What is the difference between Prime, Standby and Continuous generator ratings?

AS ISO 8528.1 defines Emergency Standby Power (ESP) for utility-outage backup with no overload and limited annual hours; Prime Power (PRP) for variable loads with unlimited hours but a ~70% average load cap; and Continuous Operating Power (COP) for constant base loads at 100% of nameplate for unlimited hours. The calculator applies 80% / 70% / 100% max-loading defaults to the respective duties.

Why is a generator typically not loaded beyond 80%?

Manufacturers such as Cummins (T-030) and Caterpillar (LEBW4977) recommend keeping standby generators at or below about 80% of nameplate for a margin against transient loads, starting kVA and engine health. Prime-rated sets target ~70% average load per ISO 8528.1 PRP, while continuous sets can run at 100%.

How is the recommended generator kVA calculated?

Running kVA = connected kW ÷ power factor, plus the motors running kVA. For each motor the worst-case starting scenario is evaluated: largest motor starting while all other motors (including remaining units of the same type) run at full load. The sizing target is max(Running, Starting) ÷ max-loading %, rounded up to the next standard alternator size (10, 15, 20, … 3000 kVA).

How do I include motor starting in generator sizing?

Enable the Motor Starting section and add each motor with its rated kW, quantity and start method. The calculator applies the AS/IEC starting kVA factor for Direct-On-Line (7.5×), Star-Delta (2.5×), Soft Starter (3.5×) or VFD/VSD (1.2×), then checks the worst-case starting scenario against the running load. Whichever is larger governs the recommended kVA. For transient voltage dip limits refer to AS ISO 8528.5.

What starting kVA multiplier should I use for DOL, Star-Delta, Soft Starter and VFD?

Typical locked-rotor / transient factors used for generator sizing are: Direct-On-Line 6–8× rated kW (default 7.5×), Star-Delta 2–3× (default 2.5×), Soft Starter 3–4× (default 3.5×) and VFD/VSD 1.0–1.5× (default 1.2×). Confirm against motor and drive data sheets — actual values depend on motor design class and soft-starter / VFD current limit settings.

How do I size the diesel fuel tank for a generator?

Required fuel volume = consumption (L/hr) × autonomy (hours) × (1 + reserve %). If a manufacturer consumption figure is not available, the calculator estimates from the recommended kVA at the chosen loading (~0.22 L/hr per kVA for modern diesel gensets). Include a 10–20% reserve to cover unusable bottom-of-tank volume, filtration margin and refuelling buffer. Day tanks, bulk tanks and bunded installations must also meet AS 1940 and local environmental requirements.

What is the typical diesel generator fuel consumption per kVA?

Modern diesel gen-sets consume roughly 0.20–0.25 L/hr per kVA at typical load factors, with a common design figure of 0.22 L/hr/kVA. Consumption scales approximately with load — a 500 kVA set at 80% load burns about 500 × 0.8 × 0.22 ≈ 88 L/hr. Always confirm with the manufacturer fuel consumption curve when available.

How many hours of autonomy should a standby generator have?

Typical design autonomies are 8–12 hours for a belly tank, 24 hours for essential-services standby (hospitals, data centres), and 48–72 hours or more for remote sites and critical life-safety installations. Local authority, insurance and AS/NZS 3009 / essential-services requirements may dictate minimum fuel reserves.

Does the calculator handle single-phase and three-phase systems?

Yes. Select 1-Phase (typically 230 V) or 3-Phase (typically 415 V); the full-load current is computed with the correct phase factor (I = kVA × 1000 ÷ V for single-phase, and I = kVA × 1000 ÷ (√3 × V) for three-phase) per AS 60038 nominal voltages.

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 four-check sizing: steady-state kW (continuous load with AS/NZS 3000 Appendix C diversity), steady-state kVA (continuous apparent power), transient voltage dip (during the largest motor start), and transient frequency dip (during the largest block load step). Pick the rating that satisfies the largest of the four checks at the standby (ESP) rating basis.

What is the difference between standby (ESP), prime (PRP) and continuous (COP) generator ratings?

Standby (ESP) is for emergency use only — typically 200 hours/year at 70% average load with no overload. Prime (PRP) is for unlimited hours of primary power at 70% average load with 10% overload for 1 hour in 12. Continuous (COP) is for unlimited hours at 100% load with no overload.

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

A soft-starter typically reduces starting current to 2–3× full-load current (vs 6–8× for DOL), cutting the starting kVA by 50–70%. For building services applications where the largest motor start governs the generator sizing, soft-starting often allows a 30–50% smaller generator.

