Car & Vehicle

Car Charger Power Calculator

What a car charger or 12 V accessory draws in amps and watts, and whether the socket fuse will carry it.

6 inputs Free, no sign-up

Your figures

A car system is nominally 12 V and sits nearer 14 V with the engine running. Enter whichever figure you are working to - the difference matters at high loads.

Leave blank to calculate it from the power.

The charger's rating in watts. Leave blank to work it out from the current instead.

Try an example

Result

Your result

Enter your figures and the result appears here.

Estimates only. Rates, fees and specifications change. Confirm against the official source before you rely on a figure — see our disclaimer.

About this calculator

A car's accessory socket is fused, and the fuse is the limit that matters. Most are 10 or 15 A, which at 12 V is 120 or 180 watts — enough for a phone charger and a dashcam, and not enough for everything at once on a splitter.

Enter any two of voltage, current and power and this page gives the third. It is the same electrical law as anywhere else; the only thing a car changes is the voltage you enter.

How to use this calculator

  1. Enter the socket voltage. 12 V is the nominal figure; with the engine running the system sits nearer 13.8–14.4 V.
  2. Enter whichever of current or power you know, and leave the other blank.
  3. Compare the current against the fuse for that socket, which is in the fuse box diagram or the handbook.

The formula

Power   = Voltage x Current
Current = Power / Voltage
Voltage = Power / Current

Working at 12 V rather than 14 V gives a slightly higher current for the same power, which is the conservative direction and the right one when you are checking a fuse.

SymbolMeaningUnit
V Socket voltage V
I Current drawn A
P Power W

Worked example

A 60 W USB-C car charger at 12 V:

Current = 60 / 12 = 5 A

Comfortably inside a 10 A fuse on its own. Add a 12 W dashcam and a 24 W tyre inflator on the same splitter and you are at 8 A, which is close enough to matter.

Notes and limits

  • The socket fuse is the real limit, and it is there to protect the wiring rather than the accessory. Fitting a larger fuse to make something work is how a wiring loom catches fire.
  • A charger's rating is what it can deliver at the USB end. It draws a little more than that from the socket, because the conversion is not lossless.
  • Many cars cut power to the accessory socket with the ignition; some do not. A dashcam left on a live socket will flatten a battery over a few days.
  • Splitters multiply what you can plug in, not what the fuse will carry. Add up the loads before assuming they all fit.
  • For a device quoting only USB output, the USB-C power calculator works out what the profile actually delivers.

What this calculation assumes

  • The calculation is ideal - conversion losses in the charger mean the socket sees a little more than the USB output rating.
  • No device's consumption is assumed. Every figure is one you entered.

Frequently asked questions

How many watts can a car cigarette lighter socket take?

It depends on the fuse, which is usually 10 or 15 A. At 12 V that is 120 or 180 watts. Check the fuse box diagram for your vehicle rather than assuming - some sockets are on a shared circuit.

How do I work out the current a car charger draws?

Divide its power rating by the socket voltage. A 60 W charger at 12 V draws 5 A, a little more in practice because the conversion has losses.

Is a car 12 V or 14 V?

Both, at different times. The battery is nominally 12 V; with the engine running the alternator holds the system at roughly 13.8-14.4 V. Using 12 V for a fuse check is the conservative choice because it gives the higher current.

Can I use a splitter to run several devices?

Only up to the fuse rating. A splitter gives you more sockets, not more current. Add up what each device draws and keep the total comfortably under the fuse.

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