Amps to Watts Converter — Free Online Calculator
Convert amps to watts for DC, single-phase, and three-phase AC circuits. Enter amps, voltage, and power factor.
How to Use This Calculator
Enter the current in amps, voltage, and select your circuit type. For AC circuits, adjust the power factor if needed (0.8 for motors, 1.0 for resistive loads).
The Formula Explained
For DC: Watts = Amps × Volts. For single-phase AC: Watts = Amps × Volts × PF. For three-phase AC: Watts = Amps × Volts × √3 × PF.
Amps to Watts: The Fundamental Power Calculation
Converting amps to watts is the most common calculation in electrical work: you measure current flowing through a circuit, and you want to know how much power is being delivered to the load. The calculation depends on whether the circuit is DC or AC, single-phase or three-phase, and whether the load is resistive or reactive. Getting it right matters for sizing, billing, and safety.
For DC: W = V × I. For single-phase AC resistive loads: W = V × I. For single-phase AC reactive loads: W = V × I × PF, where PF is the power factor (typically 0.7-0.95 for motors and older electronics, 0.95+ for modern PFC electronics). For three-phase AC: W = sqrt(3) × V × I × PF, where V is line-to-line voltage. The square root of 3 factor (about 1.732) comes from the vector sum of three phases 120 degrees apart.
Worked Example: Tesla Wall Connector Power
A Tesla Wall Connector EV charger draws 48 amps at 240V split-phase. Power calculation: 240 × 48 = 11,520 watts = 11.52 kW. Because EV chargers are purely resistive from the grid's perspective (the charging current flows through a switching power supply with PFC), power factor is essentially 1.0 and no PF adjustment is needed.
Daily energy consumption if used 4 hours per day: 11.52 × 4 = 46 kWh. At 16 cents per kWh, that is 7.35 USD per day, or 220 USD per month. Compare to gasoline for a similar car: 400 miles per week at 25 mpg = 16 gallons per week = 64 gallons per month. At 4 USD per gallon, that is 256 USD per month. EV electricity saves about 36 USD per month, or 430 USD per year.
Worked Example: Three-Phase Motor Load Calculation
A 10 HP three-phase motor at 460V has nameplate FLA (full load amps) of 14 amps and power factor 0.85. Real power: sqrt(3) × 460 × 14 × 0.85 = 9,476 watts = 9.48 kW. Compare to nameplate 10 HP × 0.746 = 7.46 kW of mechanical output. The difference (9.48 − 7.46 = 2.02 kW) is motor losses, meaning efficiency is 7.46 / 9.48 = 79%. Fine for an older motor; modern NEMA Premium motors hit 91-94% at full load.
Apparent power for wire sizing: sqrt(3) × 460 × 14 = 11,150 VA = 11.15 kVA. This is the value used to size wires, breakers, and overload relays — not the 9.48 kW of real power. Miss this distinction and you undersize the feeder, causing overheating and breaker trips.
Five Calculation Errors
1. Using peak AC voltage instead of RMS. 120V AC has a peak of 170V but RMS of 120V. Multimeters display RMS. Using peak gives 41% over-calculation of power.
2. Forgetting power factor on motor circuits. A 10A motor load at 0.8 PF delivers 20% less real power than the simple V × I calculation suggests. Important for efficiency and billing calculations; not important for wire sizing (which uses total current regardless of PF).
3. Using line-to-neutral voltage on three-phase. The sqrt(3) factor in three-phase power already assumes line-to-line voltage. Using 120V (line-to-neutral) instead of 208V (line-to-line) on a 208Y system gives wrong answers.
4. Mixing single-phase and three-phase formulas. If you have a three-phase load connected phase-to-phase (like a 240V motor on a 208Y system wired between two phases), it is actually a single-phase load at 208V, not a three-phase load. Use the single-phase formula.
5. Ignoring duty cycle on intermittent loads. A 4,800W oven is only on 30% of the time while cooking. Average power draw is 1,440W, not 4,800W. Peak watts matters for wire sizing; average watts matters for energy billing.
