supagridThe National Electricity Market (NEM) is the wholesale market where electricity is traded across eastern and southern Australia. It connects generators, transmission networks, and consumers into a single, continuously operating system that balances electricity supply and demand in real time.
Unlike many commodity markets, electricity cannot be stored or deferred at scale. The NEM exists to solve a live coordination problem: ensuring enough electricity is produced, delivered, and consumed at every moment while keeping the power system stable and secure.
What the NEM Does
At its core, the NEM performs several critical functions simultaneously. These functions are tightly linked and continuously re-optimised as conditions change.
The NEM is responsible for:
- Determining which generators should produce electricity at any moment
- Establishing the wholesale electricity price in each region
- Ensuring the power system operates within technical and security limits
Dispatch decisions affect prices, and both are constrained by the physical capabilities of the network.
Electricity must be produced at the exact moment it is consumed. Supply and demand must match in real time.
Who Runs the Market
The NEM is operated by the Australian Energy Market Operator (AEMO). AEMO's role is to operate the market and the power system safely, not to participate as a buyer or seller of electricity.
AEMO is responsible for:
- Dispatching generators every five minutes
- Maintaining system security and reliability
- Managing planned and unplanned outages
- Responding to system disturbances
- Publishing market and operational data
While AEMO enforces market rules and constraints, it does not manually set prices.
How Electricity Is Traded
Electricity in the NEM is traded through a bidding and dispatch process that runs continuously throughout the day.
Generators submit bids that specify:
- How much electricity they are willing to supply
- The price at which they are willing to supply it
Every five minutes, AEMO evaluates all bids and determines the lowest-cost combination of generators that can meet forecast demand while respecting system and network constraints.
The most expensive generator required to meet demand sets the market price for that interval.
All dispatched generators in a region receive the same regional price for a given interval, regardless of their individual bid prices.
The 5-Minute Market
The NEM operates on a five-minute dispatch cycle, making it highly dynamic and responsive to changing conditions.
Every dispatch interval involves:
- Updated demand forecasts
- Changes in generator availability
- Revised bid stacks
- Current network conditions and constraints
As a result, prices can move rapidly, particularly during periods of high demand, renewable variability, or network congestion.
Small changes in demand, availability, or network conditions can cause large price movements.
Regions and Prices
The NEM is divided into geographic regions that broadly align with the structure of the transmission network. Each region has its own supply mix, demand profile, and regional reference price.
Regions are linked by interconnectors that allow electricity to flow between them. However, these links have physical limits and are often constrained.
When interconnectors are constrained:
- Electricity cannot freely flow between regions
- Regional prices can diverge significantly
- Local supply and demand conditions dominate outcomes
Physical System vs Market
The NEM combines a financial market with a physical power system. While market outcomes are driven by bids and prices, all decisions must respect the laws of physics.
Dispatch outcomes are constrained by:
- Transmission limits
- Voltage and system strength requirements
- Contingency and reliability standards
This is why the cheapest generator is not always dispatched.
The market cannot dispatch electricity that the network cannot physically deliver.
Why the NEM Is Complex
The NEM's complexity arises from operating a real-time market across a large geographic area with diverse generation technologies.
Key sources of complexity include:
- Long transmission distances
- Network congestion and outages
- Variable renewable generation
- Rapidly changing demand patterns
Understanding the NEM requires thinking in both market and power system terms at the same time.
What to Take Away
At a high level, the NEM can be summarised by a few core ideas:
- Supply and demand are balanced every five minutes
- Prices are a result of dispatch decisions
- Physical constraints shape market outcomes
- Volatility is an expected feature of the system
This foundation makes it easier to understand the more detailed concepts covered in the sections that follow.
Continue with Regions & Interconnectors to explore how the NEM is structured geographically.