DC Inverter FAQ

1. What is an inverter?
2. How does an inverter work?
3. Why are most inverters are 115 volts
4. What size wire, fuse, or breaker will I need?

What is an inverter?

An inverter changes DC voltage from batteries or solar panels, into standard household AC voltage so that it can be used by common tools and appliances.

Converters: What are sometimes called "converters", especially in the RV world, are actually battery chargers and/or DC power supplies. Why they are called converters in RV's and no place else we have not a clue. A "converter" is basically the opposite of an inverter.

Essentially, it does the opposite of what a battery charger or "converter" does. DC is usable for some small appliances, lights, and pumps, but not much else. Most systems should include an inverter of some type, even if it is just an el-cheapo $29 Walmart thing to run the TV occasionally. Some DC appliances are available, with the exception of lights, fans and pumps there is not a wide selection. Most other 12 volt items we have seen are expensive and/or poorly made compared to their AC cousins. The most common battery voltage inputs for inverters are 12, 24, and 48 volts DC - a few models 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 feeds 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. Some, like the Sunny Boy, go up to 600 volts DC input.

A few grid tie inverters can also be used with batteries, but there will be some loss in overall efficiency for feeding the grid. How much loss can vary considerably, depending on the inverter and the size and type of batteries. If you need battery backup power for a grid tie system, we recommend the Outback Power inverters, as they have the best efficiency with batteries - you will get about a 5-10% loss. With some older inverters, such as the Xantrex SW series, that can sell back excess power to the grid overall losses can be as high as 50%.

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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 "alternating DC" into 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. They are pretty rare nowadays, some of the old 1970's Triplite and a few others, and some old military surplus is about the only place you find it now.

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 or appliances that electronic heat or speed control, or uses the AC for clocks or a timer.

What May Not Run: Appliances that use electronics to control temperature or timers may have problems with modified sine waves. This includes anything - tool or appliance - that is variable speed, bread makers, some microwaves, some washers and dryers that use electronic timing for cycling. Most computers, TV's and similar items will have no problem. Anything with a motor will use about 20% more power with a modified sine wave than with a true sine wave.

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 (such as the Exeltech or Samlex) 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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Why are most inverters 115 volts?

Most utility connected homes in North America have dual AC voltages - 115 and 230. On a typical home there are three wires coming in - 115-neutral-115. It is 230 volts across the two outside ones. The 115 is used for most things, while 230 is used for water heaters, electric clothes driers, well water pumps, and air conditioning. Since these high-power items are not practical in a solar powered home, they are either not used or are replaced with gas appliances.

Most off-grid homes have little use for 230 volt AC power - but even so many newer ones are wired just like a standard home to meet electrical and building codes. If it IS required, you can "stack" two 115 volt inverters to get 230. The one exception to the above is that many AC well pumps are 230 volt. If the well pump is the only 230 volt item you have, the best choice is probably to get a step up transformer, such as the Xantrex or Outback Power 120 to 240 step up transformer.

There are export versions of most inverters for 100 volts, 105 volts, 205 volts, and 220/230 volts, in both 50 and 60 Hz.

Inverter-Chargers:

Inverters come in two basic types - with and without built in battery chargers. The ones with built in chargers are handy if you charge your batteries from AC, especially for RV's. They are also essential if using an inverter for setting up a UPS system for backup power. But not everyone needs them - and most small inverters under 1000 watts or so are simply not available with a built in charger.

Nearly all inverter-chargers made in the past few years have 3-stage chargers, so you can usually leave them powered up all the time. Nearly all inverters with chargers also have a built in transfer relay - what that means is that if you are running from AC or shore power, the power feeds through the inverter, with some being tapped off for the battery charger. If the AC power goes out, the inverter automatically switches to battery power. In most cases you won't even see a light flicker, it is so fast.

Inverter (and other) Efficiency:

Inverter efficiency is a question we get asked about a lot. 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.

However, there is more to the story. 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.

A 90% efficient modified sine wave inverter is not 90% when running a compressor motor, for example, because electric motors are less efficient. They use about 20% more power on a modified sine wave.

Inverters are also much less efficient when used at the low end of their maximum power. For example, using a 1000 watt inverter to power a 20 watt radio may actually be using 30 to 40 watts from the battery, as the inverter itself is eating up a lot just to run. Most inverters are most efficient in the 30% to 90% power range.

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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 AC 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 on low voltage cutoff. 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.

These are the recommended cable 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 (Outback Power, Xantrex) inverters usually have a much higher maximum surge rating than electronic based (Samlex, Exeltech, Statpower), they should always use the larger if more than one size is shown. The reason some show a smaller breaker size than fuse size is that breakers do not blow as fast on a temporary surge.

The fuse should NEVER be bigger than 125% of the maximum surge power of the inverter. For example, an inverter is rated at 1000 watts, and 1800 watts surge. For a 12 volt inverter, divide 1800 by 12, which gives you 150. 150 x 1.25 = 190 amp. The nearest standard size fuse is 200 amp. You are always safe going to a smaller fuse, but if too small it might blow on heavy loads. DC breakers should be rated for about the maximum amperage draw, as they have a slight time delay on over current.

Which inverter has the best sine wave?

In general, from best down, it is Exeltech, Outback Power, Statpower, Samlex. All are good enough for 99% of all applications, but the Exeltech may be better for low power critical applications, such as recording or studio vans, or noise sensitive medical equipment. For higher power systems that need the best sine wave, either the Outback Power series or the Xantrex SW+ series.

Which is the "best" inverter?

There is no "best" for all purposes. Although the Outback Power & Xantrex are considered by many to be the top of the line, it does not make sense to spend $500 to $3000 when all you need is a little Statpower Prowatt or Exeltech 125 watt sine wave to power up a laptop. The best way to decide on what inverter is best is to work backwards - figure out what you are going to use it for, and then find one that fits those requirements. Also, some inverters have built in chargers, which may be needed in some systems. The Outback & Xantrex sine wave units include software and hardware for remote generator start, alarms, remote control and monitoring, computer data, and other functions - in many applications this is very important. If you are running pumps or other large motors, Xantrex or Outback are the only one we will recommend, even though some others might work.