What Is Electrical Power?
Electrical power, measured in watts (W), is the rate at which electrical energy is transferred by a circuit. The fundamental relationship is P = V × I, where P is power in watts, V is voltage in volts, and I is current in amperes. A 12-volt car battery delivering 5 amperes to a headlight produces 60 watts of power. An electrician in Detroit sizing a circuit for a 1500-watt microwave on a 120-volt line calculates that it draws 12.5 amperes, which determines the wire gauge and breaker rating needed.
Understanding the relationship between voltage, current, and power is essential for circuit design, appliance selection, and energy cost estimation. For the full set of Ohm's Law relationships including resistance, see our Ohms Law Calculator.
What This Calculator Does
This watt calculator solves for any one of the three variables in the power equation. Select which value you want to find, enter the other two, and the result is calculated instantly.
- Inputs: Any two of voltage (V), current (I), or power (P)
- Outputs: The third value, plus a summary of all three values
How the Calculation Works
P = V × I (power = voltage × current)
V = P / I (voltage = power / current)
I = P / V (current = power / voltage)
These three formulas are algebraic rearrangements of the same equation. Given any two values, the third is uniquely determined. This relationship holds for DC circuits and for AC circuits with purely resistive loads. For AC circuits with reactive components, the power factor must be considered. The NIST Guide to SI Units provides authoritative definitions of the watt, volt, and ampere.
Common Use Cases
Appliance sizing: Determine the current draw of an appliance from its wattage and your local voltage to select the right circuit breaker. Solar panels: Calculate the power output of a solar panel from its voltage and current ratings. LED lighting: Determine the power consumption of LED strips from their voltage and current draw. For energy cost calculations, use our Electricity Calculator.
Limitations
This calculator assumes DC circuits or purely resistive AC loads. For AC circuits with inductive or capacitive loads, the apparent power (VA) differs from real power (W) by the power factor. This calculator does not account for power factor, resistance, or three-phase power. For voltage loss over wire length, use our Voltage Drop Calculator. For AC and mechanical power calculations, see our Power Calculator.