A gas turbine generator should be sized against the electrical load it must sustain at the site’s limiting operating conditions—not simply against the largest nameplate rating available. For a continuously operating industrial plant, the governing question is whether the installation can deliver the required net power during hot weather, fuel-pressure constraints, maintenance outages, and credible equipment failures.

The selection process therefore needs several boundaries: normal demand, critical demand, allowable interruption, operating environment, and future expansion. A machine that covers the average load may leave production exposed during a disturbance. An oversized installation can create a different problem: prolonged part-load operation and unnecessary capital commitment. Gas turbine performance and economics depend on the operating point, ambient conditions, and application configuration.[1]

Define the load at the electrical delivery boundary

Start with measured demand rather than the sum of connected equipment ratings. Motors, heaters, compressors, and process auxiliaries rarely operate simultaneously at their individual maximum ratings. Conversely, monthly electricity bills can conceal short-duration events that matter to generator selection. Use interval demand records alongside motor-starting information and process operating sequences.

The load schedule should distinguish between:

• Continuous process demand: equipment that operates throughout normal production.
• Cyclic demand: loads associated with batch operations, refrigeration cycles, or intermittent compression.
• Essential demand: equipment that must remain energized after a generator or utility failure.
• Restart demand: the electrical sequence required to restore production after an interruption.

These categories answer different sizing questions. Continuous demand establishes the normal operating point. Essential demand defines the capacity needed under a selected contingency. Restart demand determines whether the surviving power system can recover the process without unacceptable voltage or frequency excursions.

Specify where power is measured. Generator-terminal output is not the same as power delivered to the plant bus. Package auxiliaries, fuel compression where required, inlet-system equipment, and electrical losses can reduce the export available to production. Keep existing plant loads separate from new generation-system auxiliaries so that neither is omitted or counted twice.

For a grid-connected plant, establish whether the turbine will follow site demand, operate at a steady output, or support an import limit. For an islanded plant, generation must also balance demand as it changes. Those operating modes require different control and transient-performance assessments.[1][4]

Convert reference ratings into guaranteed site output

Published ratings provide a comparison basis, but they are not automatically a site-capacity guarantee. ISO 3977-2 addresses standard reference conditions and ratings for gas turbine procurement. The enquiry should identify the reference basis and require performance at the actual installation conditions.[2]

Ambient temperature, elevation, inlet pressure losses, exhaust backpressure, and fuel characteristics can affect gas turbine output and heat rate. High inlet-air temperature commonly reduces available output; the magnitude is machine-specific. Heat-recovery equipment and exhaust treatment also belong in the performance calculation because their pressure losses influence the turbine’s operating conditions.[1]

Define a credible limiting case instead of combining unrelated worst values without explanation. A summer production peak may coincide with high inlet temperature. Minimum fuel supply pressure may occur under a different operating scenario. Ask the supplier to evaluate the combinations the plant could actually experience.

A useful preliminary relationship is:

Required reference gross output = (required net plant supply + generation auxiliaries and electrical losses) ÷ applicable site-output factor.

For an illustrative calculation only, assume that production requires 20 MW net, generation auxiliaries and losses total 1 MW, and the supplier’s preliminary site-output factor is 0.88. The reference gross requirement would be approximately 23.9 MW. These are hypothetical inputs, not representative product data. Final selection requires the supplier’s performance model, including any interactions that a single correction factor cannot capture.

Keep degradation separate from the clean-machine rating. Request a stated performance basis for the maintenance interval, including expected compressor fouling, cleaning assumptions, and recoverable versus nonrecoverable deterioration. Otherwise, a package sized with almost no initial margin may not retain sufficient output between maintenance interventions.[1]

Separate continuous capacity from disturbance capability

A steady-state megawatt balance does not demonstrate that the generator can start a large motor or accept a sudden process load. During a disturbance, the turbine, generator, excitation system, governor, and connected loads interact. The resulting voltage and frequency response requires a separate assessment.[4]

Prepare a disturbance schedule covering the largest motor start, the largest credible load addition, loss of utility import, and loss of an operating generator. Identify whether motors start directly, through soft starters, or through variable-frequency drives. Include transformers, significant nonlinear loads, and the voltage and frequency limits of sensitive process equipment.

The generator’s MVA capability also needs checking independently of turbine MW output. Reactive demand, power factor, ambient conditions, and the generator capability curve can constrain electrical delivery even when sufficient turbine shaft power is available. Request the relevant capability curves and the proposed operating limits rather than accepting a single power-factor statement.[4]

For isolated operation, require a dynamic study using the proposed turbine-generator and control models. The study should evaluate realistic load steps and protection settings, not assume that unused steady-state capacity is instantly available. If the process cannot tolerate the predicted response, possible remedies include staged motor starting, fast load shedding, energy storage, or a different generation arrangement. Each remedy needs its own response-time and integration assessment.

Choose the unit arrangement around the outage requirement

The correct unit count depends on what production must survive. “Continuous duty” describes operation; it does not, by itself, establish redundancy. A single unit capable of carrying the full plant load still creates a generation outage when that machine undergoes maintenance, unless another supply is available.

