Mobile & Electronics

Charger Power Calculator

What a charger actually puts out, from the volts and amps on its label - and whether that covers what you want to run.

5 inputs Free, no sign-up

Your figures

From the OUTPUT line on the label, not the input line. Multi-voltage chargers list several; work out each one.

The current beside that voltage on the label.

From the device's own charger or its input rating, if you have it. Leave blank to just read the charger's output.

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 charger's wattage is the two numbers on its output line multiplied together. A label reading 5 V 3 A is a 15 W charger. The figure matters because it is the ceiling on how fast anything plugged into it can charge — but only the ceiling, since the device decides how much of it to use.

Enter what your device needs as well and the page compares the two. That comparison is about watts and nothing else: it does not know whether the connector fits, whether the voltage suits, or whether the two ends can negotiate.

How to use this calculator

  1. Find the OUTPUT line on the charger — not the input line, which describes the mains side.
  2. Enter the voltage and the current from it.
  3. Where the label lists several pairs, work out each one. The highest is what the charger can do.
  4. Optionally enter what your device needs, to see the headroom.

The formula

Charger output = Output voltage x Output current
Headroom       = Charger output - What the device needs

A multi-voltage charger's pairs usually come to similar wattages, because the limit is the charger's total power rather than any single voltage. A charger listing 5 V 3 A, 9 V 2.22 A and 12 V 1.67 A is a 20 W charger offering 15 W at 5 V.

SymbolMeaningUnit
V Charger output voltage V
I Charger output current A
P Charger output power W

Worked example

A charger labelled OUTPUT: 5V⎓3A:

P = 5 x 3 = 15 W

Against a laptop that wants 45 W:

Headroom = 15 - 45 = -30 W

The charger is well short. It will most likely charge the laptop slowly while it is asleep and not at all while it is working.

Notes and limits

  • Read the OUTPUT line. The INPUT line describes what the charger draws from the wall and is a different, larger number.
  • A higher-wattage charger does not force power into a device. The device requests what it wants, so a 65 W charger on a phone charges it at the phone's rate, not at 65 W.
  • The cable is part of the chain. A cable not rated for the current is the commonest reason a capable charger delivers less than it could.
  • Wattage matching is not compatibility. The voltage has to be one the device accepts, the connector has to fit, and for anything above basic USB the two ends have to negotiate.
  • Charging slows as a battery fills, regardless of the charger — see the charging time calculator.

What this calculation assumes

  • Figures come from the charger's OUTPUT rating, which is a maximum rather than a measurement.
  • The comparison, where a load is entered, is of wattage only - not of voltage, connector or protocol.
  • What is actually delivered also depends on the cable and on both ends agreeing a mode.

Frequently asked questions

How do I work out my charger's wattage?

Multiply the output voltage by the output current. A 5 V 3 A charger is 15 W. Use the OUTPUT line on the label, not the INPUT line.

My charger lists several outputs. Which one is its wattage?

Work out each pair and take the highest. The lower ones are what it offers at those voltages, which a device may pick if it cannot use the higher voltage.

Will a more powerful charger damage my phone?

Not by itself. A device draws what it is designed to draw and negotiates for higher voltages rather than being given them. What matters is that the voltage is one it expects or can negotiate.

Why is my phone charging slowly on a fast charger?

Common causes are a cable not rated for the current, the two ends failing to agree on a fast-charge mode, a hot battery, or the battery being above 80% where charging always slows. The charger's rating is a ceiling, not a promise.

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