---
title: "Generator Sizing Calculator: kVA, Motor Starting and Fuel to AS ISO 8528"
url: "https://scaleset.com.au/calculator/generator-sizing"
description: "Free generator sizing calculator to AS ISO 8528: size diesel gensets for prime, standby or continuous duty with motor starting kVA and fuel tank sizing."
updated: "2026-10-13"
standards:
  - "AS ISO 8528.1"
  - "AS ISO 8528.5"
  - "AS/NZS 3000:2018"
  - "AS/NZS 3010:2017"
  - "AS 60038"
author: "Wisam Tozah (B.Eng (Electrical), MIEAust, CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE(Aus))"
site: "ScaleSet"
license: "https://scaleset.com.au/terms"
usage: "Cite and link. Do not reproduce or train on this content."
---

# Generator Sizing Calculator: kVA, Motor Starting and Fuel to AS ISO 8528

Source: https://scaleset.com.au/calculator/generator-sizing

Size a diesel or gas generator from connected load, motor starting scenarios and duty class: recommended kVA from the standard alternator ladder, plus fuel tank volume for any required autonomy.

## Key facts

- The calculator sizes a set against two conditions (the steady running load and the worst-case motor start), and the larger nameplate requirement governs.
- Steady target = the larger of running kVA and running kW ÷ 0.8, divided by the max-loading percentage for the duty class: 70% for Prime (PRP), 80% for Standby (ESP), 100% for Continuous (COP), each overridable between 30 and 100%.
- A set is rated in kVA at 0.8 power factor lagging (AS ISO 8528.1), so its engine delivers 0.8 × kVA in kW. Above 0.8 power factor the engine governs, not the alternator: 400 kW at 0.95 is 421 kVA, but a 421 kVA set is only 337 kW, so it needs 500 kVA.
- Start target = worst-case starting kVA divided by the set's short-time starting capability, taken as 2.5× nameplate by default (adjustable 1-4×) in the transient method, or by the max-loading percentage in the conservative method.
- A direct-on-line motor is taken at 7.5× full-load current, star-delta 2.5×, soft starter 3.5× and VFD/VSD 1.2×, so reduced-voltage starting typically cuts the start spike by 50-85%.
- Non-linear load (VFDs, UPS rectifiers, LED and IT equipment) is not in the sizing: its harmonic current distorts the set's voltage, and the alternator is oversized for it by an amount set by the drive or rectifier type (6-pulse or active front end), confirmed with the set supplier.
- Enter the true power factor, harmonics included, as a power quality meter reads it. A drive's manual quotes cos φ (displacement power factor), often 0.95 or more; its true power factor is lower, and the true figure is what the set carries. Name the non-linear share of the load and the result flags it as a share of the set for the set supplier to check.
- The result is rounded up to the next standard alternator size on a 10 to 3000 kVA ladder, with warnings when the target exceeds the largest standard set or when steady loading falls below 30% (wet stacking).
- Diesel fuel burn tracks real power, not apparent power: the auto-estimate is about 0.27 litres per kWh of running load, and tank volume = consumption × autonomy × (1 + reserve).

## Who this page is for

Electrical engineers, designers, contractors and facilities managers specifying standby, prime or continuous diesel and gas generator sets for Australian and New Zealand installations.

## Relevant standards

- AS ISO 8528.1
- AS ISO 8528.5
- AS/NZS 3000:2018
- AS/NZS 3010:2017
- AS 60038

## What this tool does

- Choose Prime (PRP), Standby (ESP) or Continuous (COP) duty to AS ISO 8528.1 with max-loading defaults of 70 / 80 / 100%: override per project anywhere from 30 to 100%.
- Running kVA is built from the connected load in kW at the nominated power factor, plus every motor running at rated load, and the set is held to its engine limit too: rated at 0.8 power factor (AS ISO 8528.1), so above 0.8 the set is sized on kW ÷ 0.8.
- Add any number of motors with rated kW, quantity and start method (DOL 7.5×, Star-Delta 2.5×, Soft Starter 3.5×, VFD/VSD 1.2× full-load current); every motor is tested as the one starting while the rest run, and the worst scenario governs.
- Two sizing methods: transient (default) fits the start spike inside the set's short-time capability (an adjustable 1 to 4× nameplate, 2.5× by default), while conservative treats the spike as a steady load and gives it the same max-loading headroom.
- Recommended nameplate kVA selected from the standard alternator ladder (10 to 3000 kVA) with running current for 1Ø 230 V or 3Ø 415 V, a warning when the target exceeds the largest standard set, and a wet-stacking warning below 30% loading.
- Size the fuel tank for any autonomy from manufacturer consumption data, or from an auto-estimate of about 0.27 litres per kWh of running real power, plus a reserve allowance.
- Export a branded PDF report with project details, load breakdown, motor schedule and fuel tank sizing: ready to drop into a design submission.

