Definition: Unserved Energy (USE) is the quantity of electricity demand that cannot be met by the power system due to a supply shortfall — measured in megawatt hours (MWh) or gigawatt hours (GWh). It is the standard metric used in Australia to quantify the reliability of electricity supply, representing the energy that consumers would have consumed but did not receive because of insufficient generation capacity, transmission constraints, or network failure. In the context of energy infrastructure investment, USE is used to calculate the reliability benefit of a proposed project — that is, the economic value of blackouts avoided by building new capacity or network infrastructure.

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Why Does USE Matter?

Electricity supply reliability is fundamental to modern economies. When supply falls short of demand — whether due to generator outages, network failures, extreme weather, or insufficient capacity — the consequences for consumers can be severe:

• Households face disruption to heating, cooling, lighting, refrigeration, and communications
• Businesses experience lost production, spoiled inventory, equipment damage, and reputational harm
• Hospitals and critical infrastructure face risks to public safety and essential services
• Industrial consumers — particularly mining and manufacturing operations — can suffer significant economic losses from even brief supply interruptions

USE provides a way to quantify these consequences in economic terms — converting the physical reality of supply shortfall into a dollar value that can be incorporated into cost-benefit analysis.

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How is USE Measured?

USE is expressed as a volume of energy — typically in megawatt hours (MWh) or gigawatt hours (GWh) — that was demanded but not supplied over a given period. It is calculated as:

USE = Σ (Unmet Demand_t × Duration_t)

Where:

• Unmet Demand_t = the amount of load (in MW) that could not be served at time t
• Duration_t = the length of time (in hours) that the shortfall persisted

In practice, USE is estimated through sophisticated power system modelling that simulates thousands of scenarios — varying generator availability, transmission network conditions, demand levels, and weather — to produce a probabilistic estimate of the expected USE over a given period.

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USE and the Reliability Standard

In the National Electricity Market (NEM), the Australian Energy Market Commission (AEMC) sets a Reliability Standard — the maximum level of USE that is considered acceptable for the power system as a whole. The current NEM Reliability Standard is:

0.002% of annual energy consumption

In practical terms, this means the NEM is designed to deliver supply shortfalls of no more than approximately 0.002% of total annual electricity consumption — equivalent to roughly 130 gigawatt hours (GWh) per year across the entire NEM, or less than two hours of total blackout per consumer per decade.

When AEMO’s forecasts indicate that USE is likely to exceed the Reliability Standard in a particular region, it triggers a range of regulatory responses — including the requirement for new capacity investment, the activation of emergency reserves, or the procurement of demand response.

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The Value of Customer Reliability (VCR)

To incorporate USE into cost-benefit analysis, it must be converted from a physical quantity (MWh) into an economic value (dollars). This is done using the Value of Customer Reliability (VCR) — the dollar value that electricity consumers place on avoiding one MWh of unserved energy.

The VCR is estimated by the AER through periodic consumer surveys and research, which measure the economic harm experienced by different types of consumers during supply interruptions. The VCR varies by:

• Consumer type — residential, commercial, industrial, and critical infrastructure consumers all place different values on reliability
• Time of day and season — supply interruptions during peak summer demand are typically valued more highly than off-peak interruptions
• Duration — longer interruptions generally cause disproportionately greater economic harm than brief ones

The AER publishes a national VCR that is used consistently across regulatory determinations and RIT-T assessments. As of recent determinations, the VCR for the NEM is approximately $15,900 per MWh of unserved energy — reflecting the high economic cost that Australian consumers and businesses place on electricity supply reliability.

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USE in the RIT-T

In the Regulatory Investment Test for Transmission (RIT-T), USE is used to quantify the reliability benefit of a proposed transmission investment. If a new transmission line or substation would reduce the probability or magnitude of supply shortfalls in a particular region, the value of that reduction — calculated as the avoided USE multiplied by the VCR — is included as a benefit in the NPV calculation.

For example, if a transmission investment is expected to reduce annual USE in a region by 500 MWh, and the VCR is $15,900/MWh, the annual reliability benefit is:

500 MWh × $15,900/MWh = $7.95 million per year

Discounted over the life of the asset, this reliability benefit can represent a substantial component of the total NPV of a transmission investment — particularly in regions with limited existing network capacity or high exposure to generation outages.

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USE and North West Queensland

For the CopperString 2032 Project, USE is a particularly significant metric. The North West Minerals Province currently operates as an isolated electricity network — entirely disconnected from the NEM — and relies on diesel generation that is subject to supply chain disruptions, fuel price volatility, and mechanical failure.

In an isolated network with limited generation redundancy, the consequences of a major generator outage can be severe — leading to significant USE and major economic losses for mining operations that cannot function without reliable power. The reliability benefits of connecting North West Queensland to the NEM — through the diversity, redundancy, and scale of the interconnected grid — are therefore a potentially significant component of the CopperString RIT-T cost-benefit case.

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USE vs. Other Reliability Metrics

USE is one of several metrics used to describe electricity supply reliability in Australia. It is worth understanding how it relates to other commonly used measures:

Metric	What it measures	Unit
USE (Unserved Energy)	Energy demand not met due to supply shortfall	MWh or GWh
SAIDI	Average outage duration per customer	Minutes per customer per year
SAIFI	Average number of outages per customer	Outages per customer per year
CAIDI	Average duration of each outage	Minutes per outage
Loss of Load Probability (LOLP)	Probability that demand will exceed supply	% or probability

USE is most relevant for system-wide reliability assessment and cost-benefit analysis — where the total economic impact of supply shortfalls across all consumers needs to be quantified. SAIDI and SAIFI are more commonly used for distribution network performance assessment at the individual consumer level.

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Key Takeaway

Unserved Energy is the fundamental measure of what is at stake when electricity supply fails to meet demand. By converting the physical reality of blackouts into an economic value through the Value of Customer Reliability, USE provides the critical link between power system reliability modelling and the cost-benefit analysis that drives investment decisions in Australia’s energy infrastructure. For energy planners, network operators, regulators, and communities — particularly those in remote and isolated networks like North West Queensland — understanding USE and its implications is essential to making the case for reliable, well-connected electricity infrastructure.

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Published on energyplanning.com.au | Energy Planning Glossary