Free Instant Generator kW & kVA Calculator — Size Your Generator from Load kW + Power Factor

This article explains how to size generators using kW, kVA, and power factor calculations accurately.

Practical formulas, tables, and worked examples support correct generator selection for industrial and commercial loads.

Generator kVA and kW Sizing from Load kW and Power Factor

Total active load (kW)

Average load power factor (dimensionless) Select power factor 0.80 (typical generator rating) 0.85 (typical mixed load) 0.90 (office / IT load) Custom value

Custom load power factor (dimensionless)

Advanced options

Desired generator load factor (% of rated kW)

Capacity margin for growth and transients (%)

Generator rated power factor (dimensionless)

Site altitude (m)

Ambient temperature (°C)

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Enter load kW and power factor to calculate the recommended generator rating in kVA and kW.

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Formulas used (all powers in SI units):

• Apparent load power: S_load (kVA) = P_load (kW) / PF_load
• Generator load factor: LF = load_factor_percent / 100
• Continuous generator active power before margins: P_cont (kW) = P_load / LF
• Capacity margin factor: M = 1 + (capacity_margin_percent / 100)
• Power with capacity margin: P_margin (kW) = P_cont × M
• Altitude derating fraction (above 1000 m): D_alt = max(0, altitude_m − 1000) × 0.0001
• Temperature derating fraction (above 25 °C): D_temp = max(0, ambient_temp_C − 25) × 0.001
• Total derating fraction (capped at 30 %): D_total = min(D_alt + D_temp, 0.3)
• Available fraction: F_der = 1 − D_total
• Required generator active power: P_gen (kW) = P_margin / F_der
• Generator apparent power rating: S_gen (kVA) = P_gen / PF_gen_rated

Load type	Typical power factor	Typical generator load factor	Typical capacity margin
Office / IT (UPS, servers, lighting)	0.9 – 1.0	70 – 80 %	15 – 25 %
Mixed commercial (HVAC, lighting, outlets)	0.8 – 0.9	70 – 85 %	20 – 30 %
Industrial with motors and welding	0.7 – 0.85	60 – 80 %	25 – 40 %
Residential backup	0.85 – 0.95	60 – 80 %	20 – 30 %

Technical frequently asked questions

Why is the generator rating in kVA higher than the connected kW?

The generator is rated in kVA because it must supply both active and reactive power. For loads with power factor below 1, the apparent power (kVA) is higher than the active power (kW), so the generator kVA rating must be increased accordingly.

What is the recommended load factor for continuous generator operation?

For most standby and prime power applications, a continuous load between 70 % and 85 % of the generator rated kW is recommended. This leaves margin for transients, improves reliability, and reduces thermal stress.

How do altitude and temperature affect generator sizing?

Higher altitude and ambient temperature reduce air density and cooling capability, which derates the generator. The calculator applies simple derating factors so that the selected generator rating still delivers the kW under site conditions.

Can I use this calculator for both single-phase and three-phase generators?

Yes. The kW and kVA relations are independent of the number of phases. However, three-phase industrial generators are typically rated at 0.8 power factor, while some single-phase sets may be rated at 1.0 power factor; you can adjust the generator rated power factor in the advanced options.

Fundamentals of kW, kVA, and power factor for generator sizing

Electric power for alternating-current systems is described by three related quantities: real power (kW), apparent power (kVA), and power factor (PF). Real power (kW) performs useful work and is the load designers specify. Apparent power (kVA) is the product of voltage and current without considering phase shift; generator ratings are commonly given in kVA. Power factor (PF) is the ratio of kW to kVA and indicates the phase relationship between voltage and current. Key relationships and direct implications:

• If PF < 1, the generator must supply higher current for the same real power, increasing thermal and electromagnetic stress.
• Sizing by kW alone can lead to undersized generators if PF is not included—always convert kW to kVA using PF.
• Generators are specified in kVA; after determining required kVA, select a generator whose continuous kVA rating exceeds the calculated requirement, with margins for starting currents and future expansion.

