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LoadCairn / Standby power

Generator sizing calculator

For critical-facility engineers and facility managers, estimate standby generator real-power and apparent-power requirements from the running load, power factor, planning margin, and an optional motor-start allowance.

Load assumptions

Illustrative example · editable

The starting case is 100 kW, 0.80 load PF, 20% margin, and no motor-start allowance. These are editable examples, not site operating data.

Enter the connected or expected running real-power load above zero.

Use a measured or equipment-specified value when available.

An editable planning allowance applied to both calculated requirements.

Adds transient starting kVA on top of the full entered running load. It does not subtract that motor’s running demand.

Planning estimate only

Estimated capacity

Indicative load requirements and an illustrative 0.8-PF generator reference size.

Planning estimates updated. Required 120 kW and 150 kVA. Reference size 150 kVA.

Required real power · planning only

120 kW

Required apparent power · planning only

150 kVA

Next reference size · planning only

150 kVA120 kW at 0.8 PF

The reference ladder rounds up to 150 kVA. Actual manufacturer ratings and availability vary.

Formula

Required kW = total load kW × (1 + margin % ÷ 100). Required kVA = (total load kW ÷ load PF + motor-start allowance kVA) × (1 + margin % ÷ 100). Reference size = the first ladder rating ≥ max(required kVA, required kW ÷ 0.80).

Live calculation trace · planning only

  1. Running load: 100 kW ÷ 0.8 PF = 125 kVA.
  2. Required real power: 100 kW × 1.2 = 120 kW.
  3. Required apparent power: (100 kW ÷ 0.8 PF + 0 kVA motor-start allowance) × 1.2 = 150 kVA.
  4. Size basis: max(150 kVA, 120 kW ÷ 0.80) = 150 kVA.

Assumptions and limitations

Planning estimate only

The optional motor-start value is an additive transient kVA allowance on top of the full entered running load. This simplified model does not subtract the motor’s running demand if it is already included in total load.

Motor-start performance depends on the motor, starter, concurrent load, and acceptable voltage and frequency dip. The 0.8-PF generator rating and size ladder are illustrative; actual manufacturer offerings and ratings vary.

This estimate does not model altitude or temperature derating, step loading, or harmonics. It is not equipment selection, engineering design, code compliance, or a guarantee. Have a qualified generator supplier or electrical engineer evaluate the actual load profile and site.

Source notes

Planning references · not endorsements

NFPA 110, Standard for Emergency and Standby Power Systems

The relevant system standard for emergency and standby power systems. NFPA 110 does not endorse this calculator or its simplified formula.

Cummins, “How to size a genset”

Explains the relationship between kW and kVA and why motor starting, environmental conditions, and step loading affect generator selection.

Caterpillar commercial generator sizing guide

Outlines a more detailed manufacturer process based on the actual loads and installation site.