Using an inverter to make 240-volt power for your car

You can produce 240-volt (AC) electricity fairly easily, using a device called a static inverter. Inverters, also called 'static inverters' because they don't contain moving parts, convert 12-volt DC (direct current) electricity from your car battery into a limited amount of 240-volt AC (alternating current) electricity, allowing you to run some household appliances when you're away from a source of mains-powered electricity.

However, there are limitations. The size of the inverter dictates which appliances you can run. Some larger appliances are probably better off being run from a petrol-electric generator. Also, inverters drain the car's battery if you use them when the engine is off. Heavy inverter use will require an auxilliary battery, which will add to the cost.

Smaller inverter units plug into a conventional cigarette lighter socket. These are suitable for charging mobile telephone, cameras, camcorders and PDAs. Larger units need to be hard-wired into the car's electrical system.

Selecting an inverter usually depends on what you want to do with it. Inverters are rated in watts (W).

Here is a rough guide to matching the right inverter to the appliance you wish to power up.

• 50W is good enough for basic battery recharging of phones and cameras.
• 150W will run an average laptop computer.
• 250W willl run a 19-inch LCD TV.
• 600W is enough for a power drill of grinder.
• 1500W is required for a blow drier.
• 2500W is required for a kettle. (By this time, however, you should probably be thinking about a petrol-electric generator, which could easily be cheaper and more versatile.

Most inverters produce a kind of AC electricity called 'modified sine wave' - not as clean as regular mains electricity from the power point at home. These are fine for most laptops, portable fluoro lights, camera and phone recharging, and for recharging the batteries on power tools. However, true sine wave inverters (which are more expensive) are required for medical equipment, some cheaper battery chargers and some newer variable speed power tools. Anything that says 'recharge only using mains supply' or similar needs a true sine wave inverter.

Inverters present a potential electrocution hazard similar to a domestic power point. Even a small 60W inverter will produce up to 400mA of current - which is roughly 10 times that which is required to stop a human heart and kill you via fibrillation. So the same precautions you'd normally apply to household electricity pertain with inverters (such as not using one outside, in damp conditions).

Most cigarette lighters blow their fuses above 150W of electrical load - but inverters up to 250W can be used in them provided they're not running at the inverter's maximum output. (In this example if your inverter is rated at 250 watts, and you connect a small camera battery charger to it, it will drain only a few watts from the cigarette lighter. In other words the actual power consumption of an inverter is different to the sticker that dictates its maximum rated performance. The analogy is a bit like the garden tap: wide open it might deliver 30 litres of water a minute, which is a bit like the sticker on the side of the inverter trumpeting its rating. But if you only just crack open the tap it might only deliver one litre a minute, which is a bit like the demand on the electrical system if you're only charging a small battery with an inverter with the potential to deliver more.) 

To calculate the power in watts required to run an appliance, multiply the volts by the amps. Allow a 20 per cent excess for conversion inefficiency in the inverter, and remember that some appliances pull a large amount of current briefly when they are first switched on (for example power tools and TVs). For this reason inverters usually have two ratings - a continuous rating for when the appliance is running and a 'surge' rating for when  it is first turned on. A 300W inverter (continuous) might have a 1000W surge rating.

Be aware that inverters can flatten the battery - so the best time to use a smaller one to recharge those camera batteries is when the car is running. A larger installation might warrant installing its own 'deep cycle' battery for use when the car is turned off. An auto electrician can install that if required.

If you've decided you want an inverter, plenty of places sell them. Here's a link to an Australian-based online inverter shop, or you can buy from electronics outlets like Jaycar Electronics.