Firm Power and Why It Matters

Firm Power and Why It Matters

LEARN / ENERGY & POWER

Firm Power and Why It Matters

A megawatt of nameplate capacity and a megawatt of accredited capacity are different quantities, and the gap between them is the difference between what a generator can produce and what a system can count on. There is no standard method for calculating the second, which is why the same plant is credited differently in different regions.

Key points

What firmness means

A system planner is not asking how much electricity a generator produces over a year. The question is how much of it can be relied upon during the hours when the system is most stressed, because that is when a shortfall becomes an outage. A resource is firm to the degree that its output in those hours can be scheduled and sustained.

The distinction is invisible in annual energy figures and decisive in planning. Two resources with identical annual output can contribute very differently to reliability if one produces during the evening peak and the other does not, and the accreditation process exists to put a number on that difference.

How accreditation works

Effective load carrying capability measures how much additional load a system can serve after adding a resource, while holding reliability constant at a defined target. PJM, for example, calibrates against a loss-of-load expectation of one day in ten years, a traditional resource-adequacy criterion widely used in North American planning. A resource that lets the system serve more load in the hours that matter earns a higher accreditation.

The method matters because resources interact. Solar, wind and storage complement one another, so a system holding all three is more reliable than the sum of the three evaluated separately would suggest. Accreditation has to account for that interaction, which is why simple capacity factors have been giving way to ELCC across planning regions.

DataELCC ≠ nameplate
ERCOT replaced historical average on-peak capacity factors with average effective load carrying capabilities for variable energy resources. NERC reports the resulting capacity derates largely offset the gains that load management programmes contributed to the anticipated reserve margin, and that solar ELCCs came in well below prior years. Source: NERC, report on effective load carrying capabilities, September 2025.

Series and provenanceretrieved August 9, 2026
Agency
North American Electric Reliability Corporation
Program
Resource adequacy and capacity accreditation
Series
2025 LTRA and the September 2025 accreditation report
Measure
Capacity accreditation and effective load carrying capability
Units
Accredited megawatts against nameplate
Adjustment
Method varies by planning area
Period
2025 assessment and workshop
Latest
ERCOT moved from on-peak capacity factors to average ELCC for variable resources; the resulting derates largely offset load management gains
Source tier
Primary
Retrieved
August 9, 2026

No standardised accreditation method exists across planning areas. NERC convened a 2025 workshop to identify what prevents standardisation. Accreditation varies with dominant resource types, resource diversity, regional load shapes, risk tolerance and seasonal against annual constructs, so one generator can hold different accredited capacity in two regions with neither figure being wrong.

Open the source

Measurement noteNo standard method exists. NERC assembled a workshop in 2025 to examine ELCC and other accreditation practices, identify what prevents standardisation, and look for common ground across planning areas. Accreditation varies with dominant resource types, resource diversity, regional load shapes, risk tolerance and whether the construct is seasonal or annual. The same generator can therefore hold different accredited capacity in two regions, and neither figure is wrong.

Institute analysisA change in accreditation method moves a reserve margin without a single megawatt being built or retired. ERCOT is the clearest recent case. Any comparison of reserve margins across regions, or across years within a region, has to establish that the accounting was the same, and most published comparisons do not.

What this means for a factory

An industrial facility running continuously must be served in every hour, which makes it a buyer of firmness rather than of energy. A power purchase agreement priced on annual output does not by itself secure availability during a winter morning when generation is scarce, and the difference between the two is where industrial energy risk actually sits.

The practical consequence is that a firm evaluating a site should be asking what the region’s planning reserve margin is, how the resources behind it are accredited, and whether that accreditation has recently changed. A margin that improved because the method changed is not the same as a margin that improved because capacity was added.

Common misconceptions

That a planning reserve margin is comparable across regions. Two systems reporting the same margin can carry different risk, because accreditation methods and forced outage assumptions differ.

That capacity factor and capacity value are the same. Capacity factor describes how much a unit produced. Capacity value describes what the system can count on in the hours that determine reliability.

That adding nameplate capacity adds reliability proportionally. Accreditation is not linear in nameplate, and interactive effects between resource types mean the contribution of an additional unit depends on what is already installed.

What the evidence says, and where it is contested

ContestedHow capacity should be accredited is genuinely unsettled and the stakes are financial as well as technical. One position holds that ELCC is the most accurate available representation of a resource’s reliability contribution and should be adopted broadly. A second holds that ELCC outputs are highly sensitive to modelling assumptions that are not transparent to the parties whose revenue depends on them, and that a method which cannot be replicated by an outside analyst is difficult to regulate. NERC’s own workshop framed standardisation as a barrier to be worked through rather than a settled question.

Related Institute research

What Is the Electric Grid?
Why the answer depends on which of three systems you are in.

Why Reindustrialization Requires More Electricity
Load growth, queues and the industrial siting constraint.

Sources

Reference entry maintained by the Institute for American Manufacturing & Technology. Figures are drawn from primary sources and cited above. Where the Institute states a position rather than a finding, it is marked as such.

Correction, August 31, 2026. An earlier version of this entry cited the 2025 Long-Term Reliability Assessment as the source for the capacity accreditation discussion, which caused the provenance record for the demand forecast to display here; the accreditation analysis is drawn from NERC’s September 2025 report on effective load carrying capabilities and the citation and record now reflect it. The same version described the one-day-in-ten-years criterion as carried in a NERC reliability standard; it is a traditional resource-adequacy planning criterion, and the text now says so.

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