Introduction to Inverters

By Michael Bloch

In the context of renewable energy, an inverter is a device that will convert DC battery voltage into mains type AC power.

Most medium to large scale solar power systems are designed to power everything from mains type AC voltages "inverted" from the DC battery bank. The advantage this gives us is that we can purchase conventional appliances from an electrical retailer. An energy efficient light globe can be somewhat expensive in a 12 Volt DC version and having a 24 or 48 volt lighting system makes finding a suitable light bulb even more difficult.

Inverter ratings

The three ratings that you should look at when buying an inverter are:

1. Continuous Rating: This is the amount of power you could expect to use continuously without the inverter overheating and shutting down.
2. Half Hour Rating: This is handy as the continuous rating may be too low to run a high energy consumption power tool or appliance, however if the appliance was only to be used occasionally then the half hour rating may well suffice.
3. Surge Rating: A high surge is required to start some appliances and once running they may need considerably less power to keep functioning. The inverter must be able to hold its surge rating for at least 5 seconds. TVs and refrigerators are examples of items that require only relatively low power once running, but require a high surge to start.

Types of inverters

There are basically two types of inverters: modified sine wave (aka modified square wave) and true sine wave. The differences between these two types of inverters are subtle but significant in the way they operate.

Modified sine wave inverters

A modified sine wave inverter can adequately power some household appliances and power tools. It is cheaper, but may present certain compromises with some loads such as computers, microwave ovens, laser printers, clocks and cordless tool chargers. Virtually all low cost inverters are "modified sine wave". A modified sine wave is easier and cheaper to produce than a sine wave inverter.

These low cost inverters are generally available from electrical stores, hardware stores, automotive store and electronic suppliers. They usually lack many features such as auto-start or any type of tweaking ability. The devices are usually only about 70% efficient, so expect some significant power losses if you are using a modified sine wave inverter in your system.

True Sine wave inverters

A true sine wave inverter is designed to replicate and even improve the quality of electricity supplied by utility companies. To operate higher-end electronic equipment, a true sine wave inverter is recommended. Efficiency has reached up to about 94% and the electricity from these devices is of a higher quality than grid power almost anywhere in the world.

A high quality inverter will include:

• An auto-start system. An auto start allows an inverter to switch to a low power consumption standby state when nothing is connected and turned on. This will save you a lot of manual switching and/or wasted power
• Tweaking ability. An ability to adjust parameters such as auto-start and battery depth of discharge is also helpful.
• High quality heavy-duty power transformer. A heavy inverter usually means a high quality heavy-duty power transformer.

If you are running sensitive electronic equipment, don't take the chance - get a true sine wave inverter. The extra cost will make up for itself in energy efficiency.

Michael Bloch is a consultant for Energy Matters - Wind and Solar Power Australia - a green energy equipment company offering a wide range of discounted solar panels, deep cycle batteries, power inverters and associated accessories for residential, businesses and schools. The Energy Matters site contains a wide range of resources; including a solar power system builder tool, renewable energy rebates information and free advice on off grid and grid connect solar and wind power systems

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