An introduction to inverters

Off-grid power systems with battery banks are fundamentally low-voltage, DC power sources. Which, generally, is great - low voltages are safe, and easy to wire. There are plenty of appliances that operate directly from 12V or 24V, and many people in boats and caravans find they can live quite happily with only a 12 or 24V circuit.

However, it has to be admitted that there are some advantages to mains 240V wiring as well. The ease of being able to plug any old mains appliance into a standard three pin socket is the main one of course - but going for 240V also means you can use much thinner wiring than you would need if you were operating at 12V, as you have a much smaller current for a given power. If you have long cable runs, and are operating high - powered appliances, it can make a big difference to the cost of a system.

Output from Sine Wave Inverter

Luckily, it's really very easy to produce 240V power from a 12 or 24V battery bank, with a device called an inverter. There are two types of inverters on the market. 'Sine wave' inverters - such as the Steca line that we sell - will give a waveform that is very close to that produced by the national grid. In fact, it's usually even cleaner than mains electricity! These inverters can run almost any mains appliance (although do read on to see of some possible pitfalls!).

'Modified Sine Wave' inverters tend to be considerably cheaper than true sine wave inverters. However, instead of creating a complex pure sine wave, they simply switch the power on and off, giving more of a square waveform (or 'modified sine wave' as they would have it - pretty mangled sine wave if you ask me). The Silverline inverters we sell are of this type.

Output from Modified Sine Wave Inverter

For most appliances, modified sine wave inverters are however, absolutely fine. Resistive loads - such as a kettle element or a lightbulb filament - don't give a hoot about the shape of the waveform, and they will work just as well on a modified sine wave inverter as a sine wave inverter. Motors, such as power tools and fridge or freezer compressors, also tend to work fine, although you may notice that they run just a tiny smidgeon less smoothly. There are a few appliances that may be awkward to run however - laser printers can be funny; light dimmers and motor speed controllers such as those found in sewing machines can also be difficult. Audio equipment may develop a slight hum. Fluorescent lights can also sometimes misbehave, although the technical reasons are a bit different - for a good technical article on inverters, see Jaycar's pdf on the subject. If you don't mind a bit of mucking around with a soldering iron, it can be possible to modify the lights to run on a modified sine wave inverter. Alternatively, there are plenty of fluorescent lights that will run directly from 12 or 24V, so you could just have a DC lighting circuit, and only use the inverter for other mains appliances.

Sine Wave inverters won't have a problem running any of those appliances. However, one thing to be aware of with all inverters is that some appliances take a very high current when they start up. This is particularly true of motors which start under load, such as fridge or freezer compressors. Although most inverters can deliver considerably more than their rated output over short periods, you might find that they still cannot cope with the very high current needed to start a motor under load. That said, get a big enough inverter and you won't have any problem!

Often, the only way to find out if a particular appliance will work from a particular inverter is to try it. Nine times out of ten it will work fine - but you do need to be aware of the limitations.

There is one more type of inverter to consider, by the way, which is a completely different beast altogether. 'Grid-connect' inverters are used to connect solar systems into the national grid, so that you can sell power produced by your solar array to the electricity companies.

How much power can I get from my inverter?

Inverters are usually sold with a power rating in watts. All 230V appliances will their power rating written on them somewhere. Common sense suggests that provided the inverter is rated higher than the appliance, the appliance will run.

Things are not quite that simple. Firstly, inverters (including ours) are advertised at a power rating they can sustain for a limited time, usually 30 minutes. An inverter rated at 1000W can churn out 1000W for a while but may then overheat and shut down. Run it at 800W though, and it should be able to go forever.

Secondly, not all appliances are the same. There are two types of AC load; resistive and inductive. Resistive loads such as conventional lights, heaters, toasters etc. are no problem and you can run these types of appliances right up to the rated power of the inverter.

Inductive loads, such as motors and microwaves, are more of a problem. Often these can require a significantly larger inverter than the power rating of the appliance would suggest. Technically, this is because they do not always draw their maximum current when the supply is at maximum voltage - the two are out of phase.

The best advice we can offer is to select and inverter rated at least 20% higher than the most powerful appliance you wish to use, then simply experiment see if it works!

a Steca sine wave inverter