Inverters

Modified and Pure Sine Wave Inverters

Power inverters come in two flavors, modified sine wave inverter and pure sine wave inverter.
Modified sine inverters create an approximation of the normal 240V AC power which is often a "good enough" replication for many appliances. The pure sine inverters however provide a perfect replication of the AC mains power (in fact, often better than what you usually get from the mains outlet in your house) and therefore even the most sensible applications will run without any problems.

Modified sine wave inverters are a lot lower in price as they are not nearly as complicated to build and require far less components than pure sine wave inverters.

While a modified wave inverter is great for powering simpler appliances and power tools for more complicated or sensitive electrical equipment and devices it is generally recommended to power them with a pure sine wave power inverter, as some electrical equipment will not function correctly or at all from a modified wave inverter, or if they do their life span may be reduced or it could be permanently damaged. This being the case we strongly advise checking with the manufacturers recommendations for the item(s) you want to power to make sure they will run correctly from a modified wave power inverter.
If in doubt, a pure sine inverter is the safe bet!

Below a graphical comparison of pure and modified sine wave. 

How to select the right size inverter

Inverters are rated in Watts, meaning the max power you can draw from the inverter. The basic rule is to add up the wattage of all accessories you want to power from the inverter at the same time. For example if you want to run a laptop which uses 100 Watts and some LED lights with 40 Watts you will need an inverter rated for 140 watts. So in our example you will need at least a 150W inverter but probably better to have some reserves and use a 200W inverter.

To find the wattage of your appliances, check the label or the manual. Sometimes the Watts are not listed but the Amps are. Then you can calculate 240 x Amps = Watts. For example on your TV the label says says 240V, 0.5A. So you calculate 240 x 0.5 = 120 Watts.

If you cannot find any information you can use the table below to estimate the Watts. But remember that these are only guidelines!

There is another "gotcha" when dimensioning your inverter. Some appliances, especially motors can draw 2 to 5 times the reated wattage during the start up phase. Most inverters can handle for a few seconds loads up to double the rating to compensate for starting up appliances but sometimes it can be necessary to go a size bigger in the inverter just to be able to handle the start up load.

Some other tips for using an inverter:

• Inverters have a standby current draw. Which means they do use power if they are switched on but no appliance is plugged in. Therefore always switch the inverter off or disconnect from the battery when not in use.

• The average inverter has an efficency of about 85% so you are loosing about 15% of your battery power just for the conversion. If your appliance can be powered by 240V and 12V (for example most small TV's) it is much preferable to connect it directly to the battery (12V) instead of using an inverter.

• Good quality inverters like the RedArc range or our Matson range have higher efficency and can reach > 90%.

• An inverter draws from the battery as much power it requires to run the appliance. So a 1000W inverter and a 200W inverter both powering your 150W TV set will draw much the same power from the battery. There is basically no penalty for having a bigger inverter than you need. However make sure that your wiring and battery can handle the maximum power requirements of your inverter.

• All modern inverters have features like low voltage cut-out and overload protection. This is usually effecitve in protecting the inverter and battery from damage but not all appliances like it if the power is cut. Especially computers can react with lots of problems if the power is cut without shutting down the computer properly. If you want to run computers (other than laptops) or other sensitive equipment always make sure your inverter can handle the requirements and monitor your batteries to prevent the inverter cut out because of flat batteries!

Power draw of inverters

Inverters powered by standard 12V batteries can demand an incredible current from the batteries. A 1000W inverter powering a 1000W appliance will draw close to 100A from the battery! If powered from your car starting battery it can prevent you from being able to start your car after less than 10 minutes of usage!

As a very rugh guideline we recommend not to run any inverter greater than 300W from your normal car starting batteries. If you have a standard dual battery system you can run inverters of up to about 1000W. Bigger inverters are usually limited to boats and bigger caravans with battery banks of multiple big batteries.

As a very rugh estimate the following table indicates the runtime depending on load adn battery size.

Table: Power Draw of Common Appliances

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