Volt to Watt Calculator

About this calculator

There is no conversion from volts to watts, and any page that offers you one is making up a number. Voltage describes how hard electricity is being pushed; wattage describes how much work is being done. A 5 V USB port can be delivering 2.5 W or 100 W, and the voltage is identical in both cases.

What is missing is the current. Enter it below and the wattage follows immediately. If you do not have it, the rest of this page explains where to find it.

How to use this calculator

1. Enter the voltage you have.
2. Find the current. It is on the same label as the voltage on almost every power supply, usually as a number followed by A or mA.
3. Enter it and read the wattage.
4. If your label gives watts and volts and you want the current instead, the watt to amp calculator goes that way.

The formula

Power = Voltage x Current       P = V x I

Rearranged, that is why voltage alone gives nothing: with I unknown, P is unknown. The equation has two unknowns and one of them has to come from somewhere.

The only case where a voltage alone implies a power is when the resistance is also known, in which case P = V² / R. That is a different calculation and it needs the resistance, which is not usually printed on anything.

Worked example

Three devices on the same 5 V supply:

5 V x 0.5 A  =  2.5 W     a small sensor
5 V x 2.4 A  =   12 W     a typical phone charger
5 V x 3 A    =   15 W     a fast USB-A charger

Identical voltage, wattages differing by a factor of six. That is the whole reason this conversion does not exist.

Notes and limits

• Almost every power supply prints both figures. Look for a number followed by A or mA next to the voltage.
• Where a label gives volts and watts, the current is the watts divided by the volts — no extra information needed.
• A device's wattage is what it draws, not what the supply could give. A phone on a 65 W charger is not drawing 65 W.
• If you know the resistance instead of the current, power is voltage squared divided by resistance. That applies to a heater or a resistor, not to an electronic device, whose resistance changes with what it is doing.

What this calculation assumes

• Voltage alone is not enough, and no current is assumed to make up the difference.
• A power factor of 1 treats the circuit as DC or purely resistive.
• The alternative route through resistance applies to resistive loads only.