Find the Right Inverter Size

Just How Much Power You Do You Need?

car power inverter estimate formula
Figuring out the right size for your car power inverter is actually a pretty simple formula. Bernd Opitz / Stone / Getty

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.

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 look at. The situation does get 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:

Cellular phone50
Hair dryer1,000+
Mini fridge100 (500 on startup)
Portable heater1,500
Light bulb100
Laser printer500 (2,000 peak)
LCD television250

These numbers can vary quite a bit from one device to another, so never rely entirely on a list like this 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.

The Magic Car Power Inverter Formula

After you’ve figured out exactly what you want to plug into your inverter, you can look for the input currents and voltages that they use. For devices that have AC/DC adapters, these inputs will be listed right on the power brick. (However, it’s more efficient to look for direct DC plugs for devices like that, since in that case you won’t be converting from DC to AC and then back to DC again.) Other devices will typically have a similar label located somewhere out of sight.

The key formula is:

Amps x Volts = Watts

That means you’ll need 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 that 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 50-100 watts for your TV or monitor (unless you have a video head unit or another 12V screen to play your games on.) 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.

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