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When you want to use a 230V appliance and you only have 12V or 24V available, you need an inverter. When connected to a 12V or 24V D.C. power source, such as a battery, the inverter simply converts the power into 230V A.C. which can then be used to operate any domestic or light industrial equipment.

There are two main kinds of inverter and they have significant differences:

Modified sine-wave inverters are the least expensive type. They are usually very efficient, light, and compact and are available in models supplying less than 150 watts to others which can power equipment requiring several kilowatts. They work by imitating the waveform produced by 230V mains electricity but, as can be seen from the diagram below, the waveform they produce is far from smooth. The result is that some appliances won't run as smoothly, as efficiently or as quietly as they would normally. TVs usually show interference in the form of a thin line rolling up the screen; microwave ovens may take a lot longer to cook; battery chargers may overheat or make a buzzing sound; power tools may run roughly and some equipment may be damaged or just won't work at all. For most applications however, a modified sine-wave unit will do the job and the possible disadvantages are outweighed by the low cost of a modified sine-wave unit.

Pure or True sine-wave inverters produce A.C. electricity in a smooth sine-wave shown in the diagram above - which is virtually the same as that supplied by the National Grid. As a result, any electrical appliance will run just as well on this type of inverter as it will when plugged into the mains. Their one disadvantage is that they are considerably more expensive but, if the inverter is to be used frequently or if maximum performance is essential, they may be worth the additional cost.

Whichever inverter you choose, it's essential to get one that is capable of providing the power which the appliance requires. So, if a vacuum cleaner using 1200 watts is being used, the inverter must be capable of supplying 1200 watts. Bear in mind too that a powerful 230V appliance will consume a lot of battery power. In a vehicle with a 12V battery, use the following formula to calculate exactly how much power the vacuum cleaner will use: Watts (1200) / Volts (12) = Amps (100). If the battery supplying the inverter is rated at 100Ah, you might think that the vacuum cleaner would operate for 1 hour. In practice, however, the battery will run the appliance for perhaps 15 minutes before the voltage drops to a point where the inverter will no longer function. If the battery is holding less than its maximum state of charge or is in poor condition, expect even less running time.

As a rule of thumb, your battery's Ah capacity should be at least 25% of the inverter's rated output so, if you want to run a 1200W vacuum cleaner, use a 1500W inverter supplied by batteries rated at at least 375Ah. (25% of 1500 = 375)

Many appliances draw a lot more power when starting up than they do when running. A TV, a fridge or anything using an induction motor can draw more than twice the power it needs when actually running. Make sure that your chosen inverter can deal with this surge or peak power. A TV for example, which may use 35 watts when it's running, could draw as much as 300 watts just for a fraction of a second when it's switched on.

Having chosen an inverter with the correct power rating, it is essential to fit it correctly. Some models with outputs of up to 300 watts can be connected directly to a lighter socket but make sure that the wiring to the socket and the socket itself are capable of carrying the power required. As a rule, this should not exceed 150 watts. Even in this situation, there could be problems running appliances as a result of voltage drop, especially if the appliance requires a high initial current.

Above 300 watts, an inverter should always be connected directly to a battery and as close to the battery as possible - again in order to avoid possible voltage drop. However, because the inverter will consume current even when it's not being used, ideally it should be capable of being easily switched on and off. This may not be easy if the inverter is hidden away in the battery compartment but, if left on, the inverter will eventually run the battery flat. Some inverters - usually high output models - have the option of a remote control. This can be easily located somewhere more convenient and solves the problem at a stroke.

Note however, that in vehicles with a 12V or 24V power system, it is always better to use equipment which has been designed to run on those voltages if possible. This is because inverters themselves consume power, even when switched on but not actually powering anything. Most inverters are around 80%-90% efficient depending on how they are being used. This means that 10%-20% more power is used to operate an appliance from an inverter than if the same appliance was powered directly from the battery

Inverters can be extremely useful but they are highly complex pieces of equipment and, as with all electrical products, great care should be taken when installing and using them. Never forget that, even though they only have fairly harmless 12V or 24V going in, the 230V coming out is just as lethal as 230V coming out of a mains socket at home.

Inverters, probably more than any other single product that we sell, are the source of misunderstandings and problems. It's tempting and easy to imagine that fitting an inverter to a van, boat or motorhome will solve all the domestic appliance problems. As explained above, it's not quite that simple.

There are dozens of brands of inverter and hundreds of suppliers. Some brands make claims for themselves which are, at best, bordering on the truth. Others are excellent pieces of equipment but may be just too good for the needs of someone using an inverter occasionally for leisure purposes. However, if you make sure that the inverter you buy is genuinely suitable for your requirements - even if you pay a little more than you'd intended - and if it is fitted correctly and used sensibly, you'll wonder how you ever lived without it.

For every item that will be powered by the inverter, add its Wattage until you reach a total. For a typical motorhome this could be:

4 light bulbs at 40 Watts = 160
One portable TV at 40 Watts = 40
One satellite receiver at 20 Watts = 20
Total = 220 Watts so a 300 Watt inverter should cope.

Finally, always read the instructions! Some inverters have to be warmed up for a few seconds before you can safely connect equipment. Some do not like to be connected to the battery during engine start-up because the alternator surge can damage them and because the sudden voltage drop from the battery can switch the inverter off.

 Buy an inverter here.

Copyright ©2005 RoadPro
Updated April 6, 2005
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