Effective Load Carrying Capability
Definition
Effective load carrying capability (ELCC) is a measure of how much a resource reduces the risk of supply shortfalls, expressed as an equivalent amount of perfectly reliable capacity. Grid operators use it to accredit resources for capacity markets: a resource with a 50% ELCC rating can sell about half its capacity. PJM publishes ELCC class ratings for each delivery year and applies them in its capacity auction, with resource-specific analysis where a class is too varied for one rating.
Why it matters
Accreditation converts megawatts into capacity revenue. A change in a class rating changes a project's capacity income even if nothing about the project changes. Investors in storage or solar projects that count on capacity revenue should check the rating assumed, whether it is the current published value, and how sensitive returns are to future revisions.
Common misconceptions
- •ELCC is not an efficiency or availability figure; it measures contribution to reliability at system level.
- •A rating is not fixed for a project's life; it is set each year and depends on the rest of the system.
- •Adding more of the same resource type tends to lower its marginal contribution.
- •The label is only a starting point. Investor outcomes usually depend on the governing documents, cash-flow priority, valuation policy, disclosure quality, and manager discretion surrounding the term, not just the shorthand definition used in marketing materials.
Technical details
PJM ELCC class ratings
4-hr storage: 50% (2026/2027), 58% (2027/2028).
10-hr storage: 72%, 78%.
Fixed-tilt solar: 8%, 7%. Tracking solar: 11%, 8%.
Onshore wind: 41%, 41%. Offshore wind: 69%, 67%.
Nuclear: 95%, 95%. Gas combined cycle: 74%, 74%.
How it enters revenue
Accredited capacity is roughly installed capacity times the rating, subject to resource-specific adjustments.
Capacity revenue = accredited MW x clearing price x days in the delivery year.
How investors apply the term
Investors usually apply this term by tying the definition back to the controlling documents and then asking which cash flows, rights, restrictions, or valuation assumptions it changes. A useful review looks at the term alongside fees, leverage, liquidity, reporting frequency, and manager discretion rather than treating it as a standalone concept.
The practical diligence test is simple: identify who benefits if the assumption is wrong, who has control when conditions change, and what evidence the investor receives after closing. Terms that cannot be reconciled to documents, cash movements, and monitoring data should be treated as marketing shorthand until proven otherwise.
Frequently asked questions
- Why does a battery's accredited capacity differ from its nameplate?
- Because a battery can only discharge for as long as its energy lasts, and shortfalls can run longer than that. PJM rated four-hour storage at 50% for 2026/2027, six-hour at 58%, eight-hour at 62% and ten-hour at 72%: longer duration, higher rating.
- Why is solar's rating so low?
- Because ELCC counts a resource's contribution in the hours of greatest shortfall risk, and solar produces little in many of those hours. PJM rated fixed-tilt solar at 8% and tracking solar at 11% for 2026/2027, and 7% and 8% for 2027/2028.
- Do ratings change?
- Yes, every year and sometimes sharply. Between PJM's 2026/2027 and 2027/2028 ratings, four-hour storage rose from 50% to 58% and demand resources from 69% to 92%. PJM implemented marginal ELCC accreditation for all generation and demand resources from the 2025/2026 delivery year, and its auction report notes the change makes year-over-year comparison difficult. FERC's 2025 report charts class accreditation values across CAISO, SPP, MISO and PJM.