Generator Sizing Calculator: engineer's guide
Generator sizing has to cover both steady-state load and the transient step-load caused by the largest motor start.
Open the Generator Sizing calculator →
What this calculator does
The calculator sums essential and life-safety load, applies a starting kVA multiplier for the largest motor, and rounds up to the next standard genset rating.
Formulas used
- Running kVA:
Srun = Pessential / cosφ - Starting kVA of largest motor:
Sstart = kWmotor × Kstart / (η × cosφstart) - Rated genset kVA:
Sgen ≥ max(Srun, Srun,other + Sstart)
Variables
- Kstart = motor starting factor: 6 for DOL, 3 for star-delta, 2 for VFD, 1.5 for soft start
- cosφstart = motor power factor during start, typically 0.3
Worked example
Scenario: Essential load 400 kW at cosφ 0.85, largest motor 75 kW DOL start.
- Srun = 400 / 0.85 ≈ 470 kVA
- Sstart = 75 × 6 / (0.9 × 0.3) ≈ 1667 kVA
- Srun,other = (400 - 75) / 0.85 ≈ 382 kVA
- Sgen ≥ 382 + 1667 = 2049 kVA. Round to 2250 kVA.
- If star-delta is used, Kstart drops to 3 and required genset drops to about 1220 kVA (1250 standard).
Common mistakes to avoid
- Using running kVA only and ignoring motor starting
- Neglecting UPS charging load, which appears as a sudden step
- Not derating for altitude and ambient temperature above ISO conditions
- Ignoring harmonics from VFDs and switch-mode power supplies
Frequently asked questions
Prime, continuous, or standby rating?
Standby for buildings with utility as primary; prime for sites with genset as primary supply; continuous for constant-load applications.
Do I need block loading?
For life-safety loads, yes — the fire pump and smoke fans must start on the first block. Non-essential can be shed and re-added in blocks.
Altitude derating?
About 3% per 300 m above 1000 m; check manufacturer curves.
Standards and references
ISO 8528, NFPA 110, BS 7698
Related tools on this site
Open the calculator that pairs with this guide: Generator Sizing Calculator.
Verification note
This guide is planning support only. Final design must be verified against the applicable project specification, authority requirements, manufacturer data, and the standards cited above. Consult a licensed engineer for construction-issue calculations.