Inverters

An Inverter is an electronic device that converts one voltage to a higher voltage. Travellers usually want to convert 12 volts DC (Direct Current) to 240 volts AC (Alternating Current).

Because we have 240 volts at home, we already have many appliances that run off 240 volts, whether it's a very low power charger for a camera, or a very high powered microwave oven. An Inverter allows you to use the applicances you already have when you are away from Mains power - provided you have an Inverter that puts out enough power to suit the loads and that it puts out a waveform good enough for the load.

An Inverter only changes the voltage and current - the output power (in watts = voltage multiplied by current in Amps) is always less than the input power, due to Inverter Efficiency being less than 100% - usually between 80% and 90%. The power that is lost as electrical energy escapes as heat energy in the Inverter.

Electrical Power

Electrical power is always measured in Watts (1000 W = 1 kilowatts, 1 kW).

Peak and Continuous

Two numbers are used to describe the output of an Inverter - Peak and Continuous.

Most small plugpack chargers and power supplies for cameras, laptops etc these days use Switchmode power supplies - feel the weight of it - if it feels like it is full of iron, then it has a conventional iron transformer in it (most probably needs Sinewave), if it is much lighter, then it is Switchmode. Theoretically the switchmode power supplies should work well on any waveshape input, because they firstly convert the AC to DC, however if the components inside aren’t rated adequately, then they may overheat from the inrush current 100 times a second, caused by the squarish waveshape.

Isolated Output

If you are using only double-insulated appliances (i.e. they have no third Earth-pin on the Mains plug) with your Inverter, then it doesn’t matter if the Inverter has total electrical isolation between the DC input and AC Output (provided there is no fault to earth). In larger installations with an earthed neutral, or if you have earthed appliances such as Washing Machines, then if the Inverter is not isolated, the DC input will be up to 120 volts AC above the Mains Earth - a very dangerous situation!

Soft-start

Some loads draw much more current when starting up compared with steady operation e.g. a Halogen bulb draws ten times its normal current when first turned on, and motors can also draw very high currents as they come up to operating speed. To reduce these massive current surges, some Inverters bring the output voltage up to 240 volts progressively over many seconds. This means you will be able to use an Inverter with a smaller continuous power rating, for a particular load.

Low Voltage cut-out

To prevent damage to your battery, it’s important that the Inverter switch itself off and remove all current drain, when the battery voltage drops below a set limit. It helps if the cut-off voltage can be set by the user, based on the type of battery, the depth of discharge desired and the voltage drop in the wiring. Keep in mind that as battery voltage drops, the Inverter will draw more current to keep input power constant, so this means that voltage will drop fairly quickly towards the end, and therefore manual monitoring of battery voltage is not really practical.

Automatic Fan Cooling

To get rid of waste heat in Inverters over 200 watts, it really helps to have an internal fan and usually these are automatically switched on only when needed, to minimise power used.

Earth leakage protection

Bigger Inverters may have these built-in, but they can be easily added.

Meters

You can get by without voltage and current meters, but with them it’s easier to check if your system and appliances are healthy, or if you’re close to overloading your Inverter.

Autostart

Some larger Inverters automatically sense when any appliance is connected or switched on, so that the Inverter only draws significant power when it’s actually needed. This is particularly useful in large vans, mobile homes or fixed buildings, because you don’t have to switch the Inverter off and on when using appliances.

These days, manufacturers don’t always include an Austostart feature - especially when it comes to Pure Sine wave inverters, since they will typically have a remote.

Remote Control

For installations in large vans, mobile homes or fixed buildings, it’s handy to be able to control it remotely, because the Inverter may be mounted in a cabinet near the batteries.

Inverter/Charger

For permanent Installations, the Inverter may be able to charge the batteries when mains is available, but there is no solar power available.

Grid-Feed Inverter

For permanent installations, when there is surplus solar power available, the Inverter can feed power back into the 240volt mains, resulting in electricity being ‘sold’ to the electricity company.

Overload cut-out

Not really optional, but the more sophisticated ones will protect against over-temperature as well as over-current, over-voltage or under-voltage (that can damage some appliances).

Battery capacity

All the power for the load has to come from the battery, so with high power Inverters or appliances you want to run for a long time, you need massive battery banks. If you wanted to run a small Air Conditioner that used 1000 watts on average, it would draw 100 amps - so you would need to supply 2400 amphours over a day. That means big, heavy and expensive batteries, and a way of recharging them!

Use 12 volt appliances

Many appliances are available as both 12 or 240 volt versions (fridges, microwaves etc), but the 12 volt version always uses less power and is always more expensive ! A 12 volt fridge will have thicker insulation and use a more efficient motor because mains power is cheaper than 12 volt power ! A 12 volt microwave will use less power because there are no losses in converting the 12 volts to 240 volts first. Using only 12 volt appliances, may reduce your power needs, so you won’t need a generator or Solar power to recharge your batteries on shorter outings.

Generator

These can easily generate the high power level often needed and they produce a 240 volt output directly. However an Inverter is totally silent and smaller than a Generator.

DC-DC Inverter

If you want to operate a laptop, you will already have a mains supply for it, so usually the cheapest way to run it off 12 volts is to buy a low-power 240 volt Inverter to power the laptops Mains supply pack. This means you will have two boxes and you will waste more power in the double conversion, compared with buying a DC-DC Inverter. A DC-DC Inverter directly converts the 11 to 15 volts that a car electrical system puts out to the 12 to 21 volts DC that your laptop needs to run and recharge its internal batteries.