Test redundancy at site conditions. An N+1 arrangement should retain enough available capacity after the designated unit is unavailable; installed nameplate totals alone do not demonstrate this. If only essential loads must survive, document the shedding sequence and the production consequences.

Also examine common dependencies. Multiple turbines supplied by one vulnerable fuel system, connected through one critical switchboard, or dependent on one control-power source may remain exposed to a common failure. Unit count is only one part of the availability assessment.

Match the turbine to fuel supply and useful heat demand

Fuel supply can become a sizing constraint before generator capacity does. Obtain the available pressure range, composition envelope, heating value, and applicable contaminant limits. Require the turbine supplier to confirm acceptable operation across that envelope. Where gas compression is necessary, include its electrical demand and availability in the plant balance.[1]

Fuel flexibility should be evaluated as a defined operating capability, not a general description. If backup fuel is required, establish the permitted fuel specification, changeover procedure, storage requirements, and any output or emissions limitations. The same review should identify whether a fuel-supply interruption can disable all generating units.

When comparing gas turbine solutions, technical evaluators should distinguish the turbine-generator package from the complete installed system. Inlet filtration, fuel conditioning, starting equipment, exhaust arrangements, controls, and heat recovery can materially change the usable capacity and project scope. A package comparison is meaningful only when those boundaries are consistent.

Combined heat and power can influence the preferred size. Gas turbine exhaust can support useful thermal recovery, but the benefit depends on the plant’s simultaneous heat demand and required steam or process conditions. A continuously useful thermal load can support CHP economics; surplus heat without a practical use should not be credited as an operating saving.[1]

Check operation when heat demand falls or heat-recovery equipment is unavailable. The design may need an alternative exhaust path or another operating provision, subject to engineering and permitting review. Electrical continuity should not depend on an unstated assumption that every downstream thermal system remains available.

Compare operating economics across the load profile

Full-load efficiency alone is insufficient for a continuous industrial application. Compare expected fuel consumption at the operating points the plant will actually use, including reduced-production periods and contingency dispatch. Gas turbines commonly experience lower electrical efficiency at part load, although the relationship is design-specific.[1]

Request fuel consumption on a consistent higher- or lower-heating-value basis. Include fuel-compression electricity, inlet cooling consumption where proposed, consumables, scheduled maintenance, and replacement-power costs during outages. For CHP, credit only heat that displaces a defined alternative source.

Oversizing should have an explicit purpose. Separate allowances for confirmed expansion, forecast uncertainty, degradation, and contingency capacity rather than accumulating them into one unexplained margin. If expansion is uncertain, compare installing capacity now with reserving space, electrical interfaces, and fuel infrastructure for a later unit.

A lifecycle comparison should also state fuel and electricity price assumptions. The technically acceptable arrangement with the lowest initial equipment price may not have the lowest cost under the actual dispatch schedule. Conversely, additional efficiency may not repay a larger investment if the plant operates fewer hours or uses less recovered heat than assumed.

Turn the selection into verifiable procurement requirements

The purchase specification should make the capacity claim testable. State the net delivery boundary, ambient and fuel conditions, auxiliary loads, inlet and exhaust losses, operating mode, and permitted measurement uncertainty. Define how measured results will be corrected to the agreed guarantee conditions.

ASME PTC 22 provides a performance-test framework for gas turbines, but citing it does not replace agreement on the project’s test boundary and procedures. Identify which quantities will be measured, the required instruments, correction methods, and acceptance criteria before contract award.[3]

Request a separate demonstration plan for starting, synchronization, load changes, load rejection, protection, and island operation where applicable. A factory test of individual equipment cannot by itself demonstrate the behavior of the completed plant electrical system. Site testing and engineering studies should address the functions that depend on installed equipment and interfaces.[4]

Emissions, noise, fire protection, fuel handling, and grid interconnection require project-specific review. Applicable requirements depend on location, fuel, installation, and operating mode; equipment documentation alone should not be treated as confirmation of site compliance.

The final selection should therefore state more than a turbine rating: required net continuous output, limiting conditions, surviving capacity after the agreed outage, acceptable disturbance response, and the method of verification. If those conditions cannot be written clearly, the sizing decision is not yet ready for procurement.

Sources

• [1] U.S. Environmental Protection Agency, Catalog of CHP Technologies, Section 3: “Technology Characterization—Combustion Turbines.” Reference for performance influences, part-load efficiency, fuel requirements, heat recovery, and CHP economics.
• [2] ISO 3977-2, Gas turbines—Procurement—Part 2: Standard reference conditions and ratings.
• [3] ASME PTC 22, Gas Turbines. Performance test code.
• [4] IEEE Std 3002.2-2018, IEEE Recommended Practice for Conducting Load-Flow Studies and Analysis of Industrial and Commercial Power Systems; IEEE Std 3002.7-2018, IEEE Recommended Practice for Conducting Motor-Starting Studies and Analysis of Industrial and Commercial Power Systems.