## How to size a generator with the ScaleSet Generator Sizing Calculator

1. **Set the system and duty class** — Choose 3-phase (415 V) or 1-phase (230 V), then the duty class: Prime (PRP), Standby (ESP) or Continuous (COP). The duty sets the max-loading default (70 / 80 / 100%), which you can change to any value from 30 to 100% for the project.
2. **Enter the connected load and power factor** — Enter the running load in kW, excluding any motors you list under Motor Starting, and the installation average power factor (typically 0.80 to 0.95). Run the AS/NZS 3000 maximum-demand assessment first: this calculator sizes the set from the figure you give it and does not apply diversity itself.
3. **Add the motors and their starting methods** — Enable Motor Starting and add each motor with a tag, its electrical input kW (shaft kW ÷ efficiency), quantity and start method (DOL, star-delta, soft starter or VFD/VSD). Each motor is added to the running load at full-load kVA and evaluated as the one starting while everything else runs.
4. **Choose the sizing method** — Keep the transient method to fit the start spike inside the set's short-time capability (adjust the starting capability factor from its 2.5× default to match the alternator datasheet), or switch to conservative to give the start spike the same max-loading headroom as the steady load.
5. **Read the recommended set and the breakdown** — Review the recommended standard kVA, the running current, the loading percentage and the sizing breakdown showing the steady target, the start target and which condition governs. Check the per-motor scenario table to see which motor drives the result, and act on any over-capacity or low-loading warning.
6. **Size the fuel tank** — Enable Fuel Tank Sizing, enter the required autonomy in hours and a reserve percentage, and either enter the manufacturer consumption in L/hr or leave it blank to use the estimate from the running real power. The required tank volume is returned in litres.
7. **Verify against the manufacturer data and export** — Confirm the transient voltage and frequency dip against AS ISO 8528.5 and the alternator curves, apply any altitude and temperature derating, then export the branded PDF report with the load breakdown, motor schedule and fuel tank sizing.

## Generator sizing for Australian standby and prime power installations

### What the generator sizing calculator actually does

The ScaleSet Generator Sizing Calculator takes a connected load in kW at a nominated power factor, an optional schedule of motors with their starting methods, and a duty class, and returns the nameplate kVA of the smallest standard set that can carry the installation. The connected load is converted to running kVA as kW divided by power factor, every motor is added at its own full-load kVA, the engine's kW limit is checked beside the alternator's kVA, and the recommended set is picked off the standard ladder of 10, 15, 20, 25, 30, 40, 50, 60, 80, 100, 125, 150, 175, 200, 250, 300, 350, 400, 500, 600, 750, 900, 1000, 1250, 1500, 1750, 2000, 2500 and 3000 kVA.

Two conditions are tested. The first is the steady running condition: the running kVA has to sit inside the max-loading headroom for the chosen duty. The second is the worst-case motor start: the largest transient the installation can produce has to fit inside what the set can supply for a few seconds. Each condition produces its own nameplate requirement, the larger one governs, and the result is rounded up to the next standard size. Alongside the kVA the calculator reports the running current at 1-phase 230 V or 3-phase 415 V, the resulting loading percentage, the per-motor starting scenarios, and (when enabled) the fuel tank volume for a required autonomy.

### Alternator kVA and engine kW: the 0.8 power factor rating

A generating set has two limits. The alternator carries the load's apparent power in kVA; the engine delivers its real power in kW. Sets are rated in kVA at 0.8 power factor lagging (AS ISO 8528.1), so the engine behind a 500 kVA set delivers 400 kWe. At a load power factor of 0.8 or below the load's kVA is the larger ask and the alternator governs. Above 0.8 the engine governs: 400 kW at 0.95 power factor is 421 kVA, but a 421 kVA set is only 337 kWe.

The calculator therefore takes the steady target as the larger of the alternator requirement (running kVA) and the engine requirement (running kW ÷ 0.8), each over the duty's max-loading percentage, and the breakdown and the PDF state which one governs. A load at unity power factor still needs kW ÷ 0.8 of nameplate. The loading percentage is reported on the limit that governs.

