5 min. read
Last updated Sep 29, 2025
Key takeaways
Grid reliability is under pressure. AI, data centers, and electrification are driving demand while extreme weather and aging infrastructure add stress to the grid.
Electricity generators contribute to grid reliability in varying degrees. Grid operators use effective load carrying capacity (ELCC) to assign reliability values to each generator. Ratings for solar, wind, and storage are trending downward.
Electricity carbon metrics are evolving. Carbon Direct combines ELCC and life cycle assessments (LCAs) to create ELCC-adjusted carbon intensities (EACIs), a clearer view of the climate impact of electricity.
Reliability-adjusted EACIs reshape electricity emission comparisons. In the Pennsylvania-New Jersey-Maryland (PJM) grid (home to most US data centers), solar with storage, wind with storage, and natural gas with carbon capture and storage (CCS) show similar EACIs.
The bottom line: EACIs can help policymakers, utilities, and corporate buyers make informed decisions to build a more resilient, low-carbon grid for twenty-first-century demand.
AI, data centers, and the new urgency of grid reliability
The modern economy runs on electricity, and the numbers tell a striking story. In 2025, aggregated peak demand is forecast to continue to grow at an accelerated pace. Driving this surge are the rise of AI, data centers, and widespread electrification, all adding enormous new electricity loads on the grid just as extreme weather and aging infrastructure test its limits.
Electricity is unlike most commodities; it must be generated and consumed simultaneously. While batteries allow electricity to be stored for later use, they are limited by the number of hours of power they can store, reducing their ability to provide power for long durations.
Electricity generators don’t operate in isolation; they function as part of a tightly interconnected system where a single weak link can ripple across the grid. As stressors continue to mount, reliability is already slipping. That’s why reliability can no longer be treated as a side note in climate accounting for electricity generation. To meet rising demand without increasing emissions, we need a sharper understanding of how generators will perform when the grid is under strain.
Why reliability is reshaping electricity planning
Grid operators have adapted to this reality by assigning specific ELCC reliability values to each power generator, reflecting the amount of electricity it can generate when demand spikes.
As Steve Piper, Director of Energy Research at S&P Global Commodity Insights, explains:
“Every ISO has basically done two noteworthy things. One is that they’ve revised the reliability contributions, particularly of wind, solar, and storage. And those revisions have almost uniformly been lower. A megawatt equals significantly less than a megawatt in terms of reserve contribution. And they’ve also said pretty uniformly, we want a higher target reserve margin.”
These adjustments highlight significant trends by grid operators as part of broader efforts to ensure reliability:
Reliability contributions for solar, wind, and storage are trending downwards.
Operators are contracting additional generation to meet higher reserve requirements.
Non-generating grid resources, such as demand response, virtual power plants, and flexible loads, are being examined for their grid reliability.
This is more than a technical adjustment. It signals that we cannot only examine the carbon metrics associated with an electricity generator in isolation, but we must understand its role and carbon impacts on the broader grid. If we only measure emissions without factoring in reliability, we risk missing the full story of an electricity generator’s climate impact.
What is effective load carrying capacity (ELCC)?
ELCC is the metric grid operators use to capture the reliability value of each electricity resource. It reflects a generator’s ability to deliver electricity when the grid needs it most - usually during times of highest electricity demand, like a hot summer afternoon or a winter cold snap. ELCC is all about timing: if a generator produces a lot of electricity, but it’s not when electricity demand is high and other generators are maxed out, then that generator doesn’t help “carry” the grid load. The generator is then assigned a lower ELCC reliability value. Conversely, a generator that can provide power at the most critical times of the year helps to better carry the grid load, resulting in a higher ELCC reliability score. The metric itself is straightforward:
A generator with 100 MW of capacity and an ELCC of 5% effectively contributes just 5 MW of generation to the grid during the most critical hours of the year.
A generator with 100 MW of capacity and an ELCC of 85% effectively contributes 85 MW of reliable generation to the grid during the most critical hours of the year.
Grid operators, such as the Pennsylvania-New Jersey-Maryland Interconnection (PJM), Midcontinent Independent System Operator (MISO), and Southwest Power Pool (SPP), use ELCC to plan their systems and ensure there will be enough dependable generation to meet demand during even the most extreme hours of the year.
