Glossary

Key terms for PJM capacity testing and the Capacity Test Optimizer. Definitions are based on PJM Manual 21B: Rules and Procedures for Determination of Generating Capability.

ICAP (Installed Capacity)

The net capability of a generating unit at the expected time of the PJM peak, determined by adjusting for generator site conditions (temperature, humidity, pressure) under summer or winter conditions. ICAP represents the rated capability your plant can reliably deliver.

UCAP (Unforced Capacity)

ICAP adjusted for forced outage rate. UCAP is what generators actually sell in the PJM capacity market. The relationship is: UCAP = ICAP × (1 − EFORd), where EFORd is the equivalent forced outage rate.

Accredited UCAP

The upper limit on the amount of Unforced Capacity that a resource can offer or provide in a Delivery Year. For thermal plants, this equals ICAP × ELCC Class Rating × ELCC Resource Performance Adjustment.

Net Capability

The megawatts of electric power that can be delivered by a generating unit, measured as gross output minus auxiliary and station loads. Net Capability is confirmed through seasonal capability verification testing and equals the rated ICAP.

Net Corrected Test Capability

The tested output of a generating unit, adjusted (corrected) from observed test conditions to rated conditions. This is what PJM uses to verify your claimed ICAP. The correction accounts for differences between test-day weather and the 15-year average peak conditions.

Rated conditions

The average generator site conditions (dry bulb, wet bulb, relative humidity, barometric pressure) coincident with the last 15 years of PJM summer or winter peaks at your plant location. Tests are corrected from observed conditions to these rated conditions to determine Net Corrected Test Capability.

Summer test period

June 1 through August 31. All capacity resources must complete a capability verification test during this window each year. Tests must be corrected to summer conditions—the average of the last 15 years' summer PJM peak conditions.

Winter test period

December 1 through February 28/29. All capacity resources must complete a capability verification test during this window each year. Tests must be corrected to winter conditions—the average of the last 15 years' winter PJM peak conditions.

Test duration

Per PJM Manual 21B: Simple cycle combustion turbines require 1 contiguous hour; combined cycle and steam units require 2 contiguous hours. The tested hours must be consecutive, and actual operating data can be used if available.

Eligibility

An hour is eligible for testing when observed conditions fall within the allowed bounds relative to rated conditions. For units with inlet cooling, tests should be within 10°F of rated wet bulb; without inlet cooling, within 20°F of rated dry bulb. If conditions are outside these bounds, testing is still allowed July 7–August 31 (summer) or January–February (winter) during permitted hours.

Correction factor

The adjustment applied to convert tested output at observed conditions to what the unit would produce at rated conditions. Higher correction factors during testing mean less favorable conditions—the optimizer recommends windows where conditions are closest to rated, minimizing corrections.

Multiplier

A percentage representing how favorable current conditions are relative to rated conditions. 100% means conditions match rated exactly (no correction needed). Higher multipliers indicate conditions where your unit will demonstrate more capacity.

Tier 1 vs Tier 2

Tier 1 uses formula-based correction for temperature and pressure per PJM guidelines. Tier 2 (Pro) uses plant-specific performance curves and correction equations to calculate corrected net power, providing more accurate predictions when curve data is available.

Test window

The consecutive hours used to compute a rolling average for test planning. For combined cycle units (2-hour test), we evaluate 2-hour windows; for simple cycle CTs (1-hour test), we evaluate single hours. All hours in the window must meet eligibility requirements.

Rolling average

The average of the multiplier across all hours in a test window. For a 2-hour combined cycle test, this is the average of both hours. Used to rank and select the best recommended window.

Best test window

The eligible consecutive-hour window in the 7–10 day outlook with the highest rolling average multiplier, recommended for scheduling your capacity test.

Dry bulb temperature

Ambient air temperature (°F) measured by a standard thermometer. Primary parameter for CT/CC units without inlet cooling. Rated dry bulb is the average at your site coincident with the last 15 PJM peaks.

Wet bulb temperature

Temperature (°F) accounting for evaporative cooling—always lower than dry bulb unless humidity is 100%. Primary parameter for units with inlet cooling (chillers, fogging, evaporative cooling). Rated wet bulb is the average at your site coincident with the last 15 PJM peaks.

Barometric pressure

Atmospheric pressure at your plant, typically corrected to plant elevation. Affects air density and turbine output. The optimizer uses historical pressure percentiles (p05–p95) to identify hours within the normal range for your location.

Pressure percentiles

Historical distribution of barometric pressure at your plant location. Hours with pressure between the 5th and 95th percentiles are considered within normal range. Extreme pressure days may be flagged as ineligible.

Combustion turbine (CT)

Simple cycle gas turbine generator. Test duration is 1 contiguous hour per PJM Manual 21B. Performance is primarily affected by ambient temperature and pressure. Units with inlet cooling use wet bulb as the primary correction parameter.

Combined cycle (CC)

Gas turbine(s) with heat recovery steam generator and steam turbine. Test duration is 2 contiguous hours per PJM Manual 21B. Both CT and steam components must be corrected—CT for ambient conditions, steam for cooling system conditions.

Inlet cooling

Equipment that cools compressor inlet air to increase turbine output—includes chillers, fogging, evaporative cooling, and wet compression. Units with inlet cooling must use wet bulb temperature for corrections and must test within 10°F of rated wet bulb.

Delivery Year

The 12-month period from June 1 through May 31 for which capacity is procured and delivered. Summer tests apply to the current Delivery Year; winter tests confirm capability for the winter portion of that year.

CIR (Capacity Interconnection Rights)

The MW amount a generator can inject into the transmission system. ICAP cannot exceed CIRs. CIRs are retained when summer tests meet or exceed the current CIR level at least once in a rolling 3-year period.

CAPMOD

Capacity Modification—the transaction used to increase or decrease a generator's ICAP in PJM's Capacity Exchange system. CAPMOD tests are required within 30 days when increasing ICAP.

ELCC (Effective Load Carrying Capability)

PJM's method for calculating capacity value based on how much a resource contributes to system reliability. Thermal plants receive ELCC Class Ratings based on their technology type (e.g., Gas Combined Cycle Class, Gas Combustion Turbine Class).

ELCC Class Rating

A percentage representing the capacity value of a resource class relative to a perfect resource with no outages. Applied to ICAP to determine Accredited UCAP. Ratings vary by technology—dual fuel classes typically have higher ratings than single-fuel classes.

What makes hours eligible?

Per PJM Manual 21B, eligibility depends on how close observed conditions are to your rated conditions. For units with inlet cooling, tests should be conducted when wet bulb is within 10°F of rated. For units without inlet cooling, dry bulb should be within 20°F of rated. Pressure is checked against historical percentiles (p05–p95) to flag unusually extreme days.

If conditions are outside these bounds, PJM allows testing July 7–Aug 31 (summer) or Jan–Feb (winter) during specified hours without strict temperature limits—but testing closer to rated conditions typically produces better results.