Generator Sizing — Complete Professional Guide

Proper generator sizing prevents under-sizing (failure under load) and over-sizing (wet stacking, wasted fuel). This 5-level guide covers every aspect.

Level 1: The Basic Formula

Required kVA = (Total Running kW + Largest Motor Starting kW) / (Power Factor x Derating Factor) The derating factor accounts for altitude (3% per 300m above 1000m) and temperature (1% per 5°C above 25°C).

Level 2: Connected Load Method

List every electrical device, its running kW, and starting surge multiplier. Motors draw 3-7x running current at startup. Sum all running loads, then add the LARGEST single starting surge.

Level 3: Demand Factor Analysis

Not all equipment runs simultaneously. Apply demand factors: lighting (100%), HVAC (80%), kitchen equipment (60%), office equipment (50%). This prevents over-sizing while maintaining safety margins.

Level 4: Measurement-Based Sizing

For existing facilities, use a power analyzer to record actual load profiles over 7-30 days. Identify peak demand, average load, and load factor. Size the generator at 125% of measured peak demand plus 20% future expansion.

Level 5: Dynamic Load Analysis

For facilities with large motor starting (hospitals, water treatment), perform transient stability analysis. Calculate voltage dip during motor starting: Vdip = (Motor Starting kVA) / (Motor Starting kVA + Generator Short Circuit kVA). Keep Vdip below 20% for sensitive equipment.