Inverters for 24V systems

Many appliances will run on 24V directly - pumps, lights, some TVs and stereos - but you can't beat a household 240V AC supply for convenience.

Fortunately, you can buy devices called inverters that will convert (or, technically, invert) your 24V DC supply into a 240V AC supply ready to run all your favourite household gadgets.

Inverters come in two flavours. Modified sine wave inverters are cheap but effective, though they can cause problems with some sensitive equipment including microwaves and motors with speed controls.

Pure sine wave inverters cost significantly more, but produce electricity that is identical to household mains. They will run all 240V equipment happily.

When choosing an inverter, think carefully about how large it needs to be. Inverters are usually sold by their output measured in VA or watts.

For simple devices such as incandescent lights and heaters (called resistive loads), VA are exactly the same as watts, and you can simply select an inverter with an output that matches or exceeds the rating of the largest appliance you want it to run, in watts.

But appliances with transformers and motors (inductive loads) you should allow a 50% overhead. For example, if you need to be able to power a vacuum cleaner rated at 1000W, buy a 1.5kVA or 1500 watt inverter.

Be careful also that your battery bank is up to the job. A 1.5kVA inverter running at full whack will draw around 65 amps from your batteries - at this rate they will go flat very fast indeed!

Running appliances through an inverter is not as efficient as running them directly from your 24V supply. A good inverter might be 90% efficient.

Inverters also tend to consume power even when idle, so it's well worth switching them off when not in use. More advanced models have automatic power saving modes - see the manuals.

Fitting your inverter

Install the inverter in a cool, dry location as close as possible to your batteries to minimise losses in the battery cables. However, the inverter should not be inside the battery compartment itself as batteries give off flammable gasses.

Most inverters have an option for a remote on-off switch, which is well worth considering. This allows you to have the inverter hidden away out of sight and locate the on switch somewhere convenient. The cheaper modified sine wave inverters may not have this option, but you can use a relay instead - see the wiring diagrams below.

The inverter will probably come with battery cables. If you use your own cables, make sure they are thick enough. Inverters draw a very high current (1.5kVA draws up to 65 amps), check our 24 volt wiring guide to make sure you havet the correct cables.

It is not generally necessary or practical to fit a fuse to the battery cables, but all inverters should have built in fuses as well as overload protection.

Safety

Unlike the relatively safe 24V DC from your batteries, the 240V AC output of the inverter can very easily kill you. It is essential that you connect and operate the unit safely.

Inverters are designed to power 240V appliances connected directly to their outlet only. If you plan to connect the inverter to some sort of 240V distribution system eg. a ring main and sockets, consult a qualified electrician to make sure your system is safe.

Your distribution system and the inverter body should be earthed, the output should be protected by an RCD 'trip', and the system should be designed to make it impossible to connect more than one AC source at a time, for instance with a rotary selector switch.