DC to AC Inverters
With the storm and Holiday season just around the corner, power outages are a major problem to deal with. During this part of the year, I always carry a Generator for emergencies caused by bad weather. Over the years this has saved a few parties and returned the gratitude (and tips) of many party goers.

Another solution to remote or portable AC power is the Inverter. This type of inverter can be connected to your car battery and power your equipment. How long the battery will last is determined by its size, the amount of AC power your equipment requires and the time of use. Don't forget to leave your cars engine running if your use this system for a long period of time. You may not be able to start your car when time to leave.

What is an inverter?
An inverter changes DC voltage, nearly always from batteries, into standard household AC voltage so that it can be used by common tools and appliances. Essentially, it does the opposite of what a battery charger does. A battery charger "converts" AC to DC. This AC source is usable for some small appliances, lights, and pumps and is usually suitable for small home or cabin systems, RV's and boats.

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, such as audio gear. 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 inverters put out a wave that is nothing but a square wave, and is too "dirty" for audio usage and is only good for lights and some other appliances that have only a heating element.

The square wave or modified square wave may produce AC may cause problems with audio equipment. Most audio equipment internal power supplies are not able to handle the fast edges produced from a square wave. They will not filter them properly and may just pass them though as "noise" to the speaker. This could be in the form of a 60 of 120Hz hum or click.

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 your equipment. The wiring for the AC to the equipment is sized just like you would for AC wiring in a utility connected venue. 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.

Recommended Fuses, Breakers, and Wire Sizes for Inverters
Inverter Watts
Inverter DC voltage
Input Fuse/Breaker
DC Wire Size
 20 amp/NA
 12 to 14
 30-40 amp/NA
 8 to 10
 50-60 amp/NA
 6 to 8
 110 amp/NA
 4 to 6
 200 amp/175
 2/0 to 2
 300 to 400 amp/250

These are the recommended sizes for a ten-foot distance from the batteries to the inverter. Note that the larger wire size is the recommended, the smaller wire size is the absolute minimum for safe operation. The sizes recommended are from a combination of maximum wire amperage capacity and voltage drop. You can't go wrong using bigger wire.

The fuse and breaker sizes shown are approximate. Since transformer based inverters usually have a higher maximum surge than electronic based, the you should always use the larger if more than one size is shown.
Which inverter has the best sine wave?

In general, from best down, it is Exeltech, Statpower, Trace. All are good enough for 99% of all applications, but the Exeltech or Statpower may be better for critical applications, such as recording or other audio applications, or noise sensitive medical equipment. The true sine wave inverters are always more expensive but worth it. The damage to your sensitive audio equipment from a cheap inverter will be more than you spend if you had used the proper inverter in the first place.

It is also VERY IMPORTANT to know the maximum amount of AC power your equipment requires. This will determine the size of inverter you need. If you are going to use an inverter for emergency power only, use the minimum amount of equipment to continue your show. Leave the lights off! If you want to expand your system to run lights, use a separate inverter for the light so the switching noise they generate will not be feed back into your audio equipment.