Inverters turn the extra low voltage dc output from batteries etc into the mains voltage ac (alternating current) required by electrical appliances. They are rugged and generally ultra-reliable but can be confusing to buy because there is at a least a 3:1 price range for what may seem to be similar products that do apparently similar things.

This article attempts to explain why this is so and hopefully provides hints on how to choose.

An inverter is a bit like a camel that’s made out of silicon. As any sunset on Broome’s Cable Beach will demonstrate, a good camel will carry a surprisingly heavy tourist for a brief time, a less heavy one for longer, and a normal one for ever. If you are unfortunate enough to have had dealings with camels you’ll know that nothing about them can be taken totally for granted, but here fortunately, inverters are different.

Almost all inverters work as above. And even if drastically overloaded, an inverter will simply lie down for a bit and cool off. (Unlike a camel – which may kick or bite you. Or both). It’s worth knowing this as you may otherwise end up buying an inverter that’s bigger than you need (or stand too close to a disgruntled camel).

Here’s an example. You want to run the TV/DVD/VCR, and these collectively are likely to draw 75-150 watts. You’d also like to run the odd kitchen appliance (such as a blender) and the ones you have in mind are known to draw up to 500 watts. You may want to run something else like a coffee grinder, and maybe at the same time.

Here, a good 500-watt inverter will cope with ease – including the coffee grinder (another 150 or so watts). It can do this because the blender and the coffee grinder are likely to be on for only a few minutes. The inverter will handle the total load (of a probable, but short-term 750 watts) with ease.

All good inverters have accompanying literature that indicate the output they can provide – and for how long. If such information is not available, buy elsewhere. The graph in this article shows the actual output of a good quality 1600-watt unit. As may be seen it can produce twice nominal output for close to five minutes. Lower quality units may not do so well – but all that I am aware of have some such capability.

Inverters are intended to be used this way. A short term heavy load is not necessarily an overload.

Modified Square-Wave/Sine-Wave

Where confusion mostly reigns is trying to find out which type of inverter will run your intended appliances. This article may help you reach a decision. The area of confusion concerns so-called ‘output waveforms’ – essentially the shape of the electrical output. And as most mains-voltage appliances are designed to run from a so-called ‘sine-wave’ that is really what is required from an inverter. (But a few appliances, particularly heaters, don’t care just as long as the voltage is about right).

Early dc/ac converters consisted of a dc electric motor driving an ac generator. The so-called motor-generators produced clean sine-waves but were extremely heavy, inefficient and very costly.

The advent of heavy current transistors (in the 1960s) enabled the job to be done without any moving parts. These early inverters produced what is called a square wave. This looks much as the term implies and square waves work well enough for lights, heating devices etc, but less so for motors, radios, TVs – and indeed most electronic devices.

As with motor-generators these early inverters were bulky and heavy, but generally very reliable. Forty years on, many are still in use.

As the technology did not then exist to produce affordable high output sine-wave inverters, a lot of effort was put into producing a square-wave unit that vaguely simulated a sine-wave. The resultant units were known as ‘modified square-wave’ inverters, but in recent times marketing people started to call them ‘modified sine-wave’ inverters. This they are not, and the distinction is important.

A modified square wave unit produces a series of superimposed square waves of different frequencies (the number of waveforms produced over time). If you superimpose a very large number of harmonically-related frequency square-waves the resultant wave shape is a bit like those old buildings that have a series of stepped bricks forming the facade. If you have enough of them the shape begins to look like a ragged-edged sine-wave. Good modified square-wave inverters will run most things. But not, unfortunately, everything. Known exceptions include most laser printers, most NiCad chargers, almost anything that has an inbuilt electric clock, an increasing number of electric shavers – and who knows what.

There’s really no way of knowing without first trying, and some sensitive equipment can be damaged in the process. Also, without an oscilloscope (a TV-like device that enables you to actually see waveforms), it is impossible to gauge how closely or otherwise the output is to a sine-wave.

The only totally safe advice is this. A good modified square-wave inverter may well do the job. In fact it usually does. And if it does you will certainly save a fair amount of money. That inverter may well run future purchases – but you cannot be sure. If you really know what you are doing, or can obtain truly reliable advice from someone else, then by all means buy a modified square wave inverter.

But unless you are in that position, bite the bullet and buy a good quality sine-wave unit. You may well have spent more than you needed to, but you have peace of mind.

Be wary of sales advice on this. There are sales people who know about stuff like this. But they are rare.

Power Tools

Modest power tools (drills, 4-inch angle grinders etc) can be run from inverters of 500-watts upwards. If you intend to use such tools take them with you when buying and check that the inverter can get them started: if it can, it will comfortably run them.

Microwave Ovens

Many people buy an inverter to run a microwave oven. If doing so, be aware that a microwave oven’s ‘rating’ (typically 800 watts) is its ‘cooking power’. It generates that amount of heat. But that is not what it draws in watts.

Microwaves are increasing in efficiency but few are better than 60%. Thus an 800-watt oven is likely to draw about 1330 watts – and taking in inverter losses, that’s about 110 amps at 12 volts. A typical microwave used on full power for ten minutes thus draws close to 20 amp-hours. That may be a day’s output (outside midsummer months, or in southern areas) from a 64-80 watt solar module.

Electrical Isolation

There is a further level of ‘quality’. This one affects safety. Without going too deeply into technicalities, many lower-priced inverters (both modified square wave and sine-wave) are designed so that one side of the mains-voltage output is also one side of the battery input. Where such an inverter is connected to mains wiring this can lead to a severe electric shock if the battery leads are touched.

Because of this, the better-quality inverters are built so that the mains output is fully electrically-isolated from the battery input. But this costs a fair bit more to do. ‘Electrical Isolation’ is seen as a major selling point, and it will generally be prominently stressed. By and large this is a field where you largely get what you pay for. One does however need to be aware that some imported inverters pass through several levels of distribution, each taking a cut. So whilst price is a good general guide, there are exceptions. Happily, there are some excellent and well-priced Australian made products.

Installation

This is best done by an electrician, and must be if the inverter is connected to the motorhomes’s mains wiring. You may if you wish (and have the knowledge) look after the extra low voltage side of the work. Be aware that few inverters have internal fuses and are badly damaged if the 12 or 24 volt input is connected the wrong way round. Also, be aware that a big inverter on full load draws close to light starter motor current. Correspondingly heavy cable must be used, and cable runs kept to absolute minimum. Further details on this and other related topics will be found in Collyn’s books.