The Wholesale Electricity Market
The Wholesale Electricity Market (WEM) was established in 2006 and is operated by the Australian Energy Market Operator (AEMO) for the South West Interconnected System (SWIS).
The Wholesale Electricity Market (WEM) supplies energy to the South West Interconnected System (SWIS), spanning from Kalbarri in the north, to Kalgoorlie in the east, and Albany in the South.
The WEM enables the supply and trade of wholesale electricity between retailers and generators across the SWIS and is governed by the Electricity System and Market Rules (ESM Rules). The ESM Rules guide how the SWIS and its electricity market operate and covers matters like wholesale electricity trading and dispatch, Reserve Capacity Mechanism, and Essential System Services.
The ESM Rules also include the State Electricity Objective which is to promote efficient investment in, and efficient operation and use of, electricity services for the long-term interests of consumers of electricity in relation to:
- the quality, safety, security and reliability of supply of electricity;
- the price of electricity; and
- the environment, including reducing greenhouse gas emissions.
We are a Rule Participant in the WEM in the class of Network Operator for our distribution and transmission systems. The functions required of a Network Operator are prescribed in the ESM Rules and include:
- Calculate and provide Loss Factors to AEMO;
- Provide Limit Advice to AEMO;
- Procure Non-Co-optimised Essential System Services
- Develop and publish a Transmission System Plan in conjunction with the Network Opportunity Map
- Develop WEM Procedures and WEM Procedure Changes, as required;
- Provide information and assistance to the Coordinator relating to the preparation of the Whole of System Plan by the Coordinator;
- Generator Performance Standards - Perform the functions in relation Transmission Connected Generating Systems electrically connected to the Network that the Network Operator operates
- Managing Market Information for which we are an Information Manager
- Any other functions conferred, and perform any other obligations imposed, on Network Operators under the ESM Rules
We are the Metering Data Agent of our SWIS Transmission and Distribution Networks and the Distribution System Operator of our Distribution Networks.
WEM procedure changes
If you would like to propose a change to any of Western Power’s published WEM Procedures or WEM Guidelines, please contact us.
If you’d like to receive updates on future Western Power WEM Procedure change proposals, please Subscribe for updates
Completed WEM Procedure change proposals are provided below:
Procedure change proposal documents
- Procedure Change Proposal
- Proposed amendment version Determining Loss Factors WEM Procedure – clean
- Proposed amendment version Determining Loss Factors WEM Procedure – marked up
- Procedure Change Report - Determining Loss Factors
- Determining Loss Factors WEM Procedure – final version
Submissions Received
- None
Procedure change proposal documents
- Procedure Change Proposal – NEPC_2025_02
- Proposed WEM Procedure: Technical Requirements for Standard Small User Facilities – Version 0.1 for public consultation
- Procedure Change Report – NEPC_2025_02
- WEM Procedure: Technical Requirements for Standard Small User Facilities – Version 1 to commence 01 May 2026 (final)
- WEM Procedure: Technical Requirements for Standard Small User Facilities – Version 1 to commence 01 May 2026 (changes marked)
Submissions
One submission was received during consultation, see below:
Non-Co-Optimised Essential System Services
A Non-Co-Optimised Essential System Services (NCESS) may be procured by Western Power when the Coordinator of Energy triggers an NCESS procurement process. To view a copy of the Coordinator’s determination to initiate an NCESS procurement, please visit the Coordinator Determinations. The following network support services are or have been sought under the NCESS framework by Western Power:
Underway - Refer to our Tenders and Registrations of Interest page
This NCESS procurement has concluded.
EOI documents
- Draft NCESS Service Specification – NSS Metropolitan Capacity Expansion
- NCESS EOI Submission form – NSS Metropolitan Capacity Expansion
Call for NCESS Submissions documents
- NCESS Final Service Specification – Metropolitan Capacity Expansion
- NCESS Submission form NSS Metropolitan Capacity Expansion
- NCESS - NSS draft standard contract
- Contract Departures template
Western Power’s NCESS Trigger Submission (October 2025)
Underway - Refer to our Tenders and Registrations of Interest page
Underway - Refer to our Tenders and Registrations of Interest page
This NCESS procurement has concluded.
Call for NCESS Submissions documents
- Final NCESS Service Specification – NSS Metropolitan Capacity Expansion.
- NCESS Submission form – NSS Metropolitan Capacity Expansion
- NCESS Call for Submissions - NSS Metropolitan Capacity Expansion Questions & Answers
EOI Documents
- Draft NCESS Service Specification – NSS Metropolitan Capacity Expansion
- NCESS Expression of Interest Submission form – NSS Metropolitan Capacity Expansion
- NCESS NSS EOI Metropolitan Capacity Expansion Questions & Answers
Western Power’s NCESS Trigger Submission (August 2024)
Underway - Refer to our Tenders and Registrations of Interest page
Loss Factors
Western Power calculates and provides to AEMO Loss Factors for each connection point in the network. These are determined in accordance with the ESM Rules and our WEM Procedure Determining Loss Factors.
AEMO publish the Transmission Loss Factors and Distribution Loss Factors provided by to them by Western Power and the time to which they will apply. Refer to the AEMO | WEM Loss Factors.
Limit Advice
Western Power creates and maintains Limit Advice which it provides to AEMO. These are determined in accordance with the ESM Rules and AEMO’s WEM Procedure Limit Advice Requirements and our WEM Procedure Developing Limit Advice.
The ESM Rules require AEMO to publish and maintain a Congestion Information Resource (CIR). It is intended to provide a consolidated source of information relevant to the understanding and management of network congestion in the South-West Interconnected System (SWIS). AEMO publishes Limit Advice on its website as part of the Congestion Information Resource.
Metering Data
Details on how we manage metering data under the Electricity Industry (Metering) Code 2012. Find out more.
We can also provide metering data to third parties if the end-use customer gives consent. Find out more
Market information
'Market Information' is defined as, “Any information or document that is required to be produced, provided or exchanged under these ESM Rules or a WEM Procedure.”
If you need Market Information not listed here, and for which we are the Information Manager, simply fill out the Market Information Request Form below and we'll get back to you promptly to discuss availability and disclosure details.
Please note:
- Western Power is only permitted to disclose Market Information in accordance with the ESM Rules (Chapter 10 Market Information) and the Confidentiality Classification of the requested information.
- Western Power may be required to consult relevant Information Stakeholders when reviewing the individual requests.
- Western Power may charge a fee for the preparation and disclosure of any information that is not already published or required to be disclosed.