Selecting the Right Size Generator

Selecting Your Marine Generating Set: Live Aboard Vessels

Beta Marine manufactures a range of small compact marine generating sets which are ideal for live aboard yachts and barges. These guidelines are hopefully an aid to enable you to calculate the size of Beta Marine generating set you require, and therefore the cost involved. A generator set can easily be overloaded which will result in a reduction of voltage (less power), a reduction of frequency (Hz) (motors will run slower) and even stalling of the engine.

What do you wish to power?

Your life style is an important consideration – if you wish to be able to walk onboard and switch on everything at once then the calculation is straightforward, just add up the maximum ratings on the appliance labels / nameplates and you have the size required. If you are able to change your life style and moderate / schedule your electrical power requirements / loading by considering which appliance you are switching on and the power / load – plus possibly using one machine after the other – then you can happily install a considerably smaller generating set.

To help you decide what size of ‘mains power’ generator set you might need we suggest you fill in the following chart. Fill in two operating modes, mode 1, perhaps whilst cooking lunch and mode 2 whilst relaxing/general domestic, leave blank the items that do not apply. For example:

Appliance or Load Possible Rating Operating Mode 1 Operating Mode 2
Electric cooker 5 – 10 kW
Kettle 2 – 3 kW
Water Heater (Immersion Heater in Water Tank) 3 kW
Washing Machine 1.5 – 3 kW
Tumble Dryer 2 – 4 kW
Washer/Dryer 1.5 – 3 kW
Microwave 0.5 – 2 kW
Hair Dryer 1 – 2 kW
Food Mixer 0.5 – 1.5 kW
Toaster 0.5 – 1.5 kW
Vacuum Cleaner / Hoover 0.5 – 1.5 kW
Coffee Machine 0.5 – 1 kW
Electric Hand Drill 0.2 – 1 kW
Fridge (If Mains Powered) 0.1 – 0.4 kW
Electric Blanket 0.05 – 0.2 kW
Television and Video or DVD Player 0.1 – 0.3 kW
Battery Charger / Inverter Charger 0.2 – 3 kW
Air Conditioning System 2 – 20 kW
Total Load

There Are a Few Things That You Will Need to Know

A Few Notes and Comments That May Help

To help you fill in your list you should find that each appliance has a label / name plate on it detailing the ‘ Stated Load’ i.e. maximum possible load,  generally on the back, or underneath and it is this figure you need to start with. Please note that most fridges on smaller boats are battery operated (DC, direct current).

If you buy a generating set that is too small you will suddenly become very aware of the electrical power you need, it is generally safer to err on a slightly larger size that allows for a safety factor and any electrical items you may purchase in the future. It may therefore be better to spend a little time talking to other people and getting the size calculation correct before you buy.


If you wish an electric cooker you need to consider the physical size and power when buying – a standard kitchen oven will generally have a 4kW oven and 4 hot plates from 1 kW to 2 kW each, totally about 10kW maximum possible load, and if you wish to switch everything on at once you need to allow for 10kW continuous loading. However it may be that the oven is set at 180°C and 2 hot plates are set at low and medium (all thermostatically controlled) switching in and out,  so the actual load may only be 6kW or less. A full Sunday lunch may push the loading up for a few hours.


Varies considerably dependant upon the size of motor yacht / wide beam and it’s size can vary from 2 kW, up to 20 kW. You need the exact details of the installation and the manufacturer will supply you with starting current and running current details – if in doubt please ask us for help.


Is powerful 1.5kW to 3kW, but only tales 2 or 3 minutes to boil.


If a mains powered fridge is selected it will need continuous power. This is normally via an inverter powered by your battery bank otherwise you would have to run your generator set continuously. Most fridges on boats are small and powered directly from the batteries. The generator set will have to power your battery charger or inverter charger and this should be taken into account when sizing. Usually the batteries are also charged from your main engine driven battery charging generators too.


These require from 1 to 3 kW each and the cycle can take from 1 hour to 2 hours on load.


We would always recommend that you fit a calorifier for hot water, heated by the engine cooling system when the engine is running.  This gives you ‘free’ hot water.


This is generally a personal decision – if hot water is required then there are alternatives. A solution is an immersion heater and it is normally mains AC powered and 2.0kW, requiring either a shore line, Travel Power, or a Generating set

Having calculated / guessed the actual power you need, you need to add together the worst case scenario of the possible appliances that could be on at the same moment – providing you with the minimum level of comfort you require, and you may wish to plan for the future adding a small factor for any future additions.

Some Questions You Should Be Asking Yourself Are:

Generators Typically Supplied by Beta Marine Ltd

If you ‘live aboard’ then you will need a BetaGen with an acoustic enclosure; noise is a very important issue when living right next to the generating set. We also sell good quality ‘Sound deadening’ material at reasonable prices. We have found that some customers who moderate their electrical loading and sequence their appliances / electrical load reasonably – often consider the following:

Small Narrowboat

With only interior lighting, a television, microwave and a fridge, you need a suitable domestic battery bank and an inverter that is charged by the propulsion engine during the day, possibly a Travel Power 3.5kVA.

Narrowboat 60 foot

With 3kW Washer/Dryer, TV/Video player, 0.2 kW DC Fridge, 2kW Electric Kettle, Microwave, Shower pump, inverter charger – we often supply a 3.5kW or 5kW Travel Power mounted on the propulsion engine.  If you have the space – maybe a small BetaGen.

Wide Beam / Dutch Barge

With small electric cooker, 2kW Washing machine, 3kW Tumble dryer, 2kW Immersion heater,  0.5kW Fridge/Freezer plus television, video player, battery charger – such customers often specify a 1500 rev/min BetaGen 7 or BetaGen 10.

Sailing Yacht – 25 to 35 foot

With 0.2 kW DC fridge, microwave, electrical navigation equipment, lighting, water pumps, inverter charger – we would recommend a suitable domestic battery bank and possibly a Travel Power driven by the propulsion engine.

Sailing Yacht – 60 foot

With 0.2 kW DC fridge, electrical navigation equipment, lighting, water pumps, battery charger (possibly air conditioning) – customers have taken BetaGen 4/2 or BetaGen 7.

Power Boat – 34 foot

With Air conditioning, small electric cooker, 0.2 kW fridge, electrical navigation equipment, lighting and water pumps – maybe a BetaGen 7 or 10

Power Boat – 40 foot

With Air conditioning, small electric cooker, 0.2 kW fridge, electrical navigation equipment, lighting, water pumps, battery charger – perhaps a BetaGen 10 or Beta 11.

You will probably also have 12 volt DC (direct current) electrical loads. These are usually powered from the domestic battery bank and battery charger (or inverter charger):