Generator Sizing Calculator

Parameters

Every field this calculator exposes, what it means, the range it accepts, and the traps worth knowing before you rely on the answer.

Per load fields

You can add as many loads as you like. Each row carries four numbers and a checkbox, and how they combine depends entirely on that checkbox.

Load nametext

A label only. It appears in the working and the exported report and does not affect any result. It cannot be blank.

PowerkW

Real power drawn by that load when running, in kilowatts. This is summed straight across all rows for the total running kilowatts, and divided by the power factor for the kilovolt ampere figure.

Power factorratio

Power factor for that load. The engine accepts 0.5 to 1.0; the form inputs start at 0.8. Typical values are 0.95 or higher for resistive and electronic load, 0.85 for general motors, and 0.8 or lower for lightly loaded motors. A lower value increases the kilovolt amperes for the same kilowatts.

Starting multiplierratio, 1.0 or more

Ratio of starting current to full load current for that load. Six to eight is typical for direct on line motor starting; two to three reflects star delta or soft start. Gotcha: this field is ignored entirely unless the motor checkbox is ticked.

Motorcheckbox

This is the most important field on the page. Only the single largest motor starting demand is included in the starting case, and the running demand of every other motor is dropped from that case. A load left unticked always contributes its running kilovolt amperes to both the running and starting totals. See the assumptions below, because this materially changes the answer on sites with two or more large motors.

Site fields

Supply voltagevolts

Used only to convert the selected generator rating into a full load current. The engine validates against 230, 400, 415 and 480 volts. The form also offers 600 volts and 11 kV, which are outside the validated set but still produce a current figure.

Phase typesingle or three

Three phase divides by the voltage multiplied by the square root of 3, which is 1.732. Single phase divides by the voltage alone. It has no other effect: the kilovolt ampere requirement is identical either way.

Altitudemetres above sea level

Engine range 0 to 5,000 metres. Derating starts above 1,000 metres and removes 3.5 percent for every 500 metre step or part step. Gotcha: it is a ceiling function, so 1,001 metres and 1,500 metres both give the same 3.5 percent, and 1,501 metres jumps straight to 7 percent. The factor never goes below 0.5.

Ambient temperaturedegrees Celsius

Engine range minus 10 to 50 degrees; the form allows 0 to 50. Derating starts above 40 degrees and removes 2 percent for every 5 degree step or part step, again as a ceiling function, so 41 degrees and 45 degrees both give 2 percent. The factor never goes below 0.5.

Fuel typediesel, petrol or gas

Affects only the fuel consumption estimate: 0.25 litres per kilowatt hour for diesel, 0.30 for gas, 0.35 for petrol, each applied at 75 percent of the selected rating. Gotcha: the form also offers LPG and natural gas, which the engine does not recognise, and those selections return a fuel figure of zero.

Application classstandby, prime or continuous

Recorded for the report only. It does not change the required kilovolt amperes, the selected size or any derating. There is no standby to prime rating conversion applied anywhere.

Assumptions and limits

This is a load summation and standard size selection tool. It gets you to a sensible starting rating, but a genset selection is finished in the supplier sizing software, not here.

What the tool assumes

• The required rating is simply the larger of the running kilovolt amperes and the starting kilovolt amperes. There is no separate step load or block load sequence.
• The starting case is the largest single motor at its starting multiplier, plus the running kilovolt amperes of the loads that are not ticked as motors. The running load of all other motors is not included. On a site with two comparable motors this can understate the starting demand noticeably, so if that matters, model the second motor as a non motor load at its running kilowatts.
• Motors are assumed to start one at a time and never simultaneously.
• Altitude and temperature derating factors multiply together, and the combined factor divides the required rating before a standard size is chosen. The headline required kilovolt ampere figure shown in the results is the value before derating; the derated figure is what drives the size selection.
• The standard size ladder is 15, 20, 30, 50, 60, 80, 100, 125, 150, 200, 250, 300, 350, 400, 500, 600, 750, 1,000, 1,250, 1,500 and 2,000 kilovolt amperes. It is an indicative ladder, not a manufacturer catalogue.
• The loading percentage compares the running kilovolt amperes against the selected size, not the starting kilovolt amperes. On a motor driven selection the loading will look low, because the size was set by a transient the loading figure ignores.
• The 80 percent loading limit and the derating slopes are common industry conventions, not transcribed values from any published table.

What it does not do

Treat the output as an indicative rating for early planning. Check it against the current edition of AS/NZS 3010 and the supplier sizing software, and have the responsible person for the installation review and sign off the final selection. Nothing here is validated or certified.

Worked examples

Three examples covering a motor driven three phase selection, a site with both altitude and temperature derating, and a single phase case with no motors where the loading check fails. Every figure comes from the same arithmetic the calculator runs.

Generator Sizing Guide for AS/NZS 3010:2014

Generator sizing is the process of selecting a genset with enough capacity to supply the connected electrical load while maintaining voltage and frequency within acceptable limits. In Australia, generator installations must comply with AS/NZS 3010:2014, which covers wiring requirements, protection, earthing, and fuel systems for generating sets. Whether you are specifying a standby generator for a commercial building, a prime power unit for a remote site, or a portable set for temporary supply on a construction project, the sizing process follows the same fundamental steps: calculate the total running load, account for the largest motor starting transient, apply derating factors for site conditions, and select the next standard genset size.

Key concepts

• Standby vs prime vs continuous rating. Standby rating is the maximum output available during a mains failure, typically limited to 200 to 500 hours per year. Prime rating is the maximum output for unlimited running hours at variable load. Continuous rating is for a constant load running 24/7 (base-load generation). Standby rating is typically 10 to 15 percent higher than prime for the same engine.
• Motor starting inrush. When the largest motor on the generator supply starts, the inrush current (typically 6 to 8 times full load current) causes a voltage dip proportional to the generator impedance. The generator must be large enough to hold the voltage dip below 15 to 20 percent, or sensitive equipment may malfunction or trip.
• Derating factors. Generators are derated for altitude (above 1000 m, typically 3.5 percent per 300 m), ambient temperature (above 40 degrees Celsius, typically 2 percent per degree), and fuel type (natural gas engines produce less power than diesel for the same displacement). All applicable factors must be applied to the nameplate rating before comparing to the site load.
• Fuel consumption estimation. Diesel generators consume approximately 0.27 litres per kWh at full load. At 75 percent load, which is the most efficient operating point, consumption drops to approximately 0.22 litres per kWh. These figures feed directly into fuel storage sizing and operational cost estimates.

Common scenarios

1. Commercial building standby power. A multi-storey office building with a 350 kW maximum demand and a 75 kW fire pump as the largest motor. The generator must supply 350 kW steady state while handling the fire pump inrush (approximately 225 kVA transient demand). After applying a 25 percent growth allowance, the calculated minimum is around 550 kVA standby. The next standard size up, typically 600 or 650 kVA, would be selected.
2. Remote mine site prime power. A processing plant at 1200 m altitude with 45 degree Celsius ambient temperature and a 1.2 MW base load. The altitude derating (approximately 7 percent) and temperature derating (approximately 10 percent) reduce the effective output of a 1500 kVA genset to around 1245 kVA. Two gensets running in parallel with load sharing may be required to provide both redundancy and the necessary capacity.
3. Temporary construction supply. A construction site requiring 100 kW for tower cranes, welders, and site sheds. The crane motor starting transient is the critical sizing factor. A 200 kVA portable genset is typically specified to handle both the running load and the crane starting inrush, with margin for additional temporary loads as the project progresses.