A battery can carry a 100-megawatt rating and still count for less than 100 megawatts in a grid's reliability plan. That is not necessarily a defect, a disappointing efficiency result, or a sign that the project cannot reach full output. It reflects a different question: how much does this battery help keep the system supplied during the hours when electricity is hardest to deliver? The answer is often expressed through capacity credit and a modeling approach called effective load carrying capability, or ELCC . These concepts connect battery engineering to resource adequacy, procurement, and revenue. They explain why two physically identical batteries can have different reliability values on different grids, and why a project's accredited capacity can change even when its hardware has not. The short answer Nameplate megawatts describe a power rating. Megawatt-hours describe stored energy. Capacity credit describes the contribution recognized for meeting a grid's adequacy requirement. ELCC estimates reliability contribution within a particular system, not in isolation. A four-hour battery is not automatically a four-hour reliability solution for every future grid. Hero: AI-generated conceptual illustration of battery storage serving an evening grid. It is not a photograph of a specific project. Four numbers that should not be confused Start with a hypothetical 100 MW / 400 MWh storage plant. If the energy figure represents usable discharge energy at the same measurement boundary as the power rating, it has four hours of full-power duration. That specification alone does not reveal its capacity accreditation. Nor does it tell you the battery's annual generation, utilization, or trading profit. Different measurements answer different questions; this is a conceptual comparison, not a market rule. Measure Question answered What it does not establish Power, MW How fast can the plant deliver energy? How long it can sustain that output Energy, MWh How much usable energy is available? Whether it will be charged during a shortage Capacity credit, MW or percentage How much contribution is recognized for resource adequacy? An unconditional output guarantee in every event Round-trip efficiency, percentage How much charging energy returns on discharge? Reliability contribution during critical hours Capacity factor is different again: it compares actual energy production over time with continuous full-rated output. A rarely dispatched battery may be valuable for adequacy if it is available during scarce hours. Conversely, a battery that cycles frequently can still have limited reliability contribution if its energy is exhausted before a prolonged shortage ends. Readers comparing loss measurements can use our separate round-trip efficiency explainer . What ELCC actually measures The basic ELCC question is how much additional load a system can serve, at an unchanged reliability level, after adding a resource. A planning model compares a baseline portfolio with a portfolio containing the new resource. It considers the timing of demand and available supply, including uncertainty. The comparison translates a resource's contribution into a capacity quantity rather than simply counting its equipment rating. That conceptual definition should not be mistaken for a universal settlement formula. Operators implement accreditation differently. In its 2025 ELCC and Reserve Requirement Study , PJM describes an accreditation process based on the reduction in expected unserved energy from an incremental quantity of a resource class or unit, relative to the improvement from incremental perfect capacity. The comparison is made with the system calibrated to its loss-of-load expectation criterion. PJM's model simulates supply and demand chronologically, hour by hour, across multiple annual scenarios. Weather, resource availability, and energy constraints matter. That is a substantially richer exercise than dividing a battery's duration by an assumed peak window. Expected unserved energy measures the amount of energy a system may be unable to supply; loss-of-load expectation measures expected shortage occurrence. Neither should be casually translated into a promise that a particular customer will experience a specific outage schedule. A worked example, without a fake market forecast Assume, purely for illustration, that a market recognizes 70 MW of capacity from our 100 MW battery after applying its relevant accreditation method. Its recognized capacity is then 70% of that power rating. The battery may still discharge at 100 MW when charged and operating within its limits. The 70 MW figure is a reliability-accounting result, not a software command permanently limiting its inverter. Illustrative revenue arithmetic If a hypothetical contract paid $100 per recognized kW per year, 70 MW would correspond to 70,000 kW and $7 million per year in gross capacity payments. If recognized capacity later fell to 50 MW with the same price and contract structure, the equivalent figure would be $5 million. These are invented teaching assumptions, not actual PJM rates, a forecast, or project profit. Real contracts can fix quantities, allocate accreditation risk, impose availability obligations, or use entirely different payment structures. The arithmetic exposes an important financing risk: hardware life and commercial capacity life are not the same thing. A warranty may protect a battery's usable energy against excessive degradation without protecting its future accreditation. A developer therefore needs both an equipment-performance case and a market-rule case. One cannot substitute for the other. Why adding more batteries can lower the next battery's credit Imagine a grid with a narrow evening peak. The first batteries can discharge into the highest-demand hours and sharply reduce the amount of other capacity needed. As more batteries target those same hours, the remaining shortage risk can move into adjacent hours. The residual peak becomes flatter and wider. The next four-hour system must spread a finite amount of energy over a longer problem. The laboratory report Moving Beyond 4-Hour Li-Ion Batteries: Challenges and Opportunities for Long(er)-Duration Energy Storage , published by NREL in 2023, explains this mechanism explicitly. Its historical-data examples show storage clipping peaks and widening the remaining peak period. Those examples explain the physics and portfolio effect; they are not current accreditation schedules and should not be copied into a project's financial model as today's market rule. This is where average and marginal contribution matter. Average contribution asks about the fleet's contribution relative to its size. Marginal contribution asks what an additional increment adds at the current portfolio. When resources have diminishing incremental value, the next battery's contribution can be below the existing fleet's average. An analyst should identify which concept a published percentage represents before comparing it with another study. A declining marginal credit does not mean existing batteries have suddenly become useless. It means that repeating the same resource configuration may be less effective at solving the remaining reliability problem. It also does not prove that capacity prices must fall. Accreditation quantities, capacity-market prices, energy margins, and contract terms are separate variables. Solar can help, while winter changes the question The direction is not always downward. Solar can narrow a summer net-load peak by reducing daytime demand that must be supplied by other resources. That can make a short-duration battery more useful during the remaining evening window. The same NREL report shows why additional solar can offset some of the widening caused by battery deployment. Winter can present a different pattern. Long periods of high heating demand, little overnight solar output, and limited opportunities to recharge can make the critical interval lo