Should I include AS/NZS 3000 Appendix C diversity in generator sizing?

Yes for the steady-state continuous load — the same Appendix C diversity factors apply. Do not apply diversity to the largest single motor for the transient voltage / frequency dip check, because the start of that single motor is a single event regardless of overall load diversity.

ElecAS

Generator Sizing Calculator

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.

Why this page matters

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. This static content is published so the canonical route has meaningful crawlable HTML even before the interactive application hydrates.

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 helps with

Choose Prime (PRP), Standby (ESP) or Continuous (COP) duty to AS ISO 8528.1 with manufacturer max-loading defaults (70 / 80 / 100%) — override per project.
Model motor starting with DOL (7.5×), Star-Delta (2.5×), Soft Starter (3.5×) and VFD (1.2×) factors; worst-case starting scenario governs the sizing automatically.
Add an unlimited list of motors with rated kW, quantity and start method — the calculator starts the largest while all others run.
Size the fuel day tank for any autonomy with manufacturer consumption data or an auto-estimate (~0.22 L/hr per kVA at load), including a safety reserve.
Recommended nameplate kVA selected from the standard alternator ladder (10 – 3000 kVA), plus full-load current for 1Ø 230 V or 3Ø 415 V systems.
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 for an Australian installation

List the connected loads — Enter each load as kW + power factor + start type (DOL, soft-starter, VFD, static). Distinguish always-on loads from intermittent loads.
Run the steady-state kW and kVA checks — The calculator sums the connected kW and kVA and reports the steady-state requirement, with diversity applied via AS/NZS 3000 Appendix C for the always-on portion.
Identify the largest motor or block-load step — The calculator picks the largest single load step (typically the largest DOL motor) and computes the starting kVA and starting power factor.
Apply the transient voltage and frequency dip checks — Enter the maximum acceptable voltage dip (typically 15% for normal services, 10% for medical) and frequency dip (typically 10%). The calculator picks the smallest alternator that meets the dip limits.
Review the four-check result and pick the rating — The calculator reports the kW required by each check and picks the governing rating. Choose standby, prime or continuous as appropriate. Export the branded PDF.

Generator sizing for Australian standby and prime power installations

The four generator sizing checks

A diesel or gas generator must satisfy four independent sizing checks: steady-state kW (real power continuous load), steady-state kVA (apparent power continuous load + load step capacity), transient voltage dip (the voltage sag during the largest motor start), and transient frequency dip (the frequency sag during the largest motor start or block-load step). The largest of the four governs the generator nameplate rating.

AS 3010 and AS 60034 govern the engineering for generating sets in Australian installations. Manufacturer transient performance curves (typically published for 10%, 25% and 50% load steps) determine the transient checks.

Motor-start governs most building services generators

A 22 kW direct-on-line motor draws 6–8× full-load current at start (200–250 A on a 400 V three-phase mains) at a starting power factor near 0.35 lagging. The kVA draw during start is therefore 80–100 kVAr inrush, which can dip the generator voltage by 15–25% depending on the alternator subtransient reactance.

For most building services applications (life-safety pumps, lift, smoke-exhaust fans, kitchen exhaust) the largest motor start is the governing sizing check. Reduced-voltage starting (soft starter, VFD, star-delta) cuts the starting kVA by 50–70% and is often the most economical way to reduce the generator size.

Standby vs prime vs continuous ratings

Standby (ESP) rating is for emergency standby use only — typically rated for 200 hours per year at 70% average load with no overload capability. Prime (PRP) rating is for primary power supply with utility unavailable — rated for unlimited hours at 70% average load with 10% overload for 1 hour in 12. Continuous (COP) rating is for unlimited hours at 100% load with no overload.

Most Australian building services applications use the standby rating. Off-grid mining or remote prime power applications use PRP. The ElecAS generator sizing calculator reports the required nameplate at all three rating bases.

Reviewed by

Wisam Tozah — Associate Electrical Engineer. B.Eng (Electrical), MIEAust, CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE(Aus). LinkedIn.

