How to Determine the Right Inverter Size For Your Requirements

An inverter is a device that turns the power from a 12 volt DC battery, like the one in your car or truck, into the 120 volt AC power that runs all of the electronics in your house. You can use one of these devices to power all sorts of devices in your car, but it's important to figure out how big of an inverter you need first.

How to Estimate Inverter Size Requirements

Before you buy and install a power inverter, it’s essential to determine what your power needs are going to be. It’s also important to avoid overtaxing your electrical system, which is mainly an issue when dealing with automotive applications. When installing an inverter in a car or truck , the amount of power available is limited by the capabilities of the electrical system, which—barring the installation of a performance alternator—are pretty much set in stone.

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In order to make a good estimate of your power needs, you’ll need to take a look at all of the devices you plan on plugging into your new inverter. If you only need to use one device at a time, then that’s the only one you’ll need to consider. The situation becomes more complicated as you add more devices, but it's still a relatively simple calculation.

How Much Power Is Enough for an Inverter?

The right size inverter for your specific application depends on how much wattage your devices require. This information is usually printed somewhere on electronic devices, although it may show voltage and amperage ratings instead.

If you are able to find the specific wattages for your devices, you'll want to add them together to get a bare minimum figure. This number will be the smallest inverter that could possibly suit your needs, so it's a good idea to add between 10 and 20 percent on top and then buy an inverter that size or larger.

Some common electronic devices and wattages include:

Device Watts
Cellular phone 50
Hair dryer 1,000+
Microwave 1,200+
Mini fridge 100 (500 on startup)
Laptop 90
Portable heater 1,500
Lightbulb 100
Laser printer 50
LCD television 250

These numbers can vary quite a bit from one device to another, so never rely entirely on such a list when determining power inverter size requirements. While these numbers can be useful in an initial estimate, it's important to determine the actual power requirements of your equipment before you purchase an inverter.

What Size Inverter Should You Buy?

Once you've figured out what devices you want to plug into your inverter, you can dig right in and figure out the right size inverter to buy. As an example, let's say that you want to plug in your laptop, a light bulb, a television, and still be able to run your printer.

The first thing you need to do is figure out how much power each device uses, and add them up:

Device Watts
Laptop 90 Watts
Light bulb 100 Watts
LCD television 250 Watts
Printer 50 Watts
Subtotal 490 Watts

After adding up the power requirements of each device you want to use, the resulting subtotal is a good baseline to work from. However, you'll still want to add at least 10 to 20 percent for the safety margin that we mentioned in the previous section.

If you don't give yourself a margin of error, and you run your inverter right up against the ragged edge all the time, the results won't be pretty.

490 Watts (subtotal) + 20% (safety margin) = 588 Watts (minimum safe inverter size)

What this number means is that if you want to run those four specific devices all at once, you'll want to buy an inverter that has a continuous output of at least 500 Watts.

The Magic Car Power Inverter Formula

If you aren't sure of the exact power requirements of your devices, you can actually figure that out by looking at the device or doing some pretty basic math.

For devices that have AC/DC adapters, these inputs are listed on the power brick. (However, it’s more efficient to look for direct DC plugs for those kinds of devices, since you won’t be converting from DC to AC and then back to DC again.) Other devices typically have a similar label located somewhere out of sight.

The key formula is:

Amps x Volts = Watts

That means you’ll need to multiply the input amps and volts of each device to determine its watt usage. In some cases, you can just look up the wattage for your device online. In other cases, it’s a better idea to actually look at the power supply. For example, let’s say you want to use an Xbox 360 in your car. That’s a case where you really need to look at the power supply because Microsoft has released a number of models over the years that all have different power requirements.

Looking at the power supply for my Xbox, which dates all the way back to 2005, the input voltage is listed as “100 - 127V” and the amperage is “~5A.” If you have a newer version of the console, it may draw 4.7A or even less.

If we plug those numbers into our formula, we get:

5 x 120 = 600

which means that I would need at least a 600-watt inverter to use my Xbox 360 in my car. In this particular case, the electronic device in question—the Xbox 360—draws a varying amount of power depending on what it’s doing at the time. It will use significantly less than that when you’re on the dashboard, but you have to go with the specifications on the power supply to be safe.

Go Big or Go Home: Is A Bigger Inverter Better?

In the previous example, we found out that my old Xbox 360 power supply can draw up to 600 watts during heavy usage. That means you’d need at least a 600 watt inverter to use an Xbox 360 in your car. In practice, you might get away with a smaller inverter, especially if you have a newer version of the console that isn’t quite so power-hungry.

However, you always want to go with a bigger inverter than the numbers say you need. You also have to figure in all of the devices that you want to run at once, so in the above example you’d want tack on 50 to 100 watts for your TV or monitor (unless you have a video head unit  or another 12V screen for playing your games. If you go too big, you’ll have extra room to work with. If you go too small, you’ll have another potentially expensive purchase on your hands.

Continuous vs. Peak Car Power Inverter Outputs

The other factor to keep in mind when determining the necessary size of a power inverter is the difference between continuous and peak power output.

Peak output is the wattage that an inverter can supply for short periods of time when the demand spikes, while continuous output is the limit for normal operation. If your devices draw a combined total of 600 watts, then you need to buy an inverter that has a continuous output rating of 600 watts. An inverter that is rated 600 peak and 300 continuous just won’t cut it in that situation.