Marine Generator Sizing Calculator

REF: MAR-GEN-SIZE-802

Determine the recommended electrical power output (kW / kVA) for your marine genset based on continuous operating loads, inductive motor starting surges, and a selected reserve margin. Proper sizing helps avoid both overload and persistent light-load operation.

INPUT PARAMETERS
HVAC, galley, watermaker, battery chargers.
Additional peak/inrush load from the largest motor starting event.
Applied to the continuous load before the calculation compares it with the peak starting requirement.
Calculation assumption: peak requirement = continuous load + startup surge. This is a simplified power-based screening calculator; actual generator selection should also consider motor starting current, generator transient response, voltage/frequency dip limits, and the manufacturer's rating data.
> RECOMMENDED_GENERATOR_SIZE
10.0 kW
(~12.5 kVA @ 0.8 PF)
CONTINUOUS + RESERVE 7.8 kW
PEAK REQUIREMENT 10.0 kW
CONTINUOUS LOAD 65.0% of rating

Navigating Marine Generator Sizing Rules

Selecting a marine generator requires a balance between continuous load, motor starting demand, reserve capacity, and the generator manufacturer's transient-performance limits. An undersized set may struggle during peak motor starts, while a significantly oversized set may spend more time at low load.

Key Engineering Rules & Considerations:

  • Inductive Inrush Current (Surge): Electric motors such as air-conditioning compressors, watermakers, and refrigeration units can require substantially more power/current during startup than while running. Soft-start equipment can reduce starting demand, but the exact improvement depends on the equipment.
  • Continuous Load: Generators should be selected so normal operating demand is comfortably within the manufacturer's rated capacity. The original calculator uses a 70%–80% target range as a rule of thumb, but the ideal operating point depends on the engine and genset design.
  • Wet Stacking / Underloading: Long periods of very light diesel loading can contribute to poor combustion and deposits. Avoiding chronic underloading is important, but the exact minimum load guidance should come from the specific engine/generator manufacturer.
  • Power Factor (PF): The calculator displays an approximate kVA value using kVA = kW ÷ 0.8. Actual generator selection should use the real load power factor and the generator's published kW/kVA ratings.
Genset Tip: If your vessel has large swings in demand, compare the minimum and maximum operating profiles. Parallel smaller generators can improve efficiency and redundancy when the vessel's electrical architecture supports it.