Amps to Watts Calculator
Convert electric current (amps) and voltage to power in watts for DC, single-phase AC, and three-phase AC. Includes power factor for AC circuits. All calculations run in your browser.
Volts across the load.
Table of Contents
Watts, Amps, and Volts
Electrical power in watts measures how much energy is used or delivered per second. The three core quantities — watts, amps, and volts — are always related, but the exact relationship depends on the type of circuit.
Watts (W) measure power: the rate at which energy is transferred or consumed. A 100 W light bulb uses energy twice as fast as a 50 W bulb.
Amperes (A) measure current: the flow of electric charge through a conductor. Think of it like the flow rate of water through a pipe.
Volts (V) measure electrical potential difference — the “pressure” driving current through the circuit.
For DC circuits, power is simply the product of voltage and current: P = V × I. AC circuits are more complex because voltage and current can fall out of phase with each other. The power factor (PF) accounts for this phase difference, representing the ratio of real (useful) power to apparent power. A purely resistive load like an electric heater has a PF of 1.0 — all supplied power is consumed. Motors, transformers, and fluorescent lighting typically sit between 0.7 and 0.95.
Formulas
DC
P (W) = V (V) × I (A)
Power factor is not used in DC circuits — there is no phase angle.
AC single-phase
P (W) = VRMS × IRMS × PF
VRMS and IRMS are the root mean square values of voltage and current — the figures your meter displays and what outlet ratings refer to (e.g. 120 V in the US, 230 V in Australia/EU).
AC three-phase (balanced)
P (W) = √3 × VL-L × I × PF
√3 ≈ 1.732. VL-L is the line-to-line voltage measured between any two phase conductors (e.g. 415 V in Australia, 480 V in the US). If you only have the line-to-neutral voltage, convert it first: VL-L = VL-N × √3.
PF is the power factor (dimensionless, 0–1). Use 1.0 for resistive loads, 0.8–0.95 for motors and mixed loads. If unknown, 0.8 is a safe conservative estimate.
Quick Reference Table
| Type | Amps | Voltage | PF | Watts | Typical load |
|---|---|---|---|---|---|
| DC | 10 | 12 V | — | 120 W | Car electronics |
| DC | 5 | 48 V | — | 240 W | E-bike motor |
| AC 1-phase | 10 | 120 V | 0.9 | 1,080 W | Toaster (US) |
| AC 1-phase | 10 | 230 V | 0.9 | 2,070 W | Kettle (AU/EU) |
| AC 1-phase | 20 | 230 V | 0.9 | 4,140 W | Electric oven |
| AC 3-phase | 10 | 415 V | 0.9 | 6,462 W | Industrial motor (AU) |
| AC 3-phase | 32 | 415 V | 0.9 | 20,679 W | Commercial load |
FAQ
AC voltage oscillates between positive and negative peaks rather than staying at a fixed level. RMS (Root Mean Square) is the equivalent DC voltage that would deliver the same power to a resistive load. For a standard sine wave, RMS = peak voltage ÷ √2 (about 0.707 × peak). Your wall outlet’s rated voltage — 120 V or 230 V — is always the RMS value, not the peak.
Three-phase systems have two voltage measurements: line-to-line (VL-L), measured between any two phase conductors, and line-to-neutral (VL-N), measured from a phase conductor to neutral. They’re related by √3 — so 415 V L-L equals 240 V L-N in Australian systems. The standard three-phase formula uses L-L voltage. If you only know L-N, either multiply by √3 first, or use the equivalent form: P = 3 × VL-N × I × PF.
It depends on the load. Resistive loads (heaters, kettles, incandescent bulbs) have PF ≈ 1.0. Motors at full load typically range from 0.85–0.95. Motors at partial load, older fluorescent lighting, and welding equipment can drop to 0.7–0.85. If you’re unsure and the equipment nameplate doesn’t list a PF, use 0.8 as a conservative default.
Rearrange the formula: DC → I = P ÷ V. AC single-phase → I = P ÷ (V × PF). AC three-phase → I = P ÷ (√3 × VL-L × PF). This is the calculation you need when sizing wiring or circuit breakers from a known load wattage.
Watts (W) is real power — energy actually consumed. Volt-amps (VA) is apparent power — what the supply must deliver, including reactive power that bounces back and forth in inductive or capacitive loads. W = VA × PF. Generators, UPS systems, and transformers are often rated in VA because they must handle apparent power regardless of the load’s power factor.