The power-inverter - DC to AC


My sailing yachts, motor boats, canal boats and barges, including my own have a DC/AC inverter on board. I also have an AC generator as well as the shore power. When you are anchored in some quiet cove or tied up on some peaceful canal, away from the shore power, the peace and tranquillity are disrupted when running a generator.

The inverter allows use of the video and television, the microwave and other appliances. Inverter technology has advanced considerably in the last few years. They range from small portable inverter units of just 150W up to the large fixed systems of 5kw with some units that can be parallelled with an automatic synchronisation module. The combination charger and power-inverter is also very common.

One big area of change is that transformers have been reduced in size and weight as the conversion is now done at very high frequencies. The reliability of most systems is also greatly increased over earlier units that unfairly gave power-inverters a bad reputation

About Power-Inverter Output Waveforms

Output waveforms are an important consideration when looking at proposed inverter application

The Trapezoidal Waveform The majority of inverters have a trapezoidal waveform. This is suitable for operating most equipment, but microwaves and some inductive loads such as motors do not operate at full output, dropping efficiency by some 20% or more in some cases. Fluorescent lights may also be less efficient at starting and often require a capacitor to be installed across the input, which will improve starting characteristics. Interference is also possible with these waveforms.

The Modified Sine Waveform

Some inverter units have what is called a modified or quasi sine wave output, which closely resembles the pure sine wave you have at home. It is not exactly the same and offers slightly reduced performance. Interference is also possible on these waveforms.

Appliances and equipment such as microwaves and VCR units with clocks often run either slower or faster. Battery chargers for cordless portable drills are also susceptible to early failure. If a charger gets excessively warm or hot turn it off as some incompatibility may exist. A device called a ferro-resonant line conditioner can be used to improve performance.

The Sine Waveform

Sine wave output inverter units are the ideal and offer quality better than shore mains power but are more expensive. If you are using sensitive equipment then a sine wave should be used. They are available in most ratings up to 1800W in 12-volt systems, with outputs at 230V RMS +/- 5%. Frequency stability is typically +/- 0.05%. Quality units have a low harmonic distortion, typically less than 3%, and low EMI (Electromagnetic Interference) levels.

DC Battery Loads

The typical power-inverter is capable of drawing very large DC current loads from the battery. It is essential that battery capacity is capable of supporting these loads without affecting the existing electrical system and connected loads, particularly electronics equipment. A simple method of calculation is to divide the power in watts by 10 for 12 volt systems or 20 in 24 volts systems. It should be noted that some manufacturers offer inverters for 12, 24 and 48 volts inputs depending on system voltage.

The minimum battery capacity required for an inverter is 20% of the inverter capacity, ie 2000 watts 400 Ah capacity. Most boats and barges will require a relatively large house battery bank if they intend to use the power-inverter frequently. On my barge I have a 24 volt battery bank, which is more efficient than 12 volts systems.


If another AC power source is connected across the inverter output, the power inverter electronics will be seriously damaged. Shore power supplies, generator and power-inverter outputs must never be paralleled. Most commercial AC systems have transfer switch systems. Some such as Mastervolt and Victron in the Netherlands and the US Xantrex have automatic systems

Rotary Switches The normal selection system used is the rotary cam switch. The switch should be a center off type with the inverter to the side opposite to generator and shore power. This stops any accidental paralleling of the two or more inputs.

Automatic Transfer Systems Many units are also able to automatically switch the supply from shore power or generator to the inverter on supply loss giving the same features as a UPS. Freedom series power-inverter and chargers have an automatic "load sharing" facility, which gives protection to all the connected AC loads from any variations in shore power or generator power. When the shore or generator power is connected the unit transfers the power automatically to any connected AC loads and some power is used to charge the batteries. When in the charge/transfer mode the incoming AC power is monitored and when a voltage drop (or an increase) longer than 8 milliseconds occurs the charging ceases, load transfer also ceases and the power-inverter mode starts.


Power-inverters generally have an output rating based on a resistive load and for a nominal period, typically 30 minutes. (eg. 1600 watts). The continuous rating is the normal continuous operation rating. (eg. 1000 watts). The Peak or Maximum Rating is the maximum short duration load that the inverter can withstand (eg. 3000 watts). Most units are capable of withstanding the short duration and intermittent overloads that are required, especially with motor starts.

The Surge Rating enables them to withstand short time overloads of up to approximately 200% over the continuous rating for 5 seconds. The typical inverter is now approximately in the range of 85% to 95% efficient at rated output. This efficiency level has evolved by new electronic switching technologies. Good ventilation is essential to reliable operation and the availability of full rated outputs.

1. The unit should be installed in a dry and well-ventilated area.

2. Sufficient vertical clearance should be allowed for natural convection of heat from unit.

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