When I first wrote this page just 4 years ago mains electricity on boats was a very new thing, and even then it tended to be on privately owned boats, often supplied only when moored up via a shore link. These days a look through the various hire brochures reveal many boats with at least some mains power on board.

As far as boats go there are 2 electrical systems on board, the 12 volt stuff and the mains stuff.

So let's start by looking at the 'normal' 12V stuff and I'll talk a bit about mains at the bottom of the page.

The 12V stuff

The electrical system on a boat is very similar to that of a car, unless there is provision for mains (more of which later) the only electrical power on board will be a 12 Volt DC supply. Many boats now include a cigarette lighter socket in the boat which will allow you to charge up anything that you could from a car, camcorders and mobile phones being the most common.

Like a car, if you leave the lights on too long the battery will go flat, but on a boat in addition to the lights you will also have a pump for the water system, possibly an electric fridge, a radio c/d, a TV set, in other words a normal one battery system would soon fail, and you can't push start a boat!

To get round this problem a twin battery system is fitted, one battery is exclusively for starting the engine, and a second set of batteries connected together (normally two or three) supply all of the boats' other electrical supply requirements. The system is designed so that even if all of the service batteries are exhausted there is still one to start the engine, which via the alternator will first recharge the engine's battery, then start to charge all of the service batteries. Subsequently if you stay in one place for any length of time (a day or so) you will need to run the engine for a few hours to recharge the system batteries.

Shaver points are normally provided, though these must ONLY ever be used for shavers, although they produce mains voltage it is only at the very small current needed for shavers.

The 240V stuff

Most hire boats that offer 'mains' do so by using an inverter. Ideal for short term use - bursts of a few minutes for larger items like toasters, microwaves etc. or for longer use with less power hungry items like TVs. An inverter uses electronic circuits to convert DC to AC, in simple terms it does this by alternately reversing the polarity of the supply, then using transformers to increase the voltage to something near 240. There is a direct trade off between volts and amps, so even allowing for perfect efficiency to run an appliance needing 2KW (a kettle for example) at 240 Volts you need appx 8 1/2 Amps. This equates to some 166 Amps at 12 volts, sufficient to flatten most batteries in well a matter of minutes. If you are relying on an inverter for your mains you will need to recharge the batteries frequently and in a hire boat this almost certainly means running your engine.

Also be aware that the output from cheaper inverters is not the nice clean sine wave the power companies produce, but often something more like a square wave with the potential to harm some delicate electronics. Designs are however improving all the time.