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Convert voltage and current to power (watts) for DC and AC circuits. Calculate the power consumption of your electrical devices.
P(W) = V(V) × I(A)
Power equals voltage times current
P(W) = V(V) × I(A) × PF
Power equals voltage × current × power factor
P(W) = √3 × V(V) × I(A) × PF
√3 ≈ 1.732 for three-phase calculations
| Voltage | Watts @ 5A | Watts @ 10A | Watts @ 15A | Watts @ 20A |
|---|---|---|---|---|
| 12V | 60 W | 120 W | 180 W | 240 W |
| 24V | 120 W | 240 W | 360 W | 480 W |
| 120V | 600 W | 1,200 W | 1,800 W | 2,400 W |
| 220V | 1,100 W | 2,200 W | 3,300 W | 4,400 W |
| 240V | 1,200 W | 2,400 W | 3,600 W | 4,800 W |
| 480V | 2,400 W | 4,800 W | 7,200 W | 9,600 W |
The volts to watts conversion calculates electrical power from voltage and current measurements. Power (measured in watts) represents the rate at which electrical energy is consumed or generated, and it equals voltage multiplied by current: P = V × I. This relationship, derived from Ohm's Law and Joule's first law, is the foundation of all electrical power calculations. In DC circuits, the calculation is straightforward. In AC circuits, a third variable—power factor—accounts for the phase difference between voltage and current waveforms caused by inductive and capacitive loads. For three-phase AC systems, the √3 factor (approximately 1.732) is added to account for the relationship between line voltage and phase voltage. This conversion is essential for determining circuit breaker sizes per NEC Article 210, sizing conductors per NEC Article 310, calculating energy costs, and verifying that electrical loads do not exceed the capacity of outlets, circuits, panels, and transformers.
Use a multimeter to measure voltage or refer to the circuit specifications. Common voltages: 12V DC (automotive), 120V AC (US residential), 240V AC (US appliances/international), and 480V AC (industrial three-phase).
Measure current with a clamp meter or find it on the equipment nameplate. If you know resistance instead, calculate current using Ohm's Law: I = V ÷ R. Alternatively, calculate watts directly: W = V² ÷ R.
DC: W = V × A. Single-phase AC: W = V × A × PF. Three-phase AC: W = √3 × V × A × PF. Use a power factor of 1.0 for resistive loads or the actual PF from the equipment data sheet.
Ensure the calculated watts do not exceed 80% of the circuit rating for continuous loads per NEC Article 210. A 120V/15A circuit supports 1,800W maximum but only 1,440W for continuous loads (loads running 3+ hours).
Knowing the wattage of connected devices ensures circuits are not overloaded, preventing tripped breakers, overheated wires, and potential electrical fires per NEC safety standards.
Converting volts and amps to watts lets you calculate energy consumption (watts × hours = watt-hours) and estimate operating costs for any electrical device.
Power calculations determine the required capacity of generators, UPS systems, inverters, and transformers, ensuring equipment is properly matched to the load requirements.
| Circuit Type | Voltage | Breaker (A) | Max Watts | Continuous (80%) |
|---|---|---|---|---|
| General Outlet | 120V | 15A | 1,800W | 1,440W |
| Kitchen / Bath | 120V | 20A | 2,400W | 1,920W |
| Electric Dryer | 240V | 30A | 7,200W | 5,760W |
| Electric Range | 240V | 50A | 12,000W | 9,600W |
| EV Charger (Level 2) | 240V | 40A | 9,600W | 7,680W |
| Central AC | 240V | 30A | 7,200W | 5,760W |
Multiply voltage by current: Watts = Volts × Amps. For DC circuits, this is the complete formula. For single-phase AC, add power factor: Watts = Volts × Amps × PF. For three-phase AC: Watts = 1.732 × Volts × Amps × PF. You always need at least two known values (voltage plus current or resistance) to calculate power.
Not with voltage alone. However, if you know the resistance of the load, you can use: Watts = Volts² ÷ Resistance (Ω). For example, a 120V supply across a 14.4Ω heating element produces 120² ÷ 14.4 = 1,000 watts. This is derived from combining Ohm's Law (I = V/R) with the power formula (P = V × I).
In AC circuits, inductive loads (motors, transformers) and capacitive loads cause the current waveform to shift out of phase with the voltage waveform. Power factor (0 to 1) measures how much of the apparent power (V × A) performs real work. A motor with 0.85 PF drawing 10A at 120V consumes only 1,020 real watts despite 1,200 VA of apparent power.
A standard 15A/120V outlet provides 1,800 watts maximum. NEC requires continuous loads (running 3+ hours) to stay below 80%, limiting practical continuous use to 1,440 watts. Kitchen and bathroom circuits on 20A breakers handle 2,400 watts (1,920W continuous). Never exceed these limits as it risks tripping breakers or overheating wiring.
DC circuits use the simple formula P = V × I because voltage and current are constant and always in phase. AC circuits have sinusoidal waveforms that may be out of phase, requiring the power factor correction: P = V × I × PF for single-phase. Three-phase systems add the √3 factor to relate line voltage to the power delivered across all three phases: P = √3 × VL × IL × PF.