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An inverter will provide a 230v AC supply from your boat’s 12v DC or 24v DC system enabling you to operate mains appliances or equipment, e.g. hair dryer, microwave oven, mobile phone charger, power tools, etc. Obviously the rated output of the inverter will need to exceed the total that you intend to draw at any one time.

Start by making a list of all the 230v electrical equipment that might be running simultaneously. For each item on the list allocate its consumption requirement in Watts. Watts = Volts x Amps. Be aware that appliances with motors (fridges, hairdryers, micro-waves etc) may require a larger start-up current so, check their specifications carefully and read about ‘surge capacity’ below. Add all the items on the list and you will have the total Watts required.

Surge Capacity

Some appliances momentarily require a higher current (surge) when they are switched on. Fridges are notoriously hungry starters (check specifications carefully), hair dryers, micro-waves, vacuum cleaners and basically anything with a motor will usually need a high start-up current.  A good inverter will usually cope with at least double its max output for a few seconds. Even if your inverter has adequate capacity to cope with such start-ups, if your batteries are inadequate or not fully charged, this can cause the inverter to cut out.

Battery Capacity

Many 230v AC appliances draw a lot of power and you need to be sure that you have sufficient battery capacity to cope with the demand. As a rough example, a fully charged 100Ah 12v battery will run a 1Kw 230v AC hairdryer for about 30 minutes, while a similar 24v battery would last for twice as long. Of course if the engine is running, this will extend the available time.

Bear in mind that inevitably there are system inefficiencies; a top specification inverter (including Victron and Sterling) will lose about 6-8% (at 20°C). Make sure that the inverter is installed in a well ventilated area as its efficiency will decrease as the temperature rises. You also need to be aware that all inverters draw power when in standby (quiescent) mode – this is low with both Victron and Sterling inverters, normally less than 5%. Even so, an inverter left in standby for a couple of weeks might flatten your batteries.

Sine Waves

AC power is characterised by a curved waveform called a sine wave. Now, without getting too technical, it is difficult (and expensive) to produce AC current with a true sine wave from a low voltage DC power supply. This has resulted in the availability of two kinds of inverter; pure (or true) sine wave inverters and modified (or quasi) sine wave inverters. Pure sine wave inverters are much more costly to produce than modified sine wave inverters and therefore cost a lot more.

Top specification pure sine wave inverters are capable of running virtually any type of 230v AC equipment or appliance with virtually no adverse effects. Some sophisticated telecoms equipment might be the rare exception.

Top specification modified sine wave inverters (Sterling) will run all but the most sophisticated appliances. Mobile phones, laptops, TVs, drill chargers etc. all tend to work fine as do most appliances. However there are rare (and getting rarer) occasions when the odd micro-wave, drill or vacuum cleaner will not work. Remember, it will be cheaper to change a £50 micro-wave to a different model than to buy a pure sine wave inverter instead. The firm exceptions to all this are washing machines and bread makers which will not work with modified sine wave inverters (or anything with a thyristor control). Sophisticated telecoms equipment may also have problems.

This advice is based on the Victron and Sterling Power inverters which we supply, much cheaper brands are available but expect more compatibility issues and do not expect the same performance.

Inputs & Outputs

Inverters are supplied with heavy input cables, normally 1m or 1.5m long for connecting the inverter to your batteries. As a rule, site the inverter as close to your batteries as possible and keep the input cables as short and fat as possible.

Victron inverters are normally supplied with a choice of 230v Ac output connections, including IEC plug sockets and/or terminals. Sterling inverters usually offer euro and British 230v AC output sockets.

© Boat Electricals Ltd August 2010



  • Boat Electricals Limited

  • Ocean Village Innovation Centre

  • Ocean Way

  • Southampton, SO14 3TJ

  • TEL: 0800 2343499

  • ask@boatelectricals.co.uk