Volts to Watts Calculator

You cannot from volts alone — a voltage is a pressure, not an amount of power, and the same 120 V outlet can supply a 5 W phone charger or a 1,800 W space heater. You need a second figure. With the current: watts = volts × amps, so 120 V at 15 A is 1,800 W and 12 V at 10 A is 120 W. With the resistance of a heating element: watts = volts² ÷ ohms, so a 12.8 ohm element at 240 V dissipates 4,500 W.

On AC, yes, for anything that is not a plain resistance. Volts times amps gives volt-amperes, the apparent power the wiring carries. The watts — the power doing work — are volt-amperes times the power factor. A heater has a power factor of 1, so the two are equal. A motor under load is typically around 0.85, so a compressor drawing 5 A at 120 V is 600 VA but about 510 W. On DC there is no power factor.

For a resistive appliance — a water heater element, baseboard heater, range or dryer element — about 75 percent of its rating. Its resistance is fixed, so power goes with the square of the voltage: (208 ÷ 240)² is 0.751, and a 4,500 W element delivers about 3,380 W. This is the ordinary case in an apartment on a 208Y/120 V building service. It does not apply to equipment that controls its own current, such as an EV charger, which loses about 13 percent.

Multiply by the square root of three, about 1.732, as well, using the line-to-line voltage: watts = volts × amps × 1.732 × power factor. 20 A per line at 480 V and a power factor of 0.9 is about 14,965 W. Entering the line-to-neutral voltage — 120 on a 208Y/120 system or 277 on 480Y/277 — understates the power by the same 1.732, and the tool flags both.

Only to find the ceiling, not what anything draws. 15 A at 120 V is 1,800 W, which is the most the breaker allows before it trips, not a load the circuit may carry for hours. A continuous load is limited to 80 percent of the rating, and the NEC 210.23 limits on a shared circuit are lower still for one appliance. The amps to watts calculator works through those limits from a breaker rating.

Not by itself. A higher voltage delivers the same power at less current — a 4,800 W load draws 40 A at 120 V and 20 A at 240 V — which is why large appliances run on 240 V: the smaller current needs smaller conductors. What decides the power is the load. The exception is a fixed resistance, where raising the voltage does raise the power, with the square of the change, and supplying it above its rating overdrives it.

No. It reproduces the standard conversion arithmetic as a preparation aid. It does not assess whether a circuit, conductor or breaker can carry the result, and the re-rating arithmetic is valid only for resistive loads. Confirm figures against the equipment nameplate and installation instructions, and with the authority having jurisdiction.