Quick Reference Tables
At 120V single-phase (residential outlet circuits): 5A = 600W. 10A = 1,200W. 15A = 1,800W (max on standard outlet). 20A = 2,400W (max on 20-amp outlet circuit).
At 240V single-phase (residential large appliance): 15A = 3,600W. 20A = 4,800W. 30A = 7,200W (dryer). 40A = 9,600W. 50A = 12,000W (range/oven).
At 208V three-phase (commercial): 10A × 1.732 = 3,602W per amp line-to-line. 50A three-phase 208V ≈ 18 kW (assuming PF 1.0). 100A ≈ 36 kW. 200A ≈ 72 kW.
At 480V three-phase (industrial): 10A × 1.732 × 480 = 8,314W per phase. 100A three-phase 480V ≈ 83 kW. 400A ≈ 333 kW.
NEC and Standards References
NEC does not directly use amps-to-watts conversions but does distinguish between continuous and non-continuous loads (210.19 and 215.2) and requires specific calculation methods (Article 220) for service and feeder load calculations.
For motor circuits, NEC 430 uses nameplate FLA directly rather than computed values, because motor power factor and efficiency vary with load. The nameplate FLA value captures actual operating current at rated conditions. UL 2201 covers Permanent EVSE (EV charging equipment) including power and current specifications that directly apply to household EV charger installations.
Amps to watts: backing out load from measured current
The reverse of watts-to-amps, used most often when you have a clamp meter on a wire and want to know what the load is actually drawing in watts. Same three forms (DC, single-phase AC, three-phase AC) and the same power-factor trap. A clamp meter reads RMS current including reactive current that does not do real work.
The formula and what it does
Multiply voltage by current to get VA (apparent power), then multiply by power factor to get real watts. Without a power-factor meter, assume 1.0 for known-resistive loads and 0.85 for general mixed-residential loads.
Worked example
Scenario: clamp meter reads 12 A on a refrigerator branch circuit, 120 V supply.
Apparent power: 120 x 12 = 1440 VA. A refrigerator compressor runs at PF roughly 0.85 steady state. Real power: 1440 x 0.85 = 1224 W. Matches a typical 1.2-1.5 kW running rating for a full-size fridge. The compressor briefly spikes to 40-50 A on startup (LRA), but the clamp meter average is near the running value.
Common mistakes to avoid
Assuming PF = 1.0 for everything. On motors, computers, LEDs, the real power can be 60-90 percent of apparent power. Big error.
Reading peak instead of RMS. Old non-RMS meters lie on non-sinusoidal loads (electronics, dimmers). Use a True-RMS meter for accurate readings.
Frequently asked questions
My clamp meter says one thing and the nameplate says another. Which is right?
Both, at different operating points. Nameplate is full-load running watts. Clamp readings depend on what the device is doing right now: idle, starting, running.
How do I measure power factor without a special meter?
You cannot from V and I alone. Modern multimeters with True-RMS and PF function (Fluke 87V, 117) show it directly. Cheaper option: assume 1.0 for resistive, 0.85 for mixed, accept 10-15 percent uncertainty.
Why do my readings vary so much?
Current varies with what the load is doing. Fridges cycle. HVAC blowers run at different speeds. Computers draw based on activity. Take readings over 30-60 seconds and average them.
How do I measure DC amps with a clamp meter?
Most cheap clamps do AC only. For DC you need a Hall-effect clamp (Fluke 376, Klein CL2000). They cost more but are essential for solar, battery, and automotive work.
Can I clamp around both hot and neutral together?
Yes, and you should get near zero. Equal and opposite currents cancel in the magnetic field. A non-zero reading means ground fault: current is returning through a non-neutral path.
Does the calculator account for harmonic distortion?
No. The basic P = VIPF formula assumes a clean sine wave. Loads with significant harmonic content (LED drivers, VFDs, electronics) have distortion power factor that lowers true real power below the displacement-only calculation.