Essential formulas and variable explanations

Basic conversion between kW and kVA

kVA = kW ÷ PF

Three-phase real power

kW = √3 × V × I × PF ÷ 1000

Three-phase current from kW

I (A) = kW × 1000 ÷ (√3 × V × PF)

Single-phase current from kW

I (A) = kW × 1000 ÷ (V × PF)

Apparent power from current and voltage (three-phase)

kVA = √3 × V × I ÷ 1000

Variable explanations (typical example values):

• kW: real power in kilowatts. Typical loads: lighting 0.5–5 kW, motors 1–250+ kW, HVAC 10–500 kW.
• kVA: apparent power in kilovolt-amperes. Generator ratings commonly 5 kVA to 3000 kVA.
• PF: power factor (dimensionless). Typical PF values: resistive loads 1.0, commercial mixed loads 0.85–0.95, motor-dominant plants 0.7–0.9. Use 0.8 or 0.85 for conservative sizing if unknown.
• V: line-to-line voltage in volts. Common systems: 230 V single-phase, 400 V three-phase (Europe), 480 V three-phase (North America industrial).
• I: line current in amperes.
• √3: square root of three ≈ 1.732.

Step-by-step generator sizing procedure

1. Inventory loads: list every circuit or equipment item with rated kW or rated current and operating duty (continuous, intermittent, motor starting, peak).
2. Convert all loads to real power (kW) or to kVA using known PF. If only current is given, compute kW from voltage and PF using formulas above.
3. Aggregate loads with appropriate diversity and duty factors:
   • Apply continuous load definitions: most standards use 80% or 100% depending on application.
   • Motors: include starting currents (inrush) using service factor or starting kVA multipliers, then apply diversity if not all start simultaneously.
4. Calculate required kVA: use kVA = total kW ÷ assumed PF (choose a conservative PF, e.g., 0.8 for mixed industrial loads).
5. Determine current at generator terminals: I = kVA × 1000 ÷ (√3 × V) for three-phase. Confirm conductors, switchgear, and breaker ratings.
6. Select generator with continuous kVA rating ≥ calculated kVA plus margin (typically 10–25%), considering altitude, ambient temperature derating, and transient loads.
7. Verify synchronization, paralleling capability, and automatic transfer switch (ATS) sizing for emergency and standby configurations.

Extensive tables: common generator ratings and conversions

Notes on the table: currents are approximate and calculated as I = kVA × 1000 ÷ (√3 × V). Values are rounded.

Derivation of formulas and typical values used in practice

Derivation of three-phase formulas begins with apparent power S (in VA):

S (VA) = √3 × V × I

Real power P (W) is related as:

P (W) = S (VA) × PF

Converting to kilowatts and kilovolt-amperes:

kW = (√3 × V × I × PF) ÷ 1000 and kVA = (√3 × V × I) ÷ 1000.

Example typical values (used for conservative sizing):

• Use PF = 0.8 for motor-heavy or unknown loads.
• Design margin: add 10–25% to calculated kVA to account for starting currents and future growth.
• Altitude derating: for altitudes above 1000 m, apply manufacturer derating charts (e.g., reduce rated power by 1% per 100 m above 1000 m—check datasheet).
• Ambient temperature derating: for temperatures above 25–30°C consult generator manufacturer; typical derating 1% per 5°C above rated ambient.

Practical considerations: motor starting, harmonic loads, and transient behavior

• Motor starting can require 3–7× locked-rotor current; convert starting current to starting kVA using S = √3 × V × Istart. Momentary kVA must be supported by the generator without tripping.
• For motors with across-the-line starts, size generator to withstand motor inrush by either adding starting kVA or using reduced-voltage starters, soft starters, or VFDs.
• Nonlinear loads (VFDs, UPS, IT equipment) produce harmonics that increase heating in generator windings. Oversize by 15–30% or select generators with robust alternator designs and appropriate derating.
• Transient behavior: choose generator governors and AVR (automatic voltage regulator) specifications consistent with load sensitivity (voltage dip tolerance for sensitive electronics).

Worked example 1: Small industrial workshop with mixed resistive and motor loads

Project data

• Machine tools: 3 motors, each rated 15 kW, PF 0.85, starting type: direct-on-line (DOL).
• Lighting and receptacles: 10 kW, resistive, PF ~1.0, continuous.
• HVAC package unit: 25 kW nominal, PF 0.9, starting with one compressor rated 150% starting kW equivalent.
• System voltage: 400 V three-phase.
• Assume not all motors start simultaneously; apply 50% diversity on motor starting events.

Step 1 — Steady-state kW aggregation

Motors real power: 3 × 15 kW = 45 kW.

Lighting: 10 kW.

HVAC running: 25 kW.

Total steady-state real power: 45 + 10 + 25 = 80 kW.

