This post is republished from the March installment of our Capacity Factor newsletter. Subscribe here.
Mid-Atlantic grid operator PJM recently published 2025-26 estimates of “effective load carrying capability” (ELCC) that show gas plants carrying more load (a measure of electricity demand), on average, than any of the four battery storage options during peak energy demand events. This statistic, which starkly contradicts PJM’s 2024-25 estimates, is huge: PJM might use it to argue for building more gas capacity. But why?
Utility planners calculate the ELCC by simulating grid disturbances and seeing how often a resource can meet the resulting load. If batteries provide an average of 80 MW across multiple 100 MW load event simulations, their ELCC is 80%.
ELCC is a dynamic measurement affected by technology, the weather, grid infrastructure, and the existing resource mix. In other words, it changes as the grid and load change. Four-hour batteries might meet load on a full charge for a couple peaking hours. As more batteries are added, the ELCC of each marginal battery will drop because it will be used to meet increasingly non-peak portions of load–which likely last longer than the battery’s charge.
PJM’s gas turbines don’t have this problem because, unlike batteries, they don’t need to charge. That doesn’t mean we shouldn’t build batteries and renewables. In fact, if we add them alongside additional wind and solar, battery ELCCs will increase because we’ll have added charging capacity; emissions will fall. Resource diversity is good for ELCCs!
This is not to say PJM’s ELCC estimates adequately reflect storage’s capabilities or changes in its grid over the past year. What it could mean is that their managers are increasingly spooked by reliability tail risks absent additional clean firm resources like longer-duration batteries. Or they feel renewables aren’t increasing fast enough to both charge batteries and meet load. Both are solvable industrial policy problems!
A version of the diminishing ELCC effect is shown below for solar. Marginal solar additions shift peak events into nighttime hours, where solar is increasingly unable to meet load.
But if you add both solar and 4-hour batteries together, they cover more peak load than they could alone.