Watt CalculatorWatts · volts · amps
Electrical power in watts equals volts times amps: P = V × I. For AC circuits, multiply by the power factor, and for three-phase supplies by √3 as well. Convert between watts, volts and amps for DC, single-phase and three-phase circuits, then see the kW, the current drawn and the daily running cost.
Formula verified against OpenStax — University Physics, Volume 2 · HyperPhysics, Georgia State University
By Manzoor Ahmad · Pharmacist & calculator developerReviewed & updated
Circuit values
Power factor (PF) is 1 for heaters and kettles, around 0.8–0.95 for motors and electronics.
Step-by-step working
What is watt?
A watt is the SI unit of power: the rate at which electrical energy is used or delivered, equal to one joule per second. In a circuit, power is the product of voltage and current.
Every appliance label lists its wattage. Knowing the watts, volts and amps of a load tells you what size circuit, breaker and cable it needs, and how much it costs to run.
Quick answers
How do I convert watts to amps?
Divide watts by volts: amps = watts ÷ volts. A 2,000 W hair dryer on 230 V draws 8.7 A; on 120 V it draws 16.7 A.
How do I convert amps to watts?
Multiply amps by volts: a 13 A plug at 230 V can supply up to 2,990 W. For AC motors, multiply by the power factor as well.
How many watts can a 15 amp circuit handle?
At 120 V, 15 A × 120 V = 1,800 W peak, or 1,440 W for continuous loads at the 80% rule. A 20 A circuit gives 2,400 W (1,920 W continuous).
What is the three-phase power formula?
P = √3 × VLL × I × PF. At 400 V, 16 A and PF 0.85 that is 9.42 kW. Check the cable with the voltage drop calculator.
Watts formula
Also P = I²R = V² ÷ R for a resistive load (see Ohm’s law).
| Symbol | Meaning | SI unit |
|---|---|---|
| P | Real power | W |
| V | Voltage (RMS for AC) | V |
| I | Current (RMS for AC) | A |
| PF | Power factor = cos φ | 0–1 |
| S | Apparent power | VA |
How to convert watts to amps
- Find the power rating on the device label, in watts.
- Note the supply voltage: 120 V or 230 V for most homes.
- Divide watts by volts for DC or resistive AC loads.
- For motors, also divide by the power factor, and by √3 for three-phase.
Worked example: how many amps does a 1,500 W heater draw?
A 1,500 W space heater runs on a 120 V outlet.
The same heater on a 230 V supply draws just 6.52 A. A three-phase motor at 400 V, 16 A and PF 0.85 delivers √3 × 400 × 16 × 0.85 = 9,422 W of real power.
Watts to amps quick reference
Current drawn by resistive loads (PF = 1).
| Power | At 12 V DC | At 120 V | At 230 V |
|---|---|---|---|
| 60 W | 5.0 A | 0.50 A | 0.26 A |
| 500 W | 41.7 A | 4.17 A | 2.17 A |
| 1,000 W | 83.3 A | 8.33 A | 4.35 A |
| 1,500 W | 125 A | 12.5 A | 6.52 A |
| 3,000 W | 250 A | 25.0 A | 13.0 A |
Common mistakes to avoid
- Ignoring power factor for motors. A 1 kW motor at PF 0.8 draws 25% more current than a 1 kW heater.
- Using phase voltage with the three-phase formula. √3 × V × I needs the line-to-line voltage (e.g. 400 V, not 230 V).
- Confusing kW and kWh. kW is power; kWh is energy used over time.
Manzoor Ahmad
Pharmacist turned Web Developer & Calculator Engineer · 13+ years in web developmentI’m a pharmacist by training who turned into a full-time tech geek, and for 13+ years I’ve designed and coded web tools. My pharmacy degree included physical pharmacy and pharmaceutics — states of matter, heat, flow and reaction kinetics — so the physics behind these tools is familiar ground, not a new hobby.
I’m not a physicist, so I don’t ask you to take my word for it: every formula on this page is traced to an open university textbook or NIST reference data, and the calculator is tested against published worked examples before it goes live. Spot an error? Tell me and it will be fixed and dated on the next update.
Watt Calculator: frequently asked questions
Watts = volts × amps (P = V × I) for DC and resistive AC loads. For AC with reactive loads, P = V × I × power factor.
12.5 A at 120 V, or 6.5 A at 230 V.
The ratio of real power (W) to apparent power (VA). Resistive loads have PF = 1; motors and some electronics have lower values because current and voltage are out of step.
Watts measure real power doing useful work. Volt-amperes measure apparent power, the product of RMS voltage and current. They are equal only when PF = 1.
For the same power, I = P ÷ V, so a higher voltage needs less current. That’s why long-distance power lines use very high voltages.
References & sources
- OpenStax — University Physics, Volume 2 — Ch. 9.5, Electrical Energy and Power
- HyperPhysics, Georgia State University — AC power and power factor
- U.S. Energy Information Administration — Electricity explained — power and energy units
Disclaimer: This calculator is an educational tool built on idealised textbook models. It is not a substitute for professional engineering or safety calculations.