### Duty class and the max-loading headroom

AS ISO 8528.1 (the Australian adoption of ISO 8528-1) defines the rating classes that describe how a set may be operated over its life. Emergency Standby Power (ESP) supplies a variable load for the duration of a utility outage, up to about 200 hours a year, with no overload capability. Prime Power (PRP) allows unlimited hours on a variable load, but the average load must not exceed 70% of the rating over any 24-hour period, with a 10% overload permitted for 1 hour in every 12. Continuous Operating Power (COP) allows unlimited hours at a constant 100% load with no overload. The same physical machine carries a different nameplate under each class: typically standby is around 1.10× prime and continuous around 0.90× prime, though the ratio is not universal.

The calculator turns the duty selection into a max-loading percentage that the steady load is divided by: 70% for Prime, 80% for Standby and 100% for Continuous. These are design-headroom defaults rather than clauses: diesel generator manufacturers recommend sustained operation roughly between 30% and 80% of nameplate, below which wet stacking becomes a problem and above which there is little thermal margin left for transients. The percentage is editable per project anywhere between 30 and 100%, and the calculator states which value it used and whether it was the default.

### How the worst-case motor start is found

Each motor is entered with its electrical input in kW (the nameplate shaft output divided by the motor efficiency), a quantity and a starting method. The calculator converts the motor to full-load kVA at the site power factor and multiplies by the start-current factor for its method: direct-on-line 7.5×, star-delta 2.5×, soft starter 3.5× and VFD/VSD 1.2×. These are multiples of full-load current, which is why they are applied to the full-load kVA rather than to the kW figure directly. A 22 kW motor at 0.8 power factor is 27.5 kVA at full load, so direct-on-line it presents roughly 206 kVA while it accelerates and star-delta about 69 kVA.

Rather than assume the biggest machine is the worst case, the calculator builds one scenario per motor: that motor starting while the base connected load, every other motor and the remaining units of its own type all run at rated load. The scenario with the highest total kVA is the one that sizes the set, which matters when a smaller direct-on-line motor produces a bigger spike than a larger soft-started one. Every scenario is listed in the results and in the PDF, with the governing one highlighted, so the assumption is auditable rather than hidden.

### Transient and conservative sizing methods

A motor start is a transient of a few seconds, and an alternator can supply far more than its continuous rating for that long: commonly 2 to 3 times nameplate at a voltage dip in the order of 25 to 30%. The transient method, which is the default, reflects that: the start spike is divided by a starting-capability factor (2.5× nameplate by default, adjustable between 1× and 4× to match a datasheet) while the max-loading headroom is applied only to the steady load. The nameplate needed is then the larger of steady kVA divided by max-loading, and start kVA divided by the capability factor.

The conservative method instead treats the start spike as though it were a steady load and divides it by the max-loading percentage too. That sizes the set substantially larger and is normally reserved for very dip-sensitive installations. Neither method computes the actual voltage or frequency dip: the transient performance of a specific set is a function of its alternator reactance and excitation system, so the dip must be verified against AS ISO 8528.5 and the manufacturer transient curves before the selection is locked in.

### Fuel tank sizing and why it follows kW, not kVA

When fuel tank sizing is enabled the calculator returns the tank volume needed for a required autonomy: consumption in litres per hour multiplied by the autonomy in hours, plus a reserve percentage for unusable bottom-of-tank volume, filtration margin and refuelling buffer. A manufacturer consumption figure can be entered directly; left blank, the consumption is estimated from the running load.

That estimate is taken against real power, not apparent power. The engine only ever supplies the kW (reactive power circulates between the alternator and the load and costs no fuel), so consumption is estimated at about 0.27 litres per kWh of running load, within the usual 0.25 to 0.30 L/kWh band for modern diesel engines. A 150 kW running load therefore burns roughly 40.5 L/hr, and a 24-hour autonomy with a 10% reserve needs about 1070 litres. Fuel storage itself is governed by AS 1940 and local environmental requirements, which the calculator does not assess.

### What sits outside the calculation

The connected load is an input, not a derived figure: the calculator does not apply AS/NZS 3000 Appendix C maximum-demand diversity, so the demand assessment should be done first and its result entered here. Site derating for altitude and ambient temperature is not evaluated either, and both can remove a significant fraction of a set's output at inland Australian sites: apply the manufacturer derating curves to the recommended size.

