Generating AC power

Generating AC power onboard.

Many of the electrically powered devices on our boats can operate directly from 12 volt, DC battery power. Electronic instruments, autopilots, cabin lights, engine start, and many other devices use 12 volts. However these days our boats are filled with all sorts of modern equipment, from air conditioning, microwaves, TV, refrigeration, computers etc all of which require 120-volt, 60Hz shore power (US).

In this article we will discuss the various methods of generating AC power on a boat. Each has its own features and associated costs.

Ways to generate AC power onboard

  • DC battery bank to inverters
  • Alternator from engine
  • Shore power
  • Portable genset
  • Generator AC

How to choose generating equipment largely depends on the wattage you will need to power your onboard equipment. Here are some devices and approximate power consumption. These are generic numbers so check the models you have. Heat generating devices consume the most power.

Maybe means that if you have a large enough inverter and battery bank and do not run the device for long, an inverter may work.

Energy Consumption

Table A   Inverter Generator
Water heater 3,000 watts no yes
Microwave 1,000 watts maybe yes
Refrigeration 500 watts maybe yes
Hair dryer 1,000 watts maybe yes
Air Conditioning 1,200 watts no yes
Lower consumption devices
Computer 1,200 watts yes yes
TV 45 Watts yes yes
DVD 15 Watts yes yes
Total Watts 5,680 Watts

Start up Loads

You will need to calculate starting up watts in the total watt calculations. A 100 watt TV may use 500 watts at startup. If you do not have those 500 watts available, the TV will not start. Air conditioners have large start up power requirement, which means you need to look at that not just the continuous running load.


Battery Management

Blue Seas battery switch

Warning; with an inverter you can power your AC appliances, quietly without fuss, however its will be too easy to drain your battery bank. Therefore have some check mechanism, so your not left helpless. A separate engine battery is a must, with a battery switch isolating it.

Unfortunately inverters are power hungry and take more energy from the source (Batteries) than they supply in AC power. So how much are you going to drain the batteries?

Inverters have alarms and automatic shutdown, so they will protect the inverter but not the battery. The ProSport 150 watt inverter above has a shut down of 10V DC and an alarm at 10.5V DC. This is fine for the inverter, but the batteries need a cut off voltage of 12.1-12.2 volts.

Powering a 100 watt device from an inverter for 1 hour will drain around 10 amp hours from the Battery. Next you will need to recharge the battery and assuming a 15% energy loss you will need to charge 11.5 amp hours back into the battery.

Battery management dictates we only discharge deep batteries to 50% max before recharging. Two group 27 lead acid flooded cell batteries have a combined 200 amp hours capacity. At 50% discharge, you have 100 amp hours to burn.

If you run the engine the alternator will add back some amps to limit the battery discharge. It is also possible to add solar or wind power to replace battery amps.



Inverters transform a boats DC battery power into AC. Inverter models range from 3000 watts to less than 100 watts, and depend on the amount of battery power available. Portable inverters go up to around the 300-500 watt range. For larger inverters you will need to mount them in a cool, dry, well-ventilated space close to the battery bank.

Inverters come in modified and pure sine water options. Pure sine wave is recommended for running entertainment systems and computers etc. Modified sine wave may be good for some TV sets and many radios. Your household AC is pure sine wave.

To pick an inverter there are two questions. First, what AC appliances are you going to run? Add up the wattage for each appliance and chose an inverter that delivers more watts of continuous power. It is no good having a 1000 watt inverter if the appliances you run eat up more than 1000w (assuming they are running at the same time). A 7 amp microwave @ 115V will need 805 watts, using Volts x Amps = Watts. To size an inverter add up the load requirements, see table A for ideas.

Inverter Pros Cons
Simple solution Battery drain
Cheap May not run high wattage devices
Small space requirements Short run times
No fumes or noise


Xantrex PROwatt Sine Wave Inverter.

Xantrex PROwatt

The new PROWatt SW inverter from Xantrex is a compact true sine wave inverter. PROwatt inverters come in 800, 1000 and 2000 watts. This inverter is perfect for powering Cameras, cell phones, computers and other portable equipment. The inverter measures 3.5 x 8.7 x 13.4″ for the 1000 Watt model, which makes it small enough to fit in many places without taking up valuable space. This inverter is easy to use and easy to install, just grab some Heavy Duty Inverter Cables and connect to your battery.


ProSport Cup Holder Power Inverter


This little inverter fits in a cup holder and plugs into a 12 volt receptacle (cigarette style plug. Powering up to 150 watts, the power inverter provides power for laptops, cell phone chargers, radios, portable TVs, camcorders, small power tools etc. At only $40 this is a good option.




Engine Alternator

Some equipment can be run from an engine driven alternator. An example of this is the Engine driven refrigeration. This can cool down your ice box while motoring and then keeps the ice box cool for long periods when the engine is not being used. Other devices like water heaters can do their job while you are using the engine, and hot water is available later at anchor.

