Generator Sizing Calculator

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; both ESP and PRP carry a 70% average-output limit over any 24 hours; 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 every motor running at rated load. For each motor a worst-case start is evaluated: that motor starting while the base load and all other motors (including the remaining units of the same type) run. Two sizing targets are then formed: steady target = running kVA ÷ max-loading %, and start target = worst-case start kVA ÷ the set's short-time capability factor (default 2.5× nameplate, or ÷ max-loading % if you pick the conservative method). The larger target is 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 converts each motor to full-load kVA using the site power factor you entered (so enter the motor's electrical input kW, not shaft kW, dividing the nameplate by the motor efficiency first if needed) and multiplies by the start-current factor for Direct-On-Line (7.5×), Star-Delta (2.5×), Soft Starter (3.5×) or VFD/VSD (1.2×), then evaluates each motor starting while everything else runs. The worst scenario is compared against the steady-state target and whichever demands the larger nameplate governs. The result assumes a voltage dip the set can tolerate: verify the actual dip against AS ISO 8528.5 and the alternator datasheet.

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

The transient method (default) treats the motor start as a short-time event: the spike only has to fit inside the set's starting-kVA capability, typically 2–3× nameplate, which manufacturers quote at a voltage dip of 25–30%, deeper than AS ISO 8528.5 allows for performance classes G2 (−20%) and G3 (−15%), so reduce the capability factor if your specification calls for one of those (2.5× default, adjustable 1–4×), while the max-loading headroom applies to the steady load only. The conservative method treats the start spike as though it were a steady load and divides it by the max-loading percentage as well, which sizes the set substantially larger. Use conservative for dip-sensitive installations, and confirm either result against the alternator transient curves.

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

The factors are multiples of full-load current, applied to the motor's full-load kVA. Typical values used for generator sizing are: Direct-On-Line 6–8× (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 %). Enter a manufacturer consumption figure, or leave it blank and the calculator estimates from the running real power at about 0.27 L per kWh: so a 150 kW running load burns roughly 40.5 L/hr and needs about 1070 L for 24 hours with a 10% reserve. 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?

Diesel gen-set fuel burn tracks real power output (kW), not apparent power (kVA): the engine only supplies the kW, while reactive power circulates in the alternator. Modern sets consume roughly 0.25–0.30 litres per kWh produced, and this calculator uses 0.27 L/kWh for its auto-estimate. A set running 400 kW therefore burns about 108 L/hr regardless of the power factor the load presents. Always confirm against the manufacturer fuel consumption curve, which varies with load factor, when it is available.

What does the generator sizing calculator not cover?

It sizes the set from the connected load you enter: it does not apply AS/NZS 3000 Appendix C maximum-demand diversity, so run the demand assessment first and enter the result. It does not evaluate site derating for altitude or ambient temperature, harmonic or non-linear load content (UPS and VSD front ends), step-load acceptance sequencing, the actual transient voltage and frequency dip under AS ISO 8528.5, or the installation requirements of AS/NZS 3010 such as earthing, neutral switching and changeover. Confirm those against the manufacturer data and the standards before finalising a set.

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 (230 V) or 3-Phase; 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). The tool defaults to the legacy 415 V still widely in service, though 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 the site 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?

In this calculator a soft starter is taken at 3.5× full-load current against 7.5× for direct-on-line, so the start spike falls by about 53%: but that cut applies to the spike only, since the scenario still carries the base load and every other motor running, so the nameplate usually drops a step or two rather than by the full percentage; star-delta at 2.5× cuts it by two thirds and a VFD at 1.2× by 84%. Where the motor start governs the sizing rather than the steady load, that reduction carries straight through to a smaller nameplate: often one or two steps down the standard alternator ladder. Confirm the real starting current against the drive or starter settings, since current-limit configuration changes it substantially.

Does the calculator apply AS/NZS 3000 Appendix C diversity?

No. The connected load in kW is an input, so the maximum-demand assessment should be done first (the ElecAS Maximum Demand calculator implements the AS/NZS 3000 Appendix C tables), and its result entered here. Note also that diversity should not be applied to the single motor start that governs the transient check: that start is a discrete event regardless of how diverse the rest of the installation is, which is why the motor schedule is handled separately from the connected load figure.

Why does the calculator size the fuel tank from kW rather than kVA?

Because the engine only supplies real power. Reactive power circulates between the alternator and the load without passing through the engine, so a set carrying 400 kW at 0.8 power factor burns the same fuel as one carrying 400 kW at 0.95 power factor even though the kVA differs. The auto-estimate is about 0.27 litres per kWh of running load, within the 0.25 to 0.30 L/kWh band typical of modern diesel engines; enter the manufacturer figure instead when the consumption curve for the specific set is known.

What happens if the required generator exceeds 3000 kVA?

The standard alternator ladder in the calculator stops at 3000 kVA. When the sizing target exceeds that, the result is capped at 3000 kVA and flagged as over capacity, because a single standard set cannot serve the load: the design needs paralleled sets, a custom alternator, or load segregation across multiple boards. The uncapped sizing target is still reported so the shortfall is visible.