Amps to Watts Calculator
Power in watts from current in amps and voltage in volts
Last updated:
What this calculator computes
Amps to watts conversion turns an electrical current measurement into a power measurement, answering the practical question "how much power does this draw?" given a known current and the supply voltage. The relationship comes from the definition of electrical power: power equals voltage multiplied by current, written P = V × I, where P is in watts, V is in volts, and I is in amperes. The conversion is not a fixed factor — a 10 A current draws 1200 W on a 120 V US outlet, 2300 W on a 230 V European outlet, and 120 W on a 12 V automotive circuit, so the supply voltage must always be entered alongside the current. This calculator handles the three current types found in everyday electrical work: DC circuits (batteries, solar panels, automotive accessories), AC single-phase circuits (every residential outlet in the US, UK, EU, Australia, and most of the world), and AC three-phase circuits (industrial machinery, EV chargers above 22 kW, and commercial HVAC). For AC circuits it also accepts a power factor, the dimensionless ratio between real power (watts) and apparent power (volt-amperes) that captures how much of the current is doing useful work versus circulating through reactive loads. Most resistive loads — incandescent bulbs, electric heaters, electric kettles — have a power factor close to 1, while motors, fluorescent ballasts, and switching power supplies typically run between 0.6 and 0.95. The calculator returns the real power in watts and, where relevant, the apparent power in volt-amperes so the difference is visible.
Calculator
The formula
Formula
P (W) = V × I × PF (3-phase: P = √3 × V × I × PF)
Worked example
When to use this calculator
Use this calculator any time you need to know the power draw of an appliance, motor, heater, or circuit and you have current and voltage figures rather than a wattage rating. The most common scenarios are sizing a generator or solar inverter for a known load (where appliance nameplates often list amps but not watts), checking that a household circuit can handle a planned new appliance without tripping the breaker, computing battery runtime when the battery capacity is in amp-hours and the load is in watts, and verifying the wattage of an industrial three-phase motor where the nameplate gives full-load amps and supply voltage. The calculator is also useful for back-checking the power factor of a partially-reactive load when both the apparent VA and the real W are independently known. It does not handle frequency-dependent reactive power calculations or harmonic distortion — for power-quality work on commercial loads, use a dedicated power-factor analyser rather than the simple PF input here.
Common input mistakes
- Using the wrong voltage for three-phase systems. Three-phase power formulas use the line-to-line voltage (e.g., 400 V in Europe, 480 V in US commercial) with the √3 factor; entering the line-to-neutral voltage (230 V or 277 V) without removing the √3 produces a result roughly 1.73× too low. The calculator labels the voltage field as line-to-line for three-phase mode to avoid this trap.
- Assuming a power factor of 1 for motor and electronic loads. Induction motors typically run at 0.7–0.85 PF under load, and many older fluorescent ballasts sit near 0.5 PF; using PF = 1 for these loads under-counts current and over-counts real power. When the nameplate gives only amps and voltage, a PF in the 0.8–0.9 range is a safer default for motor loads than 1.0.
Frequently asked questions
How do I convert amps to watts?
For DC and AC single-phase circuits, multiply the current in amps by the voltage in volts and, for AC, by the power factor: P = V × I × PF. For balanced three-phase circuits, multiply the line-to-line voltage by the line current by the power factor and by √3 (about 1.732), giving P = √3 × V_LL × I × PF. The result is real power in watts, which is the figure utility meters bill against and the figure used to size circuits and generators.
What voltage should I use — 120, 230, or something else?
Use the actual supply voltage of the circuit you are calculating for. US residential outlets are nominally 120 V (single-phase) or 240 V (split-phase for ranges and dryers); UK and most-of-Europe outlets are 230 V single-phase; Australia and India are 230–240 V; many automotive circuits are 12 V or 24 V DC. For three-phase, use the line-to-line voltage (208 V or 480 V in US commercial, 400 V in EU industrial), not the phase voltage.
What is power factor and when does it matter?
Power factor is the ratio of real power (watts) to apparent power (volt-amperes), and it captures how much of the current flowing through the conductor is doing useful work versus oscillating between source and reactive load. Resistive loads like incandescent lamps, electric heaters, and kettles have PF ≈ 1 and the simple V × I formula is exact. Motors, transformers, fluorescent ballasts, and switching power supplies have PF between 0.6 and 0.95, and ignoring PF over-states real power and under-states the current the wires must carry.
Why is there a √3 factor in three-phase calculations?
In a balanced three-phase system, the line-to-line voltage and the line current are 30° out of phase with each other, and the total real power summed across all three phases works out to √3 × V_LL × I × PF. The factor √3 (≈ 1.732) appears because of the geometric relationship between line-to-line and line-to-neutral voltages: V_LL = √3 × V_LN. Without the factor, three-phase calculations using line-to-line voltage understate power by 73%.
How does this differ from kW or kVA?
Watts (W) and kilowatts (kW) measure real power — the energy actually consumed and converted to heat, light, or motion — while volt-amperes (VA) and kilovolt-amperes (kVA) measure apparent power, the product of RMS voltage and RMS current without the power-factor correction. Real power and apparent power are equal only when PF = 1; otherwise W < VA. Generator and transformer ratings are in kVA because the windings must handle the apparent current regardless of PF, while utility billing is in kWh based on real power.