This marine generator sizing guide begins with a load schedule, not the sum of every appliance label. The correct capacity must handle the loads that run together, the starting surge of motors and compressors, expected duty cycle and a reasonable reserve without spending most of its life severely underloaded.
Marine generator sizing guide: load worksheet
| Load | Record | Special consideration |
|---|---|---|
| Air conditioning | Running watts/amps per unit | Compressor starting surge and staging |
| Water heater/cooking | Resistive load | High running demand but little startup surge |
| Pumps and refrigeration | Running and locked-rotor/start data | Several motors may start close together |
| Battery chargers/inverters | Maximum AC input | Demand changes with battery state and settings |
| Navigation/house loads | AC loads only for generator sizing | Some are supplied indirectly through chargers |
Running load vs startup surge
Electric motors and compressors can require substantially more current while starting than while running. The generator must maintain acceptable voltage and frequency during the event. Soft-start equipment and deliberate load sequencing can reduce the peak, but only when approved for the appliance.
Use a realistic simultaneous-load plan
A boat may not run all air conditioners, water heating, cooking and high-output battery charging at the same time. Write operating scenarios for anchoring, overnight, cooking and peak climate load. Automatic load management can shed or delay nonessential loads.
A worked load scenario
Imagine two air conditioners, a battery charger, refrigerator and occasional water heater. First total their measured or nameplate running loads for the combinations actually used. Then identify which compressor has the largest start requirement. If both AC units may call for cooling together, assume overlapping starts unless a sequencer prevents it. Finally add essential lighting, pumps and a sensible reserve.
Repeat the calculation for a quieter night mode and a heavy daytime mode. These scenarios reveal whether load management can reduce generator size without sacrificing essential service.
Voltage, frequency and regional compatibility
Confirm 120/240 V or 230 V distribution, 50 or 60 Hz frequency, phase and neutral/ground arrangement. Imported appliances or a generator built for another region may not be compatible even when the kW number looks correct.
Installation factors beyond kW
- Wet-exhaust, cooling-water intake and anti-siphon design.
- Fuel supply, filtration, return and shutoff arrangement.
- Combustion air, compartment ventilation and heat rejection.
- Sound enclosure, mounts, service clearance and lifting access.
- Fire protection, alarms and carbon-monoxide detection.
- Shore/generator transfer interlock and circuit protection.
- Manufacturer derating for temperature and installation conditions.
Avoid oversizing and undersizing
An undersized set may suffer voltage drop, overload and failed motor starts. A greatly oversized diesel generator can spend too much time at light load, which may be poor for efficiency and engine condition. The right answer balances the operating profile, not just one peak number.
Fuel, sound and maintenance access
Compare fuel type with the main engine and onboard storage, then estimate consumption at realistic load rather than maximum rating. Sound data should state test conditions; enclosure design, exhaust separation and resilient mounts influence what reaches the cabin.
Leave physical space to change oil, filters, belts, impeller and zincs where fitted. A generator that fits through the hatch but cannot be serviced is not a successful installation. Plan removal access for major components and keep hot surfaces and exhaust away from combustible materials.
Commissioning measurements
After installation, a qualified technician should verify no-load and loaded voltage and frequency, cooling flow, exhaust integrity, charging load, transfer operation and alarm shutdowns. Record baseline readings so future changes are easier to diagnose.
Bring a load worksheet, voltage, frequency, fuel and installation constraints.
Frequently asked questions
Can I add every appliance wattage to size the generator?
That can overstate normal demand. Use realistic simultaneous loads, while still accounting for the largest startup event and reserve.
Why does an AC trip the generator when it starts?
Compressor surge, low voltage, weak connections, poor frequency control or insufficient generator capacity may be responsible.
How much spare capacity is required?
Use the generator maker’s sizing method and derating guidance. The right reserve depends on load type, environment and future additions.
Technical reference: Cummins Marine Generator Sizing Tool. Final sizing and installation require a qualified marine electrical and mechanical assessment.