The idea being if you can limit the at anchor needs to small wattage devices you may get away with an inverter. This alternative works better for power boats, while sailboats may not have enough engine time. This will not help you with air conditioning however.

Warning; running the engine in neutral to power the alternator is not good for the life of the engine, and running the engine for an extended period of time on a mooring will not endear you to neighbors.

Engine alternator Pros Cons
Does not use battery Will not run Air Conditioning
Powerboats where engine is always running If you sail, not enough engine time


Shore Power

Shore Power may be the simplest way to get AC power onboard. You basically can run any device that you would have at home. Good reasons for having shore power include; you have a Marina which has Shore-Power pedestals, you use AC appliances at the dock. Even if you do not do any of the latter, you can still tie up, turn on the shore power and go home, leaving the shore power to keep your batteries fully charged. Check your Marina to see what amp service is available.

Shore power systems consists of;  
Shore Power inlet located near transom and within 10ft of breaker panel
Galvanic isolator Protect against stray currents
AC breaker panel Distributes AC power
Outlets in cabin GFCI receptacle are best
Battery charger Shore AC can be used to charge batteries

Galvanic Isolator

Typical specs of a shore power installation; 30 amp 110VAC shore power system with 6 outlets, 50ft shore power cord and ProMariner Galvanic Isolator with monitoring device.

When you connect to any shore system you are relying on the source for good quality AC power. This may not be the case and so this is why its recommended to have a Galvanic Isolator. The Galvanic Isolator protects against fluctuating voltage, power spikes, bad grounding and corrosion from neighboring boats. Galvanic Isolators cost between $100 and $500. Better than a Galvanic isolator is the Isolation transformer. These use magnetic connection rather than electrical connection but cost between $500 and $5,000.

Magnum MS Series Inverter/Chargers

Magnum MS

Incorporating a battery charger in a built-in inverter makes great sense. When you are on shore power you can charge the battery and power the cabin outlets. When away from shore power you can use the built in inverter to power AC devices.

Fixed-mount inverters are available with built-in shore-power switching systems. When an auto shore-power switching inverter is installed, shore-power flows through the inverter and is then delivered to the boats AC system. The inverter operates in standby mode when shore power is present.

To size an inverter/charger add up the load requirements, see table A for ideas.

The simplest shore power system is an extension cord

Shore power

Dockside 30A to 15A Adapter with GFCI

If you don’t have a built in shore power system, you can use this connector to the docks AC pedestal and run an extension cord to the boat.



Shore Power Pros Cons
Marina AC supply Will not help at anchor
Keep batteries fully charged Expense
Air conditioning at dock If shore power fails while you are away from the boat


Fixed Mounted Generator


If you live in an area which has very hot & humid temperatures then having a generator will let you run air conditioning at anchor. Also you will be able to run refrigeration and other devices too large for an inverter. Generators for recreational boats run from 3-15 KW and even higher if necessary. The good news is that as technology improves we are getting more KW, in a smaller package, quieter than we did 10 years ago. One of the biggest obstacles to having an onboard generator has been the size.

A genset will deliver as much AC power as you may need, for as long as you have fuel. Generators with sound shields can be very quiet, especially when fitted with exhaust systems with split cooling water & exhaust gas.

As with all the AC power options you will need to add up your equipment loads to find out how large a genset you need. see table A for ideas.

The biggest issue with a generator is the space required and the installation. The diagram below shows the installation for the red Westerbeke above. You will need thru hull fittings for water intake, and Exhaust system, muffler, anti siphon and then you need the wiring.

Generator Pros Cons
Does not drain batteries Noise
Air conditioning Space requirements
Refrigeration Expensive
Fuel consumption


A typical genset suitable for an air conditioned 30-foot boat will cost about $4,500, and that does not include installation costs.



Portable generator

Honda EU1000iA
Honda EU1000iA

The simplicity of a portable generator like this is the ability to directly plug in an appliance from lights, power tools, computers to battery charging. Honda EU1000iA generator generator

little Honda generators and the line are very quiet, self-adjusting to match the load, and easy to handle (the 1000 watt unit is 29 pounds, 43 for the 2000 watt). They also have a direct battery charge output. There is a special cable for charging batteries.

If you were to keep this on the dock, you should run a 20amp or maybe larger cable with a power strip and built in circuit breaker on to the boat and use this to plug into. Do not get any water on any cables and do not use a household extension cord. Safety is a big concern with a portable generator. You have to think about CO, hot exhaust, and Fuel spills. Fuel storage, and maintenance are some of the other issues.

To size a portable generator add up the load requirements, see table A for ideas.

Generator Pros Cons
Does not drain batteries Noise
Small space compared to a fixed generator Fuel gasoline storage
If you already have gasoline for an outboard On deck Storage
$960 and you can take it home Fumes