How this instrument works
Appliance wattage is current times voltage — the Ohm's-law power identity P = IV. It matters because a lot of real nameplates, especially on motors, pumps and older appliances, print an amp rating and a voltage rating but never state the watts directly. If you need watts — to check a circuit breaker's headroom, size an extension cord, pick an inverter, or add the device into a generator-sizing worksheet — this is the one multiplication that gets you there.
The formula is exact for a resistive or unity-power-factor load: a space heater, an incandescent bulb, a toaster. It is a close enough estimate for most single appliances quoted this way, though motors and switching power supplies draw current that isn't perfectly in phase with the voltage, so their true (real) power can run a bit lower than amps × volts suggests — the nameplate current itself is usually already the manufacturer's rated figure that accounts for this.
The same identity works at any voltage standard: a 120 V North American branch circuit, a 230 V UK or EU ring circuit, or a 12 V DC automotive or marine system. Only the numbers change; the relationship between current, voltage and power does not.
- Enter Current (A) — the amp rating from the appliance's nameplate, plug, or manual.
- Enter Voltage (V) — the voltage of the circuit or supply the appliance runs on (120 V, 230 V, 12 V DC, etc.).
- Read Power (W) — the appliance's power draw at that voltage.
- Use decimals for precise nameplate ratings, such as 4.5 A rather than rounding to 5.
- If the nameplate already lists watts, you don't need this instrument — it exists for the amps-and-volts-only case.
Worked example — a 5 A appliance on a 120 V circuit
Enter 5 into Current (A) and 120 into Voltage (V) — a typical small appliance on a standard US household circuit. Power reads 5 × 120 = 600 W, exact.
That 600 W figure is what you'd plug into a downstream calculation: on a 15 A / 1,800 W branch circuit, this appliance alone uses a third of the available headroom, leaving room to judge whether it's safe to add a second device to the same outlet.
Questions
Why does my appliance's nameplate list amps instead of watts?
Because amps is what protects the circuit — breakers and fuses trip on current, not power — so manufacturers rate motors and older appliances in amps first. Watts is the more useful number for comparing running cost or total household load, which is exactly the gap this instrument fills: multiply the nameplate amps by your supply voltage and you have watts.
Is P = I × V exact for every appliance?
It's exact for resistive loads — heaters, incandescent bulbs, toasters — where current and voltage rise and fall together. For motors and electronics with a power factor below 1, the true power drawn can be somewhat lower than the simple product, though the nameplate current rating is usually already chosen conservatively enough that this formula gives a safe, close estimate for circuit-sizing purposes.
Can I use this for 230 V UK or EU appliances?
Yes — the formula doesn't care which voltage standard you're on. A 13 A appliance on a UK 230 V ring circuit (13 A being the standard BS 1363 plug-fuse rating) draws 13 × 230 = 2,990 W, exact, the same multiplication used for a 120 V North American circuit.
Does this work for 12 V DC systems like a car or boat?
Yes. P = IV holds for DC exactly the same way it does for the resistive AC case — a 10 A device on a 12 V DC circuit draws 10 × 12 = 120 W. DC systems are actually the cleanest use case for this formula, since there's no power-factor question to worry about at all.
How do I go the other way — watts to amps?
Divide watts by voltage: amps = watts ÷ volts. That's the same P = IV relationship solved for current instead of power, useful when a spec sheet gives you wattage and you need to know what breaker size or extension-cord gauge the device actually needs.