Capacity Accreditation

A 100 MW gas plant can deliver 100 MW when the grid needs it. A 100 MW solar farm cannot, because the sun may not be shining during the peak. Capacity accreditation is how grid planners assign reliability credit to different resources, and getting it wrong either overpays unreliable capacity or underpays reliable capacity.

The core concept. Accreditation measures how much a resource actually contributes during system stress events (the hours when blackouts are most likely). A gas plant with 95% availability gets accredited near its nameplate. Solar in PJM gets roughly 40-50% accreditation (it produces during summer afternoon peaks but not winter evening peaks). Wind varies from 10-30% depending on region and season. Batteries get high accreditation for their rated duration (4-hour batteries get near 100% for 4-hour stress events) but zero beyond that.

The policy stakes. In capacity markets, accreditation determines payments. If solar is accredited at 50%, a developer receives capacity payments on only half its nameplate. Overaccrediting solar means the grid looks more reliable on paper than it is in practice. Underaccrediting it discourages investment in a cost-effective resource.

As solar penetration grows, does its accreditation rise or fall?

It falls. The more solar on the grid, the less likely that peak stress occurs during sunny hours (solar pushes the net peak to evening). This is the "effective load carrying capability" (ELCC) concept: each additional MW of solar contributes less to reliability than the last. PJM's own ELCC analyses show solar's accreditation declining steeply as penetration grows.

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