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What is an inverter?

An inverter changes DC voltage, almost always from batteries, into standard household AC voltage so that it is able to be used by common appliances. Basically, it does the opposite operation a “converter” or battery charger does. DC is usable for some small appliances, lights, and pumps and is usually suitable for small home or cabin systems, RV's and boats. However, most larger home systems should include an inverter. Although some DC appliances are available, with the exception of lights there is not a wide selection. The most common battery voltage inputs for inverters are 12, 24, and 48 volts DC - a few models from some companies are also available in other voltages. 

There is also a special line of inverters called a utility intertie or grid tie, which does not usually use batteries - the solar panels or wind generator feed directly into the inverter and the inverter output is tied to the grid power. The power produced is either sold back to the power company or (more commonly) offsets a portion of the power used. These inverters usually require a fairly high input voltage - 48 volts or more.

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How does an inverter work?

An inverter takes the DC input and runs it into a pair (or more) of power switching transistors. By rapidly turning these transistors on and off, and feeding opposite sides of a transformer, it makes the transformer think it is getting AC. The transformer changes this 12, 24, or 48 volts "alternating DC" into 115 volts AC at the output. Depending on the quality and complexity of the inverter, it may put out a square wave, a "quasi-sine" (sometimes called modified sine) wave, or a true sine wave. Square wave inverters are usually only suitable for running some type of electrical tools and motors and incandescent lights.

Quasi-sine (modified sine, modified square) wave inverters have more circuitry beyond the simple switching, and put out a wave that looks like a stepped square wave - it is suitable for most standard appliances, but may not work well with some electronics. Also, some of the chargers used for battery operated tools (such as Makita) may not shut off when the battery is charged, and should not be used with anything but sine wave inverters unless you are sure they will work.  Sine wave inverters put out a wave that is the same as you get from the power company - in fact, it is often better and cleaner. Sine wave inverters can run anything, but are also more expensive than other types. The quality of the "modified sine" (actually modified square wave), Quasi-sine wave, etc. can also vary quite a bit between inverters, and may also vary somewhat with the load. The very bottom end put out a wave that is nothing but a square wave, and is too "dirty" for all but universal motor driven tools, coffee makers, toasters, and other appliances that have only a heating element.

One solution to the problem of a few small appliances not working well with modified sine wave inverters is to get a large standard inverter, and a small true sine wave for use only with that equipment. This would also allow you to keep the small appliance (such as an answering machine) powered up without having to run the larger inverter full time.

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What is the difference between sine wave and modified sine wave?

Alternating current (AC) has a continuously varying voltage that swings from positive to negative. This has great advantages in power transmission over long distances. Power from your power company is carefully regulated to be a perfect sine wave, because that is what naturally comes out of a generator, and also because sine waves radiate the least amount of radio power.
On the other hand, a sine wave is expensive to make in an inverter, and many sine wave techniques use heavy, inefficient transformers. The most inexpensive way to make AC is to switch it on and off--a square wave. A modified sine wave is scientifically designed to simulate a sine wave in the most important respects so that it will work for most appliances. It consists of a flat plateau of positive voltage, dropping abruptly to zero for a while, then dropping again to a flat plateau of negative voltage, back to zero for a while, then returning to the positive voltage.

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How do I know if I need a sine wave, or if I can live with a modified sine wave?

The following gadgets work well with a modified sine wave: computers, motor-driven appliances, toasters, coffee makers, most stereos, ink jet printers, refrigerators, TV's, VCR's, many microwave ovens, etc.

Appliances that are known to have problems with the modified sine wave are some digital clocks, some battery chargers, light dimmers, some battery operated gadgets that recharge in an AC receptacle, some chargers for hand tools (Makita is known to have this problem). In the case of hand tools, the problem chargers usually have a warning label stating that dangerous voltages are present at the battery terminals when charging.

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What Is Inverter efficiency?

The efficiency of an inverter has to do with how well it converts the DC voltage into AC. This usually ranges from 85% to 95%, with 90% being about average.

Efficiency ratings are usually given into a resistive load (basically something like a light bulb or electric heater). When running such things as motors, the efficiency actually breaks down into two parts - the efficiency of the inverter, and the efficiency of the waveform. Waveform efficiency means that most motors and many electronic appliances run better and use less power with a sine wave. Typically, an electric motor (such as a pump or refrigerator) will use from 15% to 20% more power with a modified sine wave than with a true sine wave. When choosing an inverter based on efficiency, you should also consider what you are going to be running.

What size wire, fuse, or breaker will I need?

Inverters have two or three sets of power carrying wires to be concerned about: the wires from the battery to the inverter, the wires from the inverter to the home (or other load), and in some cases the wiring from a backup generator or other AC source. The wiring for the AC to the home and from the generator is sized just like you would for AC wiring in a utility connected home. It is usually #10, 12, or 14 standard AC wire. For the small inverters, 800 watts or less, #16 can be used but the mechanical strength of small wire leaves much to be desired.

The wire or cables from the batteries to the inverter are much more critical, and are often undersized. In some cases, the cable may be large enough to carry the "static" load of a motor, but on start up will drop so much voltage in the cable that the inverter will shut down. The same thing can happen with small inverters and TV sets - a TV may only use 100 watts, but the start up surge may be 300 watts for a few seconds. Wire lengths from the battery should always be kept as short as possible, but not so tight that there is a strain on the connections.

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