Motor & switching

Generator Sizing Calculator for ATS Backup

Estimate the standby generator kW and kVA from the diversified running load, the largest motor-starting step, a reserve margin, and site derating — then read the ATS rating that connects it. A planning starting point; confirm the transient voltage dip and step response with the generator manufacturer.

Running load (kW)

Load power factor

Largest motor (kW)

Start method

Reserve / spare (%)

Site derating (%)

ⓘDerating covers altitude above ~1000 m and high ambient temperature. Enter the combined loss as a percentage; the generator maker’s charts give the exact figure.

Generator prime rating

— kVA

Generator kW (0.8 PF)

—

Governing case

—

ATS rating @ 400 V

—

Two cases decide the size

A generator has to satisfy the larger of two demands, then keep a margin:

Steady running: the whole diversified load in kVA, plus a reserve for future growth, corrected for site derating.
Transient step: the moment the largest motor starts while the rest of the load already runs. Motor starting kVA is several times its running kVA and causes a voltage and frequency dip the set must ride through.

Running kVA = kW / PF Start step kVA = (running kVA − motor running kVA) + motor kVA × start multiple Generator kVA = max( running × (1+reserve) , start step ) / (1 − derating)

Starting multiples used here (as a multiple of the motor’s own running kVA): DOL ≈ 6, star-delta/soft ≈ 3, VFD ≈ 1.5. A VFD or soft starter dramatically lowers the step and can shrink the generator.

Why the ATS rating follows the generator

The transfer switch carries the full standby current, so it is rated from the generator’s full-load current, not just the average load:

I = kVA × 1000 / (√3 × V)

The tool rounds up to the next standard ATS frame. Confirm the AC-31/AC-33 utilization category, pole count, and whether the neutral is switched against the earthing arrangement.

⚠Transient performance is the maker’s call. The allowable voltage dip on motor start, the alternator’s ability to hold it, and block-load acceptance depend on the specific generator and AVR. Size on this estimate, then verify the step response with the supplier.

Frequently asked questions

Why is the generator bigger than my total kW?

Generators are rated in kVA at 0.8 power factor and must also swallow the motor-starting step. Both push the rating above the simple running kW, which is why kVA and the largest start dominate the sizing.

How much does a soft starter or VFD save?

By cutting the starting multiple from about 6 to 1.5–3, the transient step shrinks, so a set sized by the start case can often drop a frame. Where the running load governs instead, the saving is smaller.

Prime, standby or continuous rating?

This tool targets a prime/standby duty for backup. A continuous base-load duty derates further. Match the rating class to the run hours and load profile with the manufacturer.

Connect it with an ATS

With the generator kVA and current set, match the transfer switch rating, category, and poles for the changeover.

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Standards referencedISO 8528 (generating sets — ratings & performance) · IEC 60947-6-1 (transfer switching equipment) · IEC 60038 (voltages).

Planning estimate for guidance only. Final generator rating, transient performance, and ATS selection must be confirmed by a qualified person and the generator manufacturer against the actual load profile and site conditions.