An Inverter what is it?

Inverters seem to some like a black art or wizardry, however when fitted properly have a lot of benefits when you are off or on grid.

An inverter is an electrical device that provides a 230v AC supply at 60hz from your boat’s 12v or 24v DC system thus enabling you to operate 230v mains appliances or equipment, for example a hair dryer, microwave oven, hair dryer,  power tools, mobile phone chargers, computers etc. Obviously the rated output of the inverter will need to exceed the total power that you intend to draw at any one time. So it is important to do a power audit to see what you demand from your 230v AC inverter.

With anything on a boat, you must make a list of all the 230v electrical items that might be running simultaneously. So for each 230v electrical item on your list, allocate its consumption requirement in Watts. (Watts = Volts x Amps). However be aware that appliances with motors (fridges, hairdryers, washing machines etc) have what is called ‘surge capacity’, and will require a larger start-up current so, check the 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. This is normally an electric motor. Washing Machines and Fridges are notoriously hungry starters (check specifications carefully), micro-waves, hair dryers, vacuum cleaners and basically anything with a motor will need a high start-up current greater than it running speed.  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. See our section regarding batteries.

Battery Capacity

Due to most appliances being designed for the domestic market 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.

Victron Multi Plus Inverter

However A+++ rated items do draw less power. 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 your engine is running, and with suitable alternator(s) and in some case, special battery chargers like Stirling alternator charger this will almost be indefinite.

Please remember, 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. However if you a 230v ac land line attached, inverters like the Victron Multiplus will act as battery charger as well. They can also act as power assist to boost a landline supply if required.


To give you an idea of specification lets say go for the Victron 12/3000/120-50 the 12v version. Cost is £1505 inc Vat (year 2020)

  • Pure Sine Wave Output
  • 6000 Watt Peak Power
  • 2500 Watt / 3000 VA Continuous
  • 120 Amp Battery Charger
  • 50 Amp Power Transfer Switch (UPS)
  • Power Assist
  • Dimensions h362 x w258 x d218 mm
  • 5 year warranty
  • 18kg weight
  • Ready for parallel operation

If you need more power the Victron units can be doubled up in parallel. The below picture shows how it is done.

Inverter Victron Parallel


Sine Waves

AC power is characterised by a curved waveform called a sine wave.

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. However these are more reliable with sensitive things like computers and televisions.

Top specification pure sine wave inverters are capable of running virtually any type of 230v AC equipment or appliance with virtually no adverse effects. Victron are arguably the best.

I found this company, Onboard Energy from Springwood Haven Wharf, Mancetter Road, Nuneaton CV10 0RZ  to be very good, and are a Victron agents. If they fit the unit for you you also get discounts on the unit price.

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 suffer some problems.

The above information is based on the Victron and Sterling Power products, there are much cheaper brands available but expect more compatibility issues and do not expect the same performance.

Inputs & Outputs

Inverters are fitted with heavy input cables which must be matched to the output size of the inverter, normally 1m or 1.5m long for connecting the inverter to your batteries.

You must site the inverter as close to your batteries as possible and keep the input cables as short and fat as possible, this reduced power loss.

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.

Inverter Remote Controls

Most new inverters allow remote in boat devices, these allow you to see what the inverter unit and your batteries are performing.

Some are very simple and the latest link up multiple systems on your boat. such as the colour control GX by Victron. Go here for more information.

Mutliplus ccgx_600 Inverter Colour Control