How to get household current in your car

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Car inverter and laptop charger

Brian Cooley/Roadshow

If you want to power a laptop or other large electric device in your car, you need an inverter that turns your car’s 12-volt DC power into 120 volts of AC like you have in your home. Here’s how they work and what to look for when you buy one.

If you want to power a laptop or other large electric device in your car, you need an inverter that turns your car's 12-volt DC power into 120 volts of AC like you have in your home. Here's how they work and what to look for when you buy one.


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What a car power inverter does 

The name “inverter” doesn’t really tell you much. It should be called a “polarity flipper and voltage amplifier,” but luckily it isn’t. In simple terms, a car power inverter takes the 12-volt DC electricity in your car and alternates its polarity. Imagine if you took the red and black cables on your car’s battery and switched them back and forth quickly: You’d create a crude form of alternating current that looks like this:

Sine wave vs square wave

True sine wave AC power found at a household outlet looks like the smooth waveform at the tip. Rapidly flipping the polarity of a DC power source, like a car battery, results in the crude square wave AC power illustrated at bottom.


Brian Cooley/Roadshow

An inverter takes that crude AC and breaks it down into steps so it looks more like the sine wave AC that comes out of a household outlet, while also upping the voltage from 12 to 120. How elegantly the inverter does that is a big part of its cost.

Modified wave AC power

A DC power inverter gradually steps your car’s electric power up and down while also flipping its polarity regularly. The result, shown here in purple, starts to closely resemble the AC you get at home.


Brian Cooley/Roadshow

How to buy an inverter for your car

When buying an inverter to power electronics from your car, add up the wattage of all the devices you’ll plug into it at once and buy an inverter rated 20% higher than that wattage to be safe. If you’re going to run things that have motors or ballasts, like power tools or fluorescent lights, you need to buy a much bigger inverter to account for the high power those devices need at startup or during pulses in their operation.

DC power inverter sizing chart

This sizing chart shows that some devices require an inverter with a much higher wattage “cushion” than you need for a laptop or other small electronics.


Samlex

You’ll also need to decide how good a sine wave you need. A simpler, cheaper inverter will offer “modified sine wave” power that is acceptable for simpler electric devices, like an electric drill or light bulb. 

Amazon

Brand name modified square wave power at a low price.

For laptops and other sophisticated electronics, I recommend a “pure sine wave” inverter that puts out AC that’s virtually indistinguishable from the AC power you have at home. Such an inverter may cost substantially more, but ensures your delicate electronics work and last as they were intended.

Amazon

An inverter that faithfully replicates the sine wave found in household outlet AC and is rated to power devices up to a combined total of 300 watts.

Finally, decide how you will connect an inverter to your car. Smaller inverters that plug into a 12-volt “cigarette” outlet are limited to that outlet’s fuse rating. For example, a 12-volt outlet that is protected by a 15-amp fuse can only support 200 watts of power from an inverter, regardless of how big an inverter you connect to it. To determine this calculation for your car:

  • Look up the fuse rating for the 12-volt outlet in your car by inspecting it or checking the owner’s manual
  • Determine the voltage you will use: Your car will supply 12 volts when it’s off, 13.8 volts when it’s running
  • Note what wattage you desire in an inverter, which is its rated capacity

Plug that information into one of the various watt/amp calculators online and see if the resulting amp number is at or below the rating of the car’s 12-volt outlet fuse. If the calculation comes out above the fuse rating, you can still use the inverter, but not at its full rated watt output. To do that, you’ll need to find a more robust way to connect the inverter to your car, such as directly connecting it to the battery. Many inverters also include a connecting cable wire with battery clips to temporarily make a higher current connection when the car is stationary. 

DC power inverter cables

The battery clip cables on the left allow this 300-watt inverter to run at full capacity. But powering it via the convenient 12-volt outlet cable on the right would limit its wattage, depending on the size of the 12-volt outlet fuse in the car.


Brian Cooley/Roadshow

With an inverter in your car, you gain a decent margin of preparedness. You’ll know you can power a wide range of electric devices, no matter where unexpected plans or emergencies take you.

First published April 7.

If you want to power a laptop or other large electric device in your car, you need an inverter that turns your car's 12-volt DC power into 120 volts of AC like you have in your home. Here's how they work and what to look for when you buy one.

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