MWh overnight.

The total energy is correct.

The timing is not.

Consumers requiring electricity during the afternoon cannot use energy produced many hours later unless storage is available.

Commercial quantities may reconcile over a settlement period.

Physical balance must exist every second.

Markets operate inside physics

One of the most important ideas in modern electricity systems is this:

Engineering determines what is physically possible.

Markets determine which feasible solution is economically preferred.

Physics comes first.

Economics comes second.

Markets cannot create transmission capacity that does not exist.

They cannot eliminate thermal limits.

They cannot prevent frequency from falling after a generator trips.

They cannot force electricity to ignore electrical impedance.

Markets optimise within the feasible operating region defined by engineering.

The electricity system is a cyber-physical system

Modern electricity systems combine two worlds.

The physical world includes:

Generators.
Power lines.
Transformers.
Batteries.
Electric vehicles.
Consumers.

The digital world includes:

Markets.
Forecasts.
Prices.
Communications.
Control systems.
Settlement processes.

Neither world can operate independently.

Financial markets influence physical behaviour.

Physical conditions influence market outcomes.

Together they form what engineers call a cyber-physical system.

Understanding both is essential for designing effective electricity markets.

Why this matters for future electricity systems

Historically, electricity systems contained relatively few large generators.

Today they increasingly include millions of distributed resources.

These include:

Rooftop solar.
Home batteries.
Electric vehicles.
Heat pumps.
Flexible industrial loads.
Smart appliances.

Future electricity markets will therefore require increasingly close coordination between commercial decisions and physical operation.

Simply buying and selling electricity is no longer enough.

Markets must increasingly interact directly with the engineering realities of the network.

Bringing Module 2 together

This module has developed one central idea.

Electricity systems are governed by physical laws.

Markets exist to coordinate economic decisions within those physical limits.

Everything we study later in this course—including wholesale markets, balancing markets, flexibility markets, network pricing and retail tariffs—depends upon this distinction.

Whenever you encounter a proposed electricity market reform, ask two questions:

Does it make economic sense?
Is it physically feasible?

A successful electricity market must satisfy both.

Key takeaways

Contracts determine commercial obligations, not physical power flows.
Electricity naturally flows according to the physical properties of the interconnected network.
Markets create schedules and allocate financial responsibility.
System operators ensure those schedules remain physically feasible.
Thermal, voltage and stability constraints always take precedence over commercial intentions.
Electricity systems combine engineering, communications, control and economics into one cyber-physical system.
Markets operate within the feasible operating region defined by physics.
Understanding the distinction between commercial transactions and physical operation is fundamental to understanding electricity markets.

Module summary

You have now completed Module 2 – How Electricity Systems Physically Work.

You should now understand:

The basic electrical quantities used throughout power systems.
How electricity is generated and transported.
Why alternating current and frequency are fundamental to grid operation.
Why supply and demand must remain balanced continuously.
How transmission and distribution networks operate.
The engineering constraints that limit electricity systems.
Why electricity follows physics rather than contracts.

This physical understanding provides the foundation for the next module, where we examine how electricity markets have evolved to coordinate these engineering realities—and why many existing market designs struggle to do so effectively.