Step 2 — Convert to kVA using an assumed PF

Choose conservative site PF = 0.85 (mixed motors and lighting). Use formula: kVA = kW ÷ PF.

kVA (steady) = 80 kW ÷ 0.85 = 94.12 kVA.

Step 3 — Include motor starting and transient loads

Motor starting: assume locked-rotor starting requires 4× running current per motor. Convert to starting kVA contribution. Running kVA per motor = 15 kW ÷ 0.85 = 17.65 kVA. Starting kVA per motor = 4 × 17.65 ≈ 70.6 kVA. For three motors, simultaneous starting would be 211.8 kVA.

Apply 50% diversity (not all start same time): effective starting kVA = 211.8 × 0.5 = 105.9 kVA.

HVAC compressor starting: 150% starting kW equivalent → additional 0.5 × running kW = 12.5 kW extra transient. Convert to kVA: 12.5 ÷ 0.9 ≈ 13.89 kVA.

Total transient addition: 105.9 + 13.89 ≈ 119.79 kVA.

Step 4 — Combine steady-state and concurrent starting allowance

Many sizing methodologies consider that starting is momentary; choose design approach: ensure generator can supply steady-state plus short-duration kVA.

Required kVA for continuous operation including probable starting: 94.12 kVA steadystate + 119.79 kVA starting = 213.91 kVA peak.

Step 5 — Select generator rating with margin

Apply safety margin (20%) to accommodate harmonics, ambient conditions, and future growth.

Minimum generator kVA = 213.91 × 1.20 = 256.69 kVA.

Select nearest standard generator: 275 kVA generator at 0.8 or 0.85 PF depending on manufacturer. Check alternator design for high transient kVA and provide appropriate breaker coordination.

Step 6 — Validate currents and switchgear

For selected 275 kVA generator at 400 V three-phase: calculate current using kVA formula:

I = kVA × 1000 ÷ (√3 × V)

I = 275 × 1000 ÷ (1.732 × 400) = 275000 ÷ 692.8 ≈ 396.9 A.

Choose switchgear and cabling rated for at least 400 A continuous with appropriate interrupting rating and mechanical clearance.

Worked example 2: Data center backup generator sizing for IT load

Project data

• IT load: 320 kW critical servers and infrastructure, PF measured at 0.95 (modern UPS and server loads).
• Cooling plant: four chillers, combined steady-state 200 kW, PF 0.9. Chiller compressors start with soft-starters; assume starting penalty equivalent to additional 20% for short duration.
• Redundancy strategy: N+1 generator set architecture desired; each generator must be sized to carry full critical IT load during maintenance of one unit for the desired configuration — design chooses two-parallel 400 kVA gensets for N+1? We'll calculate single-generator requirement for full-load operation.
• System voltage: 480 V three-phase.

Step 1 — Aggregate steady-state real power

IT steady-state: 320 kW.

Chillers steady-state: 200 kW.

Total steady-state real power = 520 kW.

Step 2 — Convert to kVA using weighted PF

We can compute composite PF by weighted sum or convert each portion separately to kVA and sum:

IT kVA = 320 kW ÷ 0.95 = 336.84 kVA.

Chillers kVA = 200 kW ÷ 0.9 = 222.22 kVA.

Total steady-state kVA = 559.06 kVA.

Step 3 — Consider starting and transient penalties

Chiller starting penalty: 20% of chiller kW for short duration → additional 40 kW → convert to kVA: 40 ÷ 0.9 = 44.44 kVA.

IT transient behavior: modern UPS provides ride-through; assume negligible additional kVA if UPS input sizing matches generator; however harmonics from UPS cause derating—apply 15% derating reserve for harmonics and dynamic loading.

Additional harmonic reserve: 559.06 × 0.15 = 83.86 kVA.

Total transient/penalty kVA = 44.44 + 83.86 = 128.30 kVA.

Step 4 — Required kVA and generator selection

Total required kVA = steady-state kVA 559.06 + transient reserve 128.30 = 687.36 kVA.

Apply a final selection margin of 10% for growth and environmental derating: 687.36 × 1.10 = 756.10 kVA.

For an N+1 arrangement with two identical gensets, each generator must support full IT load when one is out. Choose a practical deployment: two 800 kVA generators (each with 800 kVA continuous rating).

Step 5 — Validate currents and ATS/parallel controls

Calculate current per generator at 480 V three-phase:

I = kVA × 1000 ÷ (√3 × V)

I = 800 × 1000 ÷ (1.732 × 480) = 800000 ÷ 831.36 ≈ 962.7 A.

