Calculating generator power involves three steps: ① Calculate continuous load (sum of kW for all simultaneously operating equipment); ② Add maximum starting surge (motor starting multipliers: direct start 6–8x / star-delta 2–3x / VFD 1.5x); ③ Multiply by a safety factor of 1.1–1.25 (for future expansion). Simultaneously, convert using kVA = kW ÷ Power Factor (0.8)—calculating only steady-state load leads to undersizing (motors consume the most power at the moment of startup).
I.
Continuous Load Calculation
List the loads: Rated kW of all equipment running simultaneously (lighting, motors, air conditioners, pumps); Demand factor: Not all equipment runs at full load (multiply by 0.8–1.0 based on actual usage); Three-phase current: I (Amps) = kW × 1000 ÷ (V × 1.732 × PF); Example: Factory continuous load of 30 kW (pumps + lighting + air compressor)—list items first, then sum them up (resistive loads have a PF of 1; inductive motors have a PF of approx. 0.8).
II. Starting Surge (Most Easily Overlooked)
Motor Starting: Starting current is 5–8 times the rated current (instantaneous power surge); Starting Multiplier: Direct-on-line (DOL) 6–8x, Star-Delta 2–3x, VFD 1.5x; Largest Single Unit: Add the starting surge of the largest motor to the total load; Example: 30 kW continuous load + one 10 kW motor (DOL start: 10 × 6 = 60 kW) → Total requirement (30 + 60) × 1.2 ≈ 108 kW → Select a 120 kW generator set—starting surge often determines the unit size (must be calculated if motors are present).
III.
kW to kVA Conversion
Formula: kW = kVA × PF (Power Factor), kVA = kW ÷ PF; PF Values: Resistive load 1.0, mixed motor load 0.8, inductive load 0.7–0.85; Example: 25 kW ÷ 0.85 ≈ 29.4 kVA; Note: Generators are often rated in kVA (standby power); check the nameplate for conversion—kW is actual power output, kVA is apparent capacity (select based on kVA and convert using PF).
IV. Environmental Derating
High Temperature: Ambient temp >40°C;
derate by 3–5% for every 5°C increase; High Altitude: Altitude >1000m; derate by 3–5% for every 500m increase; Continuous vs. Standby: Standby power (emergency) is 10–20% higher than continuous power (prime); Margin: Add 15–25% for future growth—apply derating factors when selecting units for tropical or high-altitude regions (e.g., choose a larger unit size for high-altitude locations for the same workload).
V. Calculation Steps
First, list the loads (rated kW of all equipment); second, calculate continuous kVA (total kW ÷ PF); third, add the maximum starting surge (largest motor × starting multiplier); fourth, apply environmental corrections and a safety margin (×1.1–1.25 + derating); fifth, select the nearest standard power rating (e.g., 50/75/100/120/150/200 kW)—Generator power: continuous load + maximum starting surge + 15–25% margin; kVA = kW ÷ 0.8;
derate for high temperatures and high altitudes (follow the three calculation steps and five correction steps to select the correct generator set).