Non-linear load is not sized for either. Variable speed drives, UPS rectifiers and LED and IT equipment draw harmonic current that distorts the set's output voltage, because an alternator's source impedance is far higher than the grid's, and the distortion can upset the voltage regulator. The alternator is oversized for that load by an amount set by the drive or rectifier type (a 6-pulse front end needs much more than an active front end), confirmed with the set supplier. The VFD start factor of 1.2× is about starting current only and does not cover it.

Also outside scope: step-load acceptance and load-shedding sequencing, alternator sizing for sustained fault current to hold protection discrimination, and the installation requirements of AS/NZS 3010:2017 covering earthing, neutral switching, changeover, isolation and signage. The tool answers how large the set must be for the load and its starts; the surrounding design still needs the standards.

## Key terms

- **Prime Power (PRP)** — The AS ISO 8528.1 rating for a set supplying a variable load for unlimited hours, with the average load not exceeding 70% of the rating over any 24-hour period and a 10% overload permitted for 1 hour in 12. Typical of off-grid, mining and remote primary supply. Selecting it applies a 70% max-loading default.
- **Emergency Standby Power (ESP)** — The AS ISO 8528.1 rating for a set that supplies a variable load only for the duration of a utility outage, limited to about 200 hours a year with no overload capability. Typical of life-safety, hospital and data-centre backup. Selecting it applies an 80% max-loading default.
- **Continuous Operating Power (COP)** — The AS ISO 8528.1 rating for a set running unlimited hours at a constant 100% load with no overload capability: the most conservative rating, used for base-load, grid-parallel and cogeneration duty. Selecting it applies a 100% max-loading default.
- **Max loading** — The percentage of nameplate kVA the steady running load is allowed to occupy. The calculator divides the running kVA by this figure to get the steady sizing target. It defaults from the duty class (70 / 80 / 100%) and can be set anywhere from 30 to 100% per project.
- **Running kVA** — The steady apparent-power demand on the set: the connected load in kW divided by the power factor, plus every motor in the schedule running at its full-load kVA. The alternator requirement and the running current are derived from it.
- **Rated power factor** — The power factor a set's kVA is rated at: 0.8 lagging (AS ISO 8528.1). The engine delivers 0.8 × kVA in kWe, so a load at a better power factor than 0.8 is limited by the engine, and the set is sized on kW ÷ 0.8.
- **Non-linear load** — Load that draws non-sinusoidal current, such as variable speed drives, UPS rectifiers, LED drivers and IT power supplies. On a generator its harmonic current distorts the voltage, and the alternator is oversized for it by an amount the set supplier confirms for the drive or rectifier type. This calculator does not add that oversize.
- **Starting kVA (skVA)** — The apparent power a motor draws while it accelerates, taken here as the motor full-load kVA multiplied by the start-current factor for its starting method. It lasts seconds, so it is checked against the set's short-time capability rather than its continuous rating.
- **Start-current factor** — The multiple of full-load current a motor draws during starting: 7.5× for direct-on-line, 2.5× for star-delta, 3.5× for soft starter and 1.2× for VFD/VSD in this calculator. Actual values depend on motor design class and on soft-starter or drive current-limit settings, so confirm against the data sheets.
- **Worst-case starting scenario** — The highest total apparent power the installation can present: one motor starting while the base load, all other motors and the remaining units of the same type run at rated load. Every motor is evaluated as the starting one and the largest result governs the sizing, which is not always the largest motor.
- **Starting capability factor** — How many times its nameplate kVA a set can supply briefly during a motor start, typically 2 to 3 times at a voltage dip around 25 to 30%. The transient sizing method divides the start spike by this factor (default 2.5×, adjustable 1-4×). Verify it against AS ISO 8528.5 and the alternator datasheet.
- **Transient sizing method** — The default method, in which the start spike only has to fit inside the set's short-time starting capability while the max-loading headroom applies to the steady load alone. It reflects how alternators actually behave during a few-second transient.
- **Conservative sizing method** — The alternative method, in which the start spike is treated as a steady load and divided by the max-loading percentage as well. It oversizes the set considerably and suits only very dip-sensitive installations.
- **Wet stacking** — The build-up of unburnt fuel and carbon in a diesel engine run for long periods at light load, which fouls injectors, rings and the exhaust. It is why sustained loading below about 30% of nameplate is discouraged, and why the calculator warns when the recommended set would be loaded below that.
- **Standard alternator ladder** — The commercially available nameplate kVA sizes a generating set can be bought in: 10 through to 3000 kVA in this calculator. The sizing target is always rounded up to the next size on the ladder; a target above 3000 kVA is flagged as needing paralleled sets or a custom alternator.
- **Autonomy** — The number of hours the set must run on its own fuel before refuelling. Multiplied by the consumption rate and increased by the reserve percentage, it gives the required tank volume. Common design figures are 8-12 hours for a belly tank, 24 hours for essential services and 48-72 hours or more for remote or critical sites.

