Mobile & Electronics

Adapter Power Calculator

An adapter's maximum output in watts, from the voltage and current on its label.

5 inputs Free, no sign-up

Your figures

From the OUTPUT line. This is the figure that has to match the device.

The rated current at that voltage.

From the original adapter or the device's input rating.

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

An adapter's wattage is its output voltage times its output current. A 12 V 2 A adapter is rated at 24 W, which is the most it can supply — not what any particular device will take from it.

The wattage is the least important of the three figures when deciding whether an adapter suits a device. The voltage has to match, the polarity has to match, and the plug has to fit. An adapter with ample wattage and the wrong voltage is not a substitute for anything.

How to use this calculator

  1. Read the OUTPUT line on the adapter, not the input line.
  2. Enter the voltage and the current from it.
  3. Optionally enter what the device needs, for the headroom.
  4. Before substituting one adapter for another, check the voltage matches exactly and the polarity symbol is the same.

The formula

Maximum output = Output voltage x Output current

A replacement adapter needs the same voltage and at least as much current as the original. More current is fine — the device draws what it needs. A different voltage is not fine, in either direction.

SymbolMeaningUnit
V Adapter output voltage V
I Adapter rated output current A
P Maximum nominal output W

Worked example

An adapter labelled OUTPUT: 12V 2A:

P = 12 x 2 = 24 W

Replacing an original rated 12 V 1.5 A (18 W): the voltage matches and the current is higher, so it is suitable on those two counts. The plug size and polarity still have to be checked.

Notes and limits

  • Voltage has to match. Too low and the device will not run properly; too high and it may be damaged. Wattage does not compensate for either.
  • Polarity matters on barrel connectors. The symbol on the label shows whether the centre pin is positive or negative, and getting it wrong can destroy a device.
  • More current than the original is fine. The device draws what it needs; the rating is a ceiling.
  • Plug size is a separate question again, and one that no calculation addresses.
  • This is arithmetic on two numbers, not a compatibility check. Nothing here knows what device you are plugging it into.

What this calculation assumes

  • The figures come from the adapter's OUTPUT rating, which is a maximum.
  • Any comparison shown is of wattage only, and says nothing about voltage, polarity or connector.
  • A device draws what it needs, not what the adapter could supply.

Frequently asked questions

How do I find an adapter's wattage?

Multiply the output voltage by the output current from the OUTPUT line on the label. A 12 V 2 A adapter is 24 W.

Can I use an adapter with a higher wattage?

Yes, if the voltage matches exactly and the plug and polarity are right. The device draws the current it needs and the rest of the adapter's capability goes unused. Higher wattage at a different voltage is not safe.

Can I use an adapter with a different voltage?

No. Too low and the device will misbehave or not start; too high and it can be damaged immediately. Voltage is the figure that has to match, and no amount of extra wattage makes up for it.

What does the polarity symbol mean?

It shows whether the centre pin of the plug is positive or negative. The symbol on the replacement has to match the symbol on the original, or the device can be destroyed even when the voltage is correct.

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