Electrical Power Calculator

Electrical Power

Electrical power is the rate at which electrical energy is transferred in a circuit. It is measured in watts (W). Three equivalent formulas relate voltage $V$, current $I$, resistance $R$, and power $P$.

Power Formulas

$$P = V \times I$$ $$P = I^2 \times R$$ $$P = \frac{V^2}{R}$$

Use $P = VI$ when you know voltage and current. Use $P = I^2 R$ for current through a resistor. Use $P = V^2 / R$ when voltage across a resistor is known.

Example: a 12 V source driving 2 A through a load dissipates $P = 12 \times 2 = 24$ W.

Practical Notes

Resistors and wires must be rated above the calculated power to avoid overheating. In AC circuits, use RMS values of voltage and current for average real power.

Frequently Asked Questions

Which power formula should I use?

Use P = VI when you have voltage and current. Use P = I²R when current and resistance are known. Use P = V²/R when voltage and resistance are known.

What is the unit of electrical power?

The SI unit is the watt (W). One watt equals one joule per second. Larger values use kilowatts (kW) or megawatts (MW).

Does this apply to AC circuits?

These formulas give real (dissipated) power when V and I are RMS values in resistive loads. Reactive loads need the power factor included.

How do I size a resistor for power?

Calculate dissipated power with P = I²R or P = V²/R, then choose a resistor rated at least 2× that value for safety margin.

What is the relationship between power and energy?

Energy (joules) equals power (watts) multiplied by time (seconds): E = P × t. A 24 W load running for 1 hour uses 24 Wh of energy.

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Electrical Power Calculator