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Source Capacity Utility

Generator Sizing Calculator

Generator Sizing Calculator supports engineering calculations with transparent assumptions, practical result interpretation, and links to next-step technical resources.

Generator Sizing Visualizer

Formula

Srun = Ptotal / PF

Stransient = Srun + (Smotor,start - Smotor,run)

Sbase = max(Srun × (1 + reserve), Stransient)

Sderated = Sbase / (Ftemp × Faltitude)

Sfinal = max(Sderated, Stransient / load-step-limit)

Preliminary screening model for generator frame selection. Final design should verify voltage dip, frequency recovery, alternator reactance, and site-specific standards.

Power Flow

Start Event Envelope

Enter valid load and motor startup data to render dynamic start-event envelope.

Inputs & Outputs

Total Running Load (kW)

Average Running PF (0-1 or 0-100)

Largest Motor Rated Power (kW)

Largest Motor PF (0-1 or 0-100)

Motor Start Multiplier (x run kVA)

Start Duration (s)

Reserve Margin (%)

Max Single Load Step (% of generator)

Ambient Temperature (C)

Site Altitude (m)

Candidate Generator Set (kVA, optional)

Running apparent power—

Start transient power—

Reserved running requirement—

Derated requirement—

Load-step requirement—

Final required capacity—

Recommended standard setOut of table range

Governing constraint—

Site derating factor—

Provide core load and motor inputs to estimate generator capacity.

Awaiting valid inputs

Generator Sizing Fundamentals

Generator capacity is controlled by more than just running kW. Startup transient magnitude, allowed load-step, and site derating can all dominate final kVA selection.

Steady vs Transient

Steady-state demand sets baseline loading.

Largest motor start often controls instantaneous source stress.

Environmental Derating

High ambient and altitude reduce practical deliverable capacity.

Ignoring derating can hide undersizing until commissioning.

Equation Reference

Core equations used to convert load assumptions into preliminary generator capacity requirements.

Topic	Equation	Meaning
Running apparent load	Srun = Ptotal / PF	Base continuous apparent load that the generator must support in steady operation.
Startup transient load	Stransient = Srun + (Smotor,start - Smotor,run)	Largest motor startup adds transient burden above normal running condition.
Derated requirement	Sderated = Sbase / (Ftemp × Faltitude)	Ambient temperature and altitude reduce practical generator output capability.
Load-step requirement	Sstep = Stransient / load-step-limit	If a generator accepts only limited single-step loading, larger frame size may be required.

Application Workflow Matrix

Use the calculator output to align source sizing with commissioning and reliability constraints.

Scenario	Objective	Recommendation	Critical Checks
Prime-power generator framing	Choose practical kVA frame before vendor RFQ	Use realistic running PF and largest motor startup multiplier, then apply reserve and site derating before selecting standard frame.	Voltage dip target, frequency recovery, alternator class
Standby source with motor starts	Avoid nuisance undervoltage trip during restart events	Model worst-case first-start transient and confirm load-step capability does not become the dominant sizing constraint.	ATS sequence, staggered starts, protective relays
High-altitude or high-ambient deployment	Prevent hidden undersizing from environmental derating	Treat thermal and altitude derating as explicit factors rather than post-hoc safety margin additions.	Site meteorology, enclosure ventilation, maintenance intervals