Design busbars, switchgear, breakers, and paralleling controls rated for at least 1000 A with appropriate short-circuit breaking capacity. Ensure ATS and paralleling system can handle load pickup and synchronization within data-center tolerances.

Standards, normative references, and authoritative resources

Designers should consult these standards and resources for mandatory or recommended practices and for detailed derating tables, test methods, and safety requirements:

• IEEE Std 446-1995 (IEEE Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications) — https://standards.ieee.org/
• NFPA 110 (Standard for Emergency and Standby Power Systems) — https://www.nfpa.org/
• National Electrical Code (NEC), NFPA 70 — https://www.nfpa.org/
• IEC 60034 (Rotating electrical machines) and IEC 8528 (Reciprocating internal combustion engine driven generating sets) — https://www.iso.org/ and https://www.iec.ch/
• Manufacturer datasheets and alternator sizing guides (e.g., Caterpillar, Cummins, Kohler) — use specific engine derating charts for altitude and temperature.
• IEEE Std 519 for harmonic control in power systems — https://standards.ieee.org/

Practical selection checklist and commissioning tips

1. Verify measured PF and harmonic content of actual loads during commissioning by power quality survey.
2. Select alternator with low reactance and high short-circuit capability for environments with frequent motor starting.
3. Specify automatic voltage regulation (AVR) and governor settings suitable for sensitive loads; consider digital controllers for precise transient response.
4. Include adequate fuel supply sizing and testing for continuous, standby, and prime power modes following manufacturer and code requirements.
5. Coordinate protection settings: overcurrent, differential, reverse power, and synchronizing relays for paralleling configurations.
6. Document load shedding strategy and ATS logic for staged load pickup when generator capacity is limited.
7. Plan for periodic load bank testing to validate generator capability under real loading and to detect fuel or cooling system issues.

Common pitfalls and mitigation strategies

• Underestimating starting currents — perform motor starting studies and include realistic diversity factors.
• Ignoring harmonics — obtain harmonic spectra from UPS and VFD manufacturers; specify oversizing or filters when necessary.
• Neglecting environmental derating — confirm altitude and temperature derating with manufacturer curves.
• Assuming PF = 1.0 — always use measured or conservative PF (0.8–0.95 depending on load mix).
• Failing to include future expansion — provide spare capacity or modular gensets for easy scaling.

Additional computational aids and verification

Quick validation formulas to keep on-hand:

Given total kW and PF, required kVA = kW ÷ PF.

Given kVA and V (three-phase), current I = kVA × 1000 ÷ (√3 × V).

Motor starting kVA estimate = running kVA × starting multiple (typical starting multiple 3–7 depending on motor size and starting method).

Suggested verification steps:

• Compute steady-state kVA and current to check conductor and breaker sizing.
• Compute worst-case transient kVA including simultaneous starts; verify generator transient capability.
• Run manufacturer-supplied transient response simulations if available, or request transient performance curves.
• Perform field load bank testing to verify generator performance under expected load profiles.

Summary of key equations for quick reference

• kVA = kW ÷ PF
• kW = √3 × V × I × PF ÷ 1000
• I (three-phase) = kW × 1000 ÷ (√3 × V × PF)
• kVA (three-phase) = √3 × V × I ÷ 1000

Final practical recommendations

• Always calculate generator size from consolidated kW including PF, harmonics, and starting transients.
• Prefer to size in kVA because generator capability is specified in kVA, not kW.
• Engage generator manufacturers early to obtain transient capability data, derating tables, and commissioning support.
• Document conservative assumptions (PF, diversity, margins) and keep records for future re-evaluation as loads change.

Accurate generator sizing requires a methodical approach: inventory loads, convert kW to kVA factoring PF, include starting and harmonic reserves, and select a generator with appropriate margins and manufacturer-validated transient capability. Follow applicable standards (IEEE, NFPA, IEC) and perform field testing to confirm performance under real conditions.

Useful links for further technical guidance

• IEEE Standards Association — https://standards.ieee.org/
• National Fire Protection Association (NFPA) — https://www.nfpa.org/
• International Electrotechnical Commission (IEC) — https://www.iec.ch/
• U.S. National Institute of Standards and Technology (NIST) — https://www.nist.gov/
• Major generator OEMs technical resources (e.g., Cummins, Caterpillar, Kohler)