Generator Sizing
Enter the running load, power factor and motors to start and I compute the generator capacity (kVA) with margin, the starting peak and the altitude/temperature derate.
Calculation breakdown
| Item | Value | Detail |
|---|
How it is computed · running, starting and derate
1. Running load in kVA:
kVA_run = kW / PF.
2. Recommended with margin:
kVA_rec = kVA_run × (1 + margin).
3. Starting peak of the largest motor:
kVA_start = (kW_motor / PF) × factor
(DOL 6×, Y-Δ 3×, soft starter 2×). The generator must cover
max(kVA_rec, kVA_start).
4. Site derate: altitude above 1000 m
−3.5 % / 300 m; temperature above 40 °C
−2 % / 5 °C. Combined factor
f = (1 − d_alt) × (1 − d_temp).
5. Required nominal:
kVA_nominal = kVA_rec / f.
! This is an estimate. The starting peak is transient (seconds) and depends on the load torque; check the manufacturer's voltage-dip curve and the motor type. Validate the real PF and expected growth before you buy.
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How it works
The calculator sizes a generator set from the running load in kW and the power factor: it converts to apparent power with kVA = kW / PF, adds a configurable reserve margin (typically 20–30 %) and compares that value against the starting peak of the largest motor, computed as (motor kW / PF) × factor depending on the method: direct-on-line DOL ≈ 6×, star-delta ≈ 3× and soft-starter ≈ 2×. The generator must cover the larger of the two.
It then applies the site derate: above 1000 m of altitude it subtracts 3.5 % per 300 m and above 40 °C ambient it subtracts 2 % per 5 °C, combining both factors as f = (1 − d_alt) × (1 − d_temp). The recommended nameplate rating is kVA / f, and the breakdown shows whether the size is driven by the running load or by the starting transient. It is an estimate: the peak lasts seconds and depends on the load torque, so validate against the manufacturer voltage-dip curve.
Example: 40 kW load with a 10 kW motor on direct-on-line start
- Running:
40 kW / 0.8 = 50 kVA; with a 25 % margin:62.5 kVA. - Motor start:
10 / 0.8 = 12.5 kVA × 6 (DOL) = 75 kVApeak. - With no derate (below 1000 m and 40 °C), the rating is
max(62.5, 75) = 75 kVA: here the starting peak, not the running load, sets the size.