How to Choose and Install Inverters

Advice, Tips and Suggestions when working with inverters

Inverters are an essential purchase for everyone who wants to be able to get normal mains power from a battery.   If you're a caravan, motorhome, camping or boat enthusiast, or someone who lives "off-grid" then an inverter will be top of your wish list.  Inverters are easy to install and operate, but there are few basis rules to follow when choosing, installing and using inverters. 

What is an inverter?

Batteries use and store DC - Direct Current and have a low voltage output usually in the range of 12 - 24 volts. Virtually all modern appliances operate on AC - Alternating Current and work on 240 volts. An inverter is a device that takes the power from your DC battery source and through special technology boosts it to household AC electricity giving you the power to run appliances such as televisions, lights, computers, and power tools wherever you may be. Simply, an inverter increases your 12/24/48-volt battery power to 110/240 AC power.

Pure Sine Wave vs. Modified Sine Wave - What's the difference?
There are two forms of electric wave output generated by an inverter:

i. pure or true sine wave;
ii. modified sine wave.

To understand the difference we need to look at their performance.
Pure Sine wave is the same as the power you have in your home. It is very clean and is the optimum for performance. This smooth pure sine wave is most efficient. Therefore, appliances such as TVs, computers and stereos run cleaner and easier. Also, Pure Sine wave generates less heat than other waveforms.
Modified Sine wave is an output that tries to imitate pure sine waves but comes up a little short. Nevertheless, modified sine wave inverters have their advantages. Modified sine wave inverters are great to run appliances and equipment such as power tools, non-digital microwave oven, lights, and other motor driven loads.  

The main downside to modified sine wave output is slight interference. This interference can be seen on some televisions and computers. It will not cause any damage, but can be a little annoying. The upside to modified sine wave inverters is they will run most appliances, and are very affordable.

What size inverter do I need?

This depends on what you want to operate.  An inverter rated up to 1000W will be able to power a small travel type hairdryer.
If you are a boat owner you would want to be able to power a small microwave, and this can be done with an inverter rated around 1500W or more. NOTE: make sure you read your microwave power rating properly as cooking power and NOT the power going into the microwave, which is likely to be at least double that figure.

If you want to run a 600-watt drill, a 150-watt inverter will not do the job. When choosing an inverter determine what you want to run and then get the inverter that delivers the appropriate power you need. Please look at the specifications to see which inverter gives you the power you need.

Installation
Installing an inverter is quite straightforward, but if in doubt consult a professional.  There are certain key points that you must be aware of:

1. Large power inverters, rated around 2000W or 2kW, are capable of drawing up 160-200A from your batteries, which is a considerable amount of current, so large diameter cables are needed to carry the current. For higher output models, it is advisable to use 50mm2 cables, no more than 1.5m long. All terminations must be done properly with crimped lugs. Using smaller cable or increasing the length of run will reduce the performance of the inverter.

2. Check if the inverter has a main input fuse fitted. If they don't then you will need to fit one. A 250A fuse fitted on the supply side of the inverter is a good idea. It is also advisable to fit a separate master switch in line so that the inverter can be disconnected and completely isolated. The master switch must be able to handle a 250A continuous load.

3. Depending on the inverter the terminals will either be 8mm2 or 10mm2. Check the diameter of your battery studs as they may be 8mm whilst the inverter has 10mm terminals. 

Connecting to Batteries 

The condition of your battery bank is very important. Once the inverter is connected to the batteries they start to discharge, and the voltage drops along the cable run. Hence why the length of cable and cable size affects volt drop. If you have used a long cable run and small size cables you may find that the inverter will fall below its low voltage cut-off point.

If you are using a 1kW inverter, you should use a minimum of 200Ah of batteries. For the more powerful, 2kW or more inverter, you need to ensure that your battery bank is rated at a minimum of 400Ah batteries, even better 600Ah.  Always make sure that the correct polarity is used when connecting the inverter to the batter bank and be aware that there will be a spark when you first connect the the batteries to the inverter as the internal capacitors in the inverter charge up.

Will it run my batteries dead?

Running an inverter certainly takes power from your batteries, but most inverter protects you against running your battery completely dead. Once your battery drops to certain point the inverter will sound an alarm to let the user know battery power is getting low. If the inverter continues to operate, drawing more power, the inverter will turn itself off to ensure you have enough power in your battery to start your car, truck, boat or RV. It is important to remember that to get the best performance from an inverter a good battery source is needed. Please view the chart overleaf that describes different power requirements for everyday equipment.

Most inverters draw a continuous current when they are switched on, even when nothing is connected to the output. This residual standby current could be around 1-2A.  This may not sound like a lot, but if the inverter is left switched on for 24 hours, then it could drain 50Ah from the batteries. 

Depending on what you want to do with the inverter you may find some extra features invaluable. For example, a remote control, enabling you to turn the inverter on/off when you need to without having to manually switch it on/off. Spending a bit more and you can get a nice battery monitor included. Some models have a power save feature that sends the inverter to sleet when nothing is connected to it and plugging in equipment brings it back to life. If using the inverter on board a boat, for example, you should also fit a changeover switch in the inverter output circuit to enable switching to take place between shore power supply, generator and inverter.