## Frequently asked questions

### What is the difference between prime, standby and continuous generator ratings?

AS ISO 8528.1 defines three duties. Emergency Standby Power (ESP) backs up a utility outage for limited annual hours with no overload; Prime Power (PRP) runs a variable load for unlimited hours; Continuous Operating Power (COP) runs a constant load at 100% of its rating for unlimited hours. ESP and PRP both limit the average output over any 24 hours to 70% of the rating. The calculator turns the duty into a max-loading default of 80% for standby, 70% for prime and 100% for continuous, adjustable from 30 to 100%.

### Why are generators sized to run at no more than 80% load?

An 80% ceiling keeps a standby set a margin for load steps, motor starts and engine life: generator manufacturers commonly recommend sustained loading between about 30% and 80% of nameplate. Above that there is little headroom for a transient; below about 30% a diesel engine wet-stacks. Prime-rated sets work to a 70% average load under AS ISO 8528.1 and continuous-rated sets to 100%. The calculator applies those as its defaults and warns when the recommended set would run below 30% load.

### How do I calculate what size generator I need in kVA?

Take the larger of two targets and round it up to the next standard alternator size. Steady target: the running kVA (connected kW ÷ power factor, plus every motor running) ÷ the max-loading percentage. A set is rated in kVA at 0.8 power factor (AS ISO 8528.1), so when the load power factor is above 0.8 the engine governs instead and the steady target is running kW ÷ 0.8 ÷ max loading. Start target: the worst motor-start scenario ÷ the set's short-time starting capability (2.5× nameplate by default, adjustable from 1 to 4×), or ÷ max loading on the conservative method. The ladder runs from 10 to 3000 kVA.

### How do I calculate motor starting kVA for generator sizing?

Starting kVA = the motor's full-load kVA × the start-current factor for its starting method: 7.5× direct-on-line, 2.5× star-delta, 3.5× soft starter and 1.2× VFD/VSD. Enable Motor Starting and enter each motor's electrical input kW (nameplate shaft kW ÷ efficiency), quantity and start method. The calculator converts it to full-load kVA at the true power factor you entered, then tests every motor as the one starting while the base load and every other motor run, and the worst scenario governs. A VFD-started motor is also counted as non-linear load in the result. Verify the actual voltage dip against AS ISO 8528.5 and the alternator datasheet.

### What is the difference between the transient and conservative sizing methods?

The transient method (the default) treats a motor start as a few-second event: the start spike only has to fit inside the set's short-time starting capability, 2.5× nameplate by default and adjustable from 1 to 4×, while the max-loading headroom applies to the steady load alone. The conservative method also divides the start spike by the max-loading percentage, as if it were a steady load, which sizes the set much larger and suits dip-sensitive installations. Manufacturers usually quote starting capability at a 25 to 30% voltage dip, deeper than AS ISO 8528.5 allows for performance classes G2 (20%) and G3 (15%), so lower the factor where the specification names one of those classes, and confirm either result against the alternator transient curves.

### What starting current multiplier should I use for DOL, star-delta, soft starter and VFD motors?

Typical multiples of full-load current for generator sizing are 6 to 8× for direct-on-line, 2 to 3× for star-delta, 3 to 4× for a soft starter and 1.0 to 1.5× for a VFD/VSD. The calculator uses 7.5×, 2.5×, 3.5× and 1.2× respectively, applied to the motor's full-load kVA. Real values depend on the motor design and on the soft starter or drive current-limit setting, so confirm them against the motor and drive data sheets.

### How do I size a diesel generator fuel tank or day tank?

Tank volume = fuel consumption (L/hr) × autonomy (hours) × (1 + reserve %). Enter the manufacturer's consumption, or leave it blank and the calculator estimates it from the running real power at about 0.27 L per kWh: a 150 kW running load burns roughly 40.5 L/hr and needs about 1070 L for 24 hours with a 10% reserve. A reserve of 10 to 20% covers unusable bottom-of-tank volume, filtration margin and a refuelling buffer. Day tanks, bulk tanks and bunding must also meet AS 1940 and local environmental requirements.

