This type produce AC electricity in a smooth sine-wave shown in the diagram (black line) which is virtually the same as that supplied your home. 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 drill using 950 watts is being used, the inverter must be capable of supplying at least 950 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 drill will use: Watts (950) / Volts (12) = Amps (79.1). If the battery supplying the inverter is rated at 80Ah, you might think that the drill would operate for an hour. In practice, however, the battery will run the appliance for perhaps 15-20 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 950W drill, use a 1000W inverter supplied by batteries rated at at least 250Ah. (25% of 1000 = 250). Of course you can always leave your engine running to keep the battery topped up.
Many appliances draw a lot more power when starting up than they do when running. A Television, Refrigerator 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 Television, 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 cigarette 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 (at 12 volts). 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 and as the lighter socket is, quite often, not a good tight fit to the plug.
Above 300 watts, an inverter should always be connected directly to the battery and mounted as close to the battery as possible, with the shortest run of the thickest cable you can use - 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. If remote control is not an option for your invertor then use the cricuit shown for controlling a transceiver on the Mobile Installation page (reproduced here):
The "Switched live to radio" will be where you connect the inverters + supply and the "Ignition or Accessory live" will be connected to a switch, the other side of the switch being connected to the battery (or other switched feed if required).
IMPORTANT The displayed circuit shows a standard vehicle type relay, these can handle currents of up to 20-30 amps (depending on relay), you will need to use a relay that can handle at least the current that the inverter requires at full load. High current relays are available, the terminals may be different but the principle is the same.
Note: 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.
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.
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.
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.