Frequently asked questions

What is the difference between Prime, Standby and Continuous generator ratings?: AS ISO 8528.1 defines Emergency Standby Power (ESP) for utility-outage backup with no overload and limited annual hours; Prime Power (PRP) for variable loads with unlimited hours but a ~70% average load cap; and Continuous Operating Power (COP) for constant base loads at 100% of nameplate for unlimited hours. The calculator applies 80% / 70% / 100% max-loading defaults to the respective duties.
Why is a generator typically not loaded beyond 80%?: Manufacturers such as Cummins (T-030) and Caterpillar (LEBW4977) recommend keeping standby generators at or below about 80% of nameplate for a margin against transient loads, starting kVA and engine health. Prime-rated sets target ~70% average load per ISO 8528.1 PRP, while continuous sets can run at 100%.
How is the recommended generator kVA calculated?: Running kVA = connected kW ÷ power factor, plus the motors running kVA. For each motor the worst-case starting scenario is evaluated: largest motor starting while all other motors (including remaining units of the same type) run at full load. The sizing target is max(Running, Starting) ÷ max-loading %, rounded up to the next standard alternator size (10, 15, 20, … 3000 kVA).
How do I include motor starting in generator sizing?: Enable the Motor Starting section and add each motor with its rated kW, quantity and start method. The calculator applies the AS/IEC starting kVA factor for Direct-On-Line (7.5×), Star-Delta (2.5×), Soft Starter (3.5×) or VFD/VSD (1.2×), then checks the worst-case starting scenario against the running load. Whichever is larger governs the recommended kVA. For transient voltage dip limits refer to AS ISO 8528.5.
What starting kVA multiplier should I use for DOL, Star-Delta, Soft Starter and VFD?: Typical locked-rotor / transient factors used for generator sizing are: Direct-On-Line 6–8× rated kW (default 7.5×), Star-Delta 2–3× (default 2.5×), Soft Starter 3–4× (default 3.5×) and VFD/VSD 1.0–1.5× (default 1.2×). Confirm against motor and drive data sheets — actual values depend on motor design class and soft-starter / VFD current limit settings.
How do I size the diesel fuel tank for a generator?: Required fuel volume = consumption (L/hr) × autonomy (hours) × (1 + reserve %). If a manufacturer consumption figure is not available, the calculator estimates from the recommended kVA at the chosen loading (~0.22 L/hr per kVA for modern diesel gensets). Include a 10–20% reserve to cover unusable bottom-of-tank volume, filtration margin and refuelling buffer. Day tanks, bulk tanks and bunded installations must also meet AS 1940 and local environmental requirements.
What is the typical diesel generator fuel consumption per kVA?: Modern diesel gen-sets consume roughly 0.20–0.25 L/hr per kVA at typical load factors, with a common design figure of 0.22 L/hr/kVA. Consumption scales approximately with load — a 500 kVA set at 80% load burns about 500 × 0.8 × 0.22 ≈ 88 L/hr. Always confirm with the manufacturer fuel consumption curve when available.
How many hours of autonomy should a standby generator have?: Typical design autonomies are 8–12 hours for a belly tank, 24 hours for essential-services standby (hospitals, data centres), and 48–72 hours or more for remote sites and critical life-safety installations. Local authority, insurance and AS/NZS 3009 / essential-services requirements may dictate minimum fuel reserves.
Does the calculator handle single-phase and three-phase systems?: Yes. Select 1-Phase (typically 230 V) or 3-Phase (typically 415 V); the full-load current is computed with the correct phase factor (I = kVA × 1000 ÷ V for single-phase, and I = kVA × 1000 ÷ (√3 × V) for three-phase) per AS 60038 nominal voltages.
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 four-check sizing: steady-state kW (continuous load with AS/NZS 3000 Appendix C diversity), steady-state kVA (continuous apparent power), transient voltage dip (during the largest motor start), and transient frequency dip (during the largest block load step). Pick the rating that satisfies the largest of the four checks at the standby (ESP) rating basis.
What is the difference between standby (ESP), prime (PRP) and continuous (COP) generator ratings?: Standby (ESP) is for emergency use only — typically 200 hours/year at 70% average load with no overload. Prime (PRP) is for unlimited hours of primary power at 70% average load with 10% overload for 1 hour in 12. Continuous (COP) is for unlimited hours at 100% load with no overload.
How much does soft-starting a motor reduce generator size?: A soft-starter typically reduces starting current to 2–3× full-load current (vs 6–8× for DOL), cutting the starting kVA by 50–70%. For building services applications where the largest motor start governs the generator sizing, soft-starting often allows a 30–50% smaller generator.
Should I include AS/NZS 3000 Appendix C diversity in generator sizing?: Yes for the steady-state continuous load — the same Appendix C diversity factors apply. Do not apply diversity to the largest single motor for the transient voltage / frequency dip check, because the start of that single motor is a single event regardless of overall load diversity.