Most boats that need a generator at all land somewhere between 4.5kW and 38kW, and the right size inside that range comes down to two numbers: the load your vessel draws continuously, and the surge when the largest motor on board starts. Get those two right and the model almost picks itself. This guide walks through how we work them out, and the traps worth knowing about before you buy.
Start with a load count, not a boat length
Length is a rough proxy at best. A 45-footer with air conditioning, an electric galley and a watermaker needs far more power than a 55-footer that runs none of them. So the first step is a list: every AC appliance on board, its nameplate wattage, and whether it runs continuously or in bursts.
Air conditioning is usually the biggest single draw, and the galley is usually second. Typical figures, as a starting point only:
| Load | Typical draw | Pattern |
|---|---|---|
| Air conditioning, per 16,000 BTU unit | roughly 1.5kW running | Continuous in summer |
| Induction cooktop or electric oven | 1.8kW to 3kW | Bursts, mealtimes |
| Hot water system | 1.2kW to 1.8kW | Bursts |
| Watermaker | roughly 1kW to 2.5kW | Continuous while making water |
| Battery charger | 0.5kW to 2kW | Continuous after a night at anchor |
| Pumps, lighting, refrigeration, electronics | 0.5kW to 1kW combined | Continuous |
Your own nameplates are the real data. The exercise is to add up what genuinely runs at the same time on a hot afternoon at anchor: air conditioning on, batteries charging after the night before, someone cooking. That worst realistic hour is your continuous load. For the cruiser described above it might come to 6kW or 7kW; a vessel without air conditioning might struggle to reach 2kW.
The second number: starting surge
Electric motors draw a multiple of their running current for the second or two it takes them to spin up, commonly two to three times. On a vessel the loads that matter here are the air-conditioning compressors and the larger pumps. A generator sized exactly to your continuous load can stall or trip when a compressor kicks in on top of everything else already running.
The working rule: the generator carries your continuous load with enough headroom to start the largest motor on board on top of it. Compressor soft-starters can shave that surge and are worth having on marginal installations, but headroom in the generator is the honest fix.
Why oversizing is the more common mistake
Undersizing announces itself: lights dim and breakers trip. Oversizing is quieter and more common, because bigger sounds safer. It isn't. A generator spends most of its life at partial load, and a marine diesel run lightly loaded for long periods glazes its cylinder bores, carbons up and wet-stacks. The engine that never works hard wears out faster than the one that does.
As a guide, you want your typical evening load to sit somewhere near the middle of the generator's rating, not down at 15 per cent of it. This is why we size to a calculated load rather than adding a generous margin "to be safe". The margin has a real cost in engine life, and in fuel.
50 Hz or 60 Hz in Australia
Australian shore power and Australian-supplied appliances run at 230 V / 50 Hz, so Australian-flagged vessels almost always want the 50 Hz rating. Vessels built for the US market, and some that cruise internationally, are wired for 60 Hz throughout. The same generator carries a different rating at each frequency (a 9kW machine at 60 Hz is a 7kW machine at 50 Hz, because it runs at 1500 rpm instead of 1800), so the frequency decision changes the sizing arithmetic, not just the wiring. Check what your vessel's equipment actually is before assuming; a mixed installation is an electrical job to resolve, not a generator selection problem.
Typical sizes by vessel
With those caveats made, the ranges below are where vessels commonly land. Treat them as a sanity check on your load count, not a substitute for it.
| Vessel | Common range |
|---|---|
| Coastal cruiser to about 40ft, no air conditioning | 4.5kW to 6kW, if a generator is needed at all |
| 40ft to 55ft cruiser with air conditioning | 7kW to 12kW |
| 55ft to 70ft motor yacht, full house loads | 14kW to 20kW |
| Larger motor yachts and superyachts | 20kW to 38kW, often twin installations |
One honest caveat at the small end: if your vessel runs no air conditioning and no electric galley, a generator may be the wrong purchase altogether. A good inverter and a properly sized house battery bank cover many sub-40ft vessels better, for less money and less maintenance. We will tell you so if that is what your load count says.
What we check before recommending a model
A load calculation is the start of the job, not the whole of it. Before we put a model number on a quote we also measure the engine room space and access route the generator has to fit through, look at how the exhaust and raw-water plumbing will run, and talk through sound: where you sleep, where the generator sits, and whether the optional sound enclosure is worth it for how you use the vessel. It usually is for liveaboards.
Marine Solutions Australia is an authorised Northern Lights dealer, and the range spans 4.5kW to 38kW, so the sizing work isn't steering you toward a model we happen to stock; the answer falls wherever your loads put it. We install and commission what we supply, sea-trial it under real load, and service it afterwards from our Gold Coast City Marina base.
If you are weighing up a generator, send us your vessel's details and what you run on board through the contact page. The load calculation costs you a list of appliances and a phone call.
