When you’ve already figured out what you would connect to power inverter, you should check the input currents and voltages that they use. For applications that have AC/DC adapters, these inputs would be listed right on the nameplate. Or you can just check these DC plug, which one should also list rated currents and voltages.

Here the magic formula is:

Amps x Voltages = Wattages
That means you’ll need times the input amps and volts of each device to get its wattage consumption.

In some cases, you can just search the wattage for your device online. In other cases, it would be better to actually look at the power supply.

For example, if you would like to use a electric appliance in your car, you look at the power supply for this device, which shows the input voltage is listed as “100 – 127V” and the amperage is “~5A.”

Let’s plug those numbers into key formula, we can get:

5 x 120 = 600
That means you would need at least a 600W power inverter to use this device in my car.

In this particular case, the electronic device in question draws a varying amount of power depending on what it’s doing at the time. It will use significantly less than that when just standby, but you’d better to go with the specifications on the power supply to be safe.