What is Electrical Power?

Electrical power is the rate at which electrical energy is transferred in a circuit. Measured in watts (W), it describes how quickly a device consumes or produces energy. Understanding power is essential for sizing cables, selecting fuses, calculating electricity bills, and designing safe electrical installations.

Power Formulas

Power can be calculated from any two of the three basic electrical quantities:

• P = V × I (from voltage and current)
• P = I² × R (from current and resistance)
• P = V² / R (from voltage and resistance)

How to Use This Calculator

Select what you know — voltage and current, current and resistance, or voltage and resistance — and enter the values. The calculator returns the power in watts, along with the third unknown quantity.

Practical Examples

Example 1: A 230V kettle drawing 10 A consumes 2,300 W (2.3 kW).

Example 2: A 100Ω resistor with 5 A flowing through it dissipates P = 25 × 100 = 2,500 W. This would immediately destroy a standard resistor — always verify power ratings.

Example 3: A 9V battery powering a 100Ω circuit: P = 81 / 100 = 0.81 W.

Why Power Calculations Matter

• Cable sizing: Cables must carry the current for the load's power without overheating.
• Fuse selection: Fuses protect against excess current, which ties directly to power.
• Cost calculation: Monthly energy cost = Power (kW) × Hours × Tariff ($/kWh).
• Heat dissipation: Resistors, transistors, and MOSFETs must be rated above the power they dissipate.

AC vs DC Power

For DC circuits, P = V × I is exact. For AC circuits, this gives apparent power (VA). Real power (W) = V × I × power factor (PF). For purely resistive AC loads (heaters, incandescent bulbs), PF = 1 and the formulas are identical.