### How much diesel does a generator use per hour?

About 0.25 to 0.30 litres per kWh produced: fuel burn follows the set's real power output in kW, not its kVA, because the engine supplies only the kW while reactive power circulates between the alternator and the load. The calculator uses 0.27 L/kWh for its estimate, so a set running 400 kW burns about 108 L/hr whatever the load power factor. Confirm against the manufacturer's fuel consumption curve, which varies with load factor.

### What does the generator sizing calculator not cover?

It sizes the set from the load you enter, so it does not apply AS/NZS 3000 Appendix C maximum-demand diversity: run the demand assessment first and enter the result. It does not derate for altitude or ambient temperature, and it does not size the alternator for harmonics: non-linear load is flagged as a share of the set for the set supplier to check, not added to the kVA. Step-load acceptance, the actual transient voltage and frequency dip under AS ISO 8528.5, and the AS/NZS 3010 installation requirements such as earthing, neutral switching and changeover are also outside it.

### How many hours of autonomy should a standby generator have?

Common design figures are 8 to 12 hours for a belly tank, 24 hours for essential-services standby such as hospitals and data centres, and 48 to 72 hours or more for remote sites and critical life-safety installations. Local authority, insurer and AS/NZS 3009 or essential-services requirements can set a minimum fuel reserve.

### Does the calculator handle single-phase and three-phase generators?

Yes. Choose 3-Phase (415 V) or 1-Phase (230 V) and the running current is worked with the matching phase factor: I = kVA × 1000 ÷ (√3 × V) for three-phase and I = kVA × 1000 ÷ V for single-phase. The voltage follows the selection: 415 V is the legacy figure still widely in service, while AS 60038 gives 230/400 V as the nominal.

### Which Australian Standards apply to generator set installations?

AS ISO 8528.1 defines duty ratings and application of reciprocating IC engine driven AC generating sets. AS ISO 8528.5 covers transient voltage and frequency performance on motor starting. AS/NZS 3010:2017 sets out installation, earthing, neutral switching and changeover requirements. AS/NZS 3000:2018 (Wiring Rules) applies to the downstream installation. AS 1940 governs fuel storage.

### How do I size a generator for a building services standby application?

Run the AS/NZS 3000 maximum-demand assessment first, then enter that demand in kW with its true power factor and select Standby (ESP) duty, which applies an 80% max-loading headroom. Add the life-safety motors (pumps, lifts, smoke-exhaust and kitchen exhaust fans) with their starting methods so the worst-case start is evaluated against the steady load. The calculator returns the larger of the two nameplate requirements rounded up to the next standard alternator size; verify the transient dip against AS ISO 8528.5 and the manufacturer curves before selecting the set.

### How much does soft-starting a motor reduce generator size?

A soft starter cuts that motor's start spike by about 53% against direct-on-line, 3.5× full-load current instead of 7.5×; star-delta at 2.5× cuts it by two thirds and a VFD at 1.2× by 84%. The set shrinks only where the motor start governs the sizing, and by less than those percentages, because the start scenario still carries the base load and every other motor running: typically one or two steps down the standard alternator ladder. A VFD is non-linear load, so the alternator is oversized for its harmonics by an amount the set supplier confirms. Confirm the real starting current against the starter or drive current-limit setting.

### Why does the calculator recommend more kVA than my load draws?

Besides the max-loading headroom for the duty, a set is rated in kVA at 0.8 power factor lagging (AS ISO 8528.1), so its engine delivers only 0.8 × kVA in kW. When the load's power factor is better than 0.8, the engine is the limit: 400 kW at 0.95 power factor is 421 kVA of load, but needs a 500 kVA (400 kWe) set before any headroom. The sizing breakdown names which limit, the alternator or the engine, governs.

### Which power factor do I enter for a generator?

The true power factor: real power over apparent power with the harmonic current included, which is what a power quality meter reads as PF. Not the cos φ in a drive's manual. cos φ is the displacement power factor, the phase shift at 50 Hz only, and a drive often quotes 0.95 or more. The set also carries the drive's harmonic current, so its true power factor is lower and its kVA higher. Entering the cos φ undersizes the alternator, and the harmonic current then distorts the set's voltage, which can upset its voltage regulator and damage the drives' rectifiers. The calculator asks for the true figure, and for the non-linear share of the load, which it reports as a share of the set for the set supplier to check against the drives' harmonic data.

## Related ScaleSet pages

- [Maximum Demand](https://scaleset.com.au/calculator/max-demand)
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