Reserve margin calculator
The reserve margin is the capacity a power system holds above its peak demand, as a share of that peak: (capacity - peak) / peak. The capacity margin divides by capacity instead.
Data as of October 14, 2026
MW
MW
%
Reserve margin
15.0%
Capacity above peak, as a share of peak
Capacity margin
13.0%
Capacity above peak, as a share of capacity
Meets the 15.0% target exactly
Method and data
Formula
Reserve margin = (C - P) / P Capacity margin = (C - P) / C Accredited capacity = sum of (MW_i × accreditation_i) + demand response + firm imports - firm exports Surplus or shortfall (MW) = C - P × (1 + target) C is capacity and P is peak demand, both in MW
Worked example
| Case | Installed (MW) | Accredited (MW) | Peak (MW) | Reserve margin | Capacity margin |
|---|---|---|---|---|---|
| All capacity firm (100%) | 115,000 | 115,000 | 100,000 | 15.0% | 13.0% |
| 30,000 MW of the capacity is solar at 7% | 115,000 | 87,100 | 100,000 | -12.9% | -14.8% |
| Technology | PJM class | Accreditation |
|---|---|---|
| Onshore wind | Onshore Wind | 41% |
| Offshore wind | Offshore Wind | 67% |
| Solar, fixed tilt | Fixed-Tilt Solar | 7% |
| Solar, tracking | Tracking Solar | 8% |
| Landfill gas | Landfill Intermittent | 48% |
| Run-of-river hydro | Hydro Intermittent | 39% |
| Storage, 4 hours | 4-hr Storage | 58% |
| Storage, 6 hours | 6-hr Storage | 67% |
| Storage, 8 hours | 8-hr Storage | 70% |
| Storage, 10 hours | 10-hr Storage | 78% |
| Nuclear | Nuclear | 95% |
| Coal | Coal | 83% |
| Gas combined cycle | Gas Combined Cycle | 74% |
| Gas turbine | Gas Combustion Turbine | 61% |
| Gas turbine, dual fuel | Gas Combustion Turbine Dual Fuel | 77% |
| Diesel | Diesel Utility | 92% |
| Steam | Steam | 72% |
| Waste to energy | Waste to Energy Steam | 83% |
| Oil-fired turbine | Oil-Fired Combustion Turbine | 80% |
Ratings are PJM's published figures for the 2027/2028 base residual auction. They apply to PJM's system and fleet and are examples, not values for other systems.
Assumptions
- Peak demand is the forecast coincident peak: the highest hour of the system as a whole, not the sum of each customer's own peak.
- Installed capacity is the sum of the resources' MW plus demand response and firm imports, less firm exports, because exports reduce the capacity left for the home system.
- Accredited capacity weights each resource by the share of its MW that counts towards reliability. Demand response, imports and exports are counted at 100%, so enter them as the MW that count.
- In the resource table the surplus or shortfall against the target is shown on both bases, accredited first. Compare the target with the basis it is set on.
- The accreditation presets are one publisher's class ratings for the delivery year shown in the table, offered as examples. Other systems use other methods, and a class rating is not a forecast for a single plant.
- No country benchmarks are shown, because published targets differ by system and by year. The calculator takes the target you enter.
PJM Interconnection ELCC Class Ratings for the 2027/2028 Base Residual Auction · Data as of October 14, 2026
Definition: Reserve Margin
Definition: Resource Adequacy
Definition: Effective Load Carrying Capability (ELCC)
Definition: Loss of Load Expectation (LOLE)
Tera Intelligence maps the plants, interconnectors and grids behind a system's capacity, so a reserve margin can be read against the assets that make it up. Explore the map
Questions
- What is the difference between reserve margin and capacity margin?
- Both measure the cushion of capacity above peak demand, but they divide by different things. The reserve margin divides by peak demand: (C - P) / P. The capacity margin divides by capacity: (C - P) / C. For 115,000 MW against a 100,000 MW peak, the reserve margin is 15.0% and the capacity margin is 13.0%. When the cushion is positive, the capacity margin is the smaller of the two. Check which one a target or a report uses before comparing figures.
- What is a typical reserve margin target?
- There is no universal figure. The planning authority of each system sets its own target, and it derives the target from a reliability standard instead of choosing a margin directly. A common standard is a loss-of-load expectation (LOLE) of 0.1 days per year, which means about one day of lost load in ten years. The reserve margin that meets such a standard depends on the system's mix of resources, its interconnections and the shape of its demand, so it differs between systems and changes from year to year. Use the target your planning authority publishes for your system and year.
- Why does accredited capacity differ from installed capacity?
- Installed capacity is nameplate MW. Accredited capacity counts only the share of each resource that can be relied on when the system is short, usually measured as effective load carrying capability (ELCC). That share depends on the technology and on the rest of the fleet. In PJM Interconnection's class ratings for the 2027/2028 delivery year, fixed-tilt solar counts for 7% of its installed MW, gas combined cycle for 74% and nuclear for 95%. In the worked example, replacing 30,000 MW of firm capacity with solar at 7% leaves 87,100 MW accredited and a reserve margin of -12.9%, against 15.0% on the installed basis. Other systems rate their resources differently, so enter the values your planning authority uses.
- How do imports count towards the reserve margin?
- A firm import counts as capacity only when a contract and transmission capacity will deliver it at the time of the peak; non-firm imports should be left out. Firm exports reduce the capacity available at home, so the calculator subtracts them. Imports raise the margin, but they also depend on the exporting system having a surplus in the same hour, so a system that relies on imports is exposed when its neighbours are short too.
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