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Generator Sizing Calculator

Calculate the exact required kVA / kW capacity for standby or prime power diesel generators to safely sustain surge currents.

Project Specifications
Calculated Output
Recommended Genset Size
0 kVA
0 kW
Total Surge Load (kW)
0 kW
Required Engine (kW)
Comprehensive Guide

Genset Engineering & Motor Staring Physics

Understand why generators must be oversized to account for induction motor locked-rotor currents.

The Physics of Motor Starting

When an induction motor (like a large water pump or HVAC compressor) first turns on, its internal rotor is physically stopped. Because there is no back-Electromotive Force (EMF) generated yet, the motor acts almost like a dead short, pulling massive amounts of current from the grid. This is called the Locked-Rotor Current (LRA). A generator must have a massive enough alternator end (the copper windings) to physically supply this surge of energy without its output voltage collapsing.

The Sizing Equation

$$\text{Peak Surge kW} = \text{Base kW} + (\text{Largest Motor kW} \times \text{Multiplier})$$ $$\text{Required Engine (kW)} = \text{Peak Surge kW} \times (1 + \text{Margin \%})$$ $$\text{Genset Rating (kVA)} = \frac{\text{Required Engine (kW)}}{\text{Power Factor}}$$

Why Power Factor dictates Genset Size (kVA)

Generators consist of two distinct parts: an engine (which supplies real mechanical kW) and an Alternator (which limits maximal electrical flow measured in kVA). Since the alternator must deal with circulating reactive energy (kVAR) that merely bounces back and forth without doing work, it limits total generating capacity. Therefore, all generators are rated primarily in kVA based on a standard $0.8$ Power Factor threshold.