ELCC values vary across regions and technologies
ELCC is not one-size-fits-all. It varies depending on the technology, season, and region. These differences demonstrate why regional context matters: a megawatt of solar or wind energy in one location contributes significantly differently to reliability than a megawatt of solar energy in another. A few examples:
Electricity-generating technology | |||||
Solar | 10-14% | 7% | 62.1% (summer); 39.1% (winter) | 50% (summer, fall, spring); 5% (winter) | 5.5-27.2% (evening-afternoon; summer);
2.0-2.6% (evening-morning; winter) |
Wind | 38-62% | 12% | 15.4% (summer); 25.1% (winter) | 18.1% (summer); 15.6% (fall); 53.1% (winter); 18.0% (spring) | 8.3-33.6% (evening-afternoon; summer); 15.2-32.6% (winter) |
Natural Gas Combined Cycle | 78% | - | - | - | - |
Traditional carbon intensity of electricity generators
To understand why reliability matters for emissions, it helps to first look at how carbon intensity is traditionally measured.
A life cycle assessment (LCA) framework can include all emissions for electricity production, for example, in a ‘cradle-to-gate’ system boundary (including generator manufacturing, construction, fuel production, transport, and operations).
Emissions tallied in this way are used to calculate the carbon intensity (CI) of electricity (expressed in kg CO2e/MWh). For more on how CI is calculated, see our earlier blog: Electricity emissions accounting: GHG Protocol and LCA explained.
Figure 1. Cradle-to-gate boundary (dark green or dark + light green bars) for LCA of electricity generation. The light green bar represents a gate-to-gate LCA for electricity production (includes only power generation).
ELCC-adjusted carbon intensity (EACI) of electricity generators
Traditional carbon intensity calculations for a single electricity generator do not consider the role of the generator in the context of the broader power grid. For instance, if a solar generator has an ELCC of 5%, a grid operator would need to contract many multiples of its nameplate capacity and add energy storage to ensure a supply of generation that will be reliably available whenever the grid needs it most. The additional emissions from this “extra” capacity to meet reliability targets aren’t typically included in emissions calculations—but Carbon Direct believes it should be.
To address this gap, Carbon Direct combines the LCA framework with the ELCC metric to create the ELCC-adjusted carbon intensity (EACI) metric. EACI provides a clearer picture of a generator’s true climate impact once grid reliability is taken into account.
PJM case study: Demonstrating EACI’s climate impact
To see the difference EACI makes in understanding the climate impact of electricity generators, our electricity experts analyzed the PJM grid. We compared the lifetime climate impact of each generator type: coal, natural gas turbine (NGT), natural gas combined cycle (NGCC), solar and battery, natural gas combined cycle with carbon capture (NGCC+CCS), wind and battery, and nuclear. We then adjusted for the reliability contribution (ELCC) of each type of generator to calculate the (EACI).
The results (Figure 2) tell an important story:
Coal and natural gas turbines remain the highest-emitting resources, even adjusting for ELCC.
Solar PV with four-hour storage (solar and battery) and wind with four-hour storage (wind and battery) have EACIs with higher emissions than traditional (unadjusted) carbon intensities for these technologies.
As a result, solar and battery, and wind and battery have EACIs in the same range as NGCC and CCS.
Nuclear continues to stand out with the lowest EACI, underscoring its role as a highly reliable, low-carbon resource.
Figure 2. EACIs for different electricity-generating types in PJM: coal, natural gas turbine (NGT), natural gas combined cycle (NGCC), solar and battery, natural gas combined cycle with carbon capture and storage (NGCC and CCS), wind and battery, and nuclear.
The key takeaway is that the carbon intensity of electricity-generating technologies will change depending on the generator’s role in the broader grid. EACI provides grid planners and electricity buyers with a more holistic understanding of the climate impacts of different electricity-generation technologies.
Conclusion: From PJM to the future grid
The PJM case study makes one thing clear: traditional carbon intensity analyses of a single generator are not enough. This matters now more than ever. With demand surging from AI data centers and electrification, overlooking reliability risks is undercutting both climate goals and grid stability.
EACI offers decision-makers a more complete view of electricity’s climate impact. It helps policymakers, utilities, and corporate buyers:
Compare the climate impact of electricity resources on a reliability-adjusted basis.
Better communicate the benefits of different types of generation, including natural gas with carbon capture and storage.
Build portfolios that balance decarbonization and reliability.
The path forward is not about choosing a single “winner.” It’s about building a resilient, low-carbon grid that can meet twenty-first-century demand. EACI helps point the way.
Read more in our white paper, Meeting Data Center Electricity Demand: Mapping carbon capture potential for natural gas-fired generators in the US and Canada.
