Power Inverters

This page is written for the benefit of people who are looking for a source of 120 volt AC electricity for use in an emergency or while camping. If you are already an expert in the subject, you'll probably be quick to point out that this page contains a few oversimplifications, and you would be quite right. However, this page is intended for people with only limited experience, and it is intentionally written quite simply. If you're an expert on the subject, then this page probably won't be particularly helpful. But if you don't have much background on the subject, and simply want an inexpensive way to solve a particular problem, then this page and the links here will probably be very helpful.

The most important thing to know about power inverters is that they are a good solution for relatively small electrical devices. And by "small", I mean devices that have low power needs. Most electronic devices fit in this category. However, most devices that generate heat are not good candidates for using an inverter. No matter how small they are, devices that generate heat do so by using large amounts of electric power. Therefore, if you are looking for a way to run your coffee pot, toaster, hairdryer, curling iron, or electric heater, then a power inverter is a very poor choice. You might be able to come up with something that works, but it will probably be expensive. And more importantly, it will run down the battery you intend to use very quickly.

If you need to do something that generates heat, then you should probably stop thinking about an inverter. (If you want to make coffee, I have more information on my How To Make Coffee Without Electricity page.)

An inverter is an electronic device that converts 12 volts DC (the electricity that is supplied by your car) to 120 volts AC (the electricity that comes from the plugs in your house). The price can range from about $10 to hundreds of dollars, depending upon your needs. You hook the inverter up to your car, either by hooking it directly to the battery, or, in the case of smaller units, by plugging it into the car's cigarette lighter socket. The inverter has a standard household AC outlet on it, and you plug your 120 volt device directly into that outlet.

The cost of the inverter will depend on two factors. First, it will depend on the number of watts it supplies. Also, it will depend on how "clean" you require the power to be. We'll discuss that later in this article.

Because the cost can vary considerably, you want to determine what your needs are. If a $10 inverter will serve your needs, then it's a waste of money to buy an inverter costing hundreds of dollars. And more importantly, in many cases, the cheap inverter will actually do the job better than the expensive one, because in many cases, your car's battery will last longer using the cheap inverter.

As you probably guessed, the smaller (usually cheaper) inverters supply less power--a lower number of watts--than the larger, more expensive ones. But because of this, the smaller inverters usually draw less "juice" from the 12 volt battery. And in many cases, the larger inverter will continue drawing more "juice" even when nothing is plugged in. Therefore, you will usually get more battery life if you use the smallest possible inverter for the job. And usually, this means that the less expensive inverter will actually be better than the more expensive one.

Therefore, the best possible advice when buying an inverter is to buy one that will supply just slightly more watts than are necessary to do the job. In other words, if you need to run a 40 watt item, then you should probably use a 60 watt inverter rather than a 100 watt model. If you need to run 100 watts, then you can't use the 60 watt model. But if you have a choice between a 150 watt model and a 300 watt model, then the 150 watt model is almost certainly the best choice.

This also means that you might be best buying two inverters. You can use the small one for small loads, and the larger one for large loads. Of course, if you need to run the small device only for a few minutes, then the difference in how fast you run down the battery wouldn't be a major concern, and it would be OK to use the larger one. But before you buy, you should give some thought to how you will be using the inverter.

Calculating the Number of Watts you Need

The first step, even before you start shopping, is to calculate the number of watts you will be using at any given time. Whatever you can do to minimize the number of watts will save you money when buying an inverter, and will give you the maximum battery life. For example, if you have two 40 watt items and it's possible to run them at different times, then this will be preferable to trying to run both of them at the same time.

To determine the number of watts you need, the first step is to look on the back of the electrical device. Usually, this number is given. Then, you simply add up the total number of watts of the items you will be running together. So if you have a 40 watt item and a 60 watt item, this will be 100 watts, and you will need to buy an inverter that puts out at least 100 watts.

In some cases, the device will not indicate how many watts it uses. In those cases, you might be able to find that information in an owner's manual or by searching online. For example, most product descriptions on Amazon will list the number of watts.

In some cases, this information will be given in terms of the number of amps. All you need to remember is that watts equals amps times volts. Since we are dealing with 120 volts, all you need to do is multiply the number of amps by 120. For example, if the device is marked "2 amps" or "2 amperes", you multiply this by 120, and know that the device uses 240 watts.

This advice works for most devices, although there is an exception in the case of devices that have a motor. In many cases, a device that has an electric motor will draw more amps (and therefore, more watts) for a split second when it is starting up. Therefore, if you are right at the limit, the motor will not be able to start. For very small motors, this is not a big issue, because the additional starting current is usually quite small. But in the case of power tools or a refrigerator, this could be significant, and the inverter might need to be several times larger than would be necessary otherwise. If you are looking for an inverter for a use such as this, then you need to look at the specifications for words such as "peak current" and make sure they are adequate.

For this reason, an inverter is rarely a good choice for use with a large refrigerator. And you would almost never use an inverter to run an air conditioner, because the size of the inverter would make it extremely expensive.

Once you know the size of the inverter, it's a relatively easy matter to start shopping. Remember, get one that supplies a number of watts that is more than what you calculated you will need. But don't get one that is significantly larger than what you need. Not only will you waste money, but you will run down your battery faster than necessary.

Here are some inverters currently available on Amazon. There are many more available, but these will give you some idea of what is available in certain watt ranges.

Under 100 Watts

150-200 Watts

300-500 Watts

1000 Watts and Higher

As you can see, the smaller inverters generally plug into the car's cigarette lighter socket. The larger units need to be hooked directly to the battery. This is because the cigarette lighter socket is fairly limited in how much power it can supply. If you tried to use one of the larger units plugged into the cigarette lighter, you would quickly blow a fuse in the car.

"Clean" power

In some cases, you will need a more expensive inverter to supply "cleaner" power. When you look at the specifications, some of these inverters state that they supply a "pure sine wave", which is the cleanest type of power. Some electronic devices will not function without this "clean" power. However, in my experience, the vast majority of electrical devices and most electronic devices will work fine without worrying about this factor. Some inverters are advertised as a "modified sine wave", which means that they are not quite as "clean". The ones that don't mention this at all are probably the least "clean". However, for most applications, they will work fine, and will usually represent a big savings in money. The most likely culprits to require "clean" power are the power supplies for laptop computers.

Avoiding An Inverter Entirely

There are many devices that require 120 volts. However, if you can get away without 120 volts, you are almost always better off doing so. Therefore, if you have a small electronic device, then instead of buying an inverter, you are probably better off buying a car adapter for that particular device. If the device has a USB hookup, then one of these adapters is the best option:

Not only will you save money doing it this way, but you will get more battery life. The inverter will always introduce a certain amount of inefficiency, and it's best to avoid this. However, having the inverter and using the existing AC adapter can be very convenient, and since many electronic devices use very little electricity to start with, the added inefficiency is sometimes quite negligible.

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