What Is Electrical Power? (Watts)
Electrical power is the energy consumed or transferred per unit of time in a circuit; its unit is the watt (W). The basic relation is P = V × I (power = voltage × current). Combined with Ohm's law, P = I² × R = V² / R. One watt means one joule of energy per second.
Electrical power is the energy dissipated (or produced) per unit time in a circuit; that is, how fast work is done. Its unit is the watt (W), and its most basic formula is P = V × I (power = voltage × current).
The Power Formula: P = V × I
Multiply the voltage applied to a component by the current through it and you get the power it dissipates. For example, a lamp drawing 0.5 A at 12 V dissipates: P = 12 × 0.5 = 6 W.
Derived Power Formulas
Combined with Ohm's law (V = I × R), power can be written three ways:
| Known | Formula | Example |
|---|---|---|
| V and I | P = V × I | 5 V × 0.1 A = 0.5 W |
| I and R | P = I² × R | 0.1² × 100 = 1 W |
| V and R | P = V² / R | 5² / 100 = 0.25 W |
Worked Example: Choosing a Resistor's Power Rating
If 0.05 A flows through a resistor and it's 220 Ω, the power it dissipates is: P = I² × R = 0.05² × 220 = 0.55 W. In this case a 0.25 W resistor isn't enough; you need to choose at least a 1 W resistor. Otherwise it will overheat and burn.
Power Turns Into Heat
The power a component dissipates mostly appears as heat. That's why components running at high power (regulators, power transistors) need a heat sink.
Common Mistakes
- Neglecting the power rating: choosing a component with the right ohm value but insufficient wattage leads to burning.
- Confusing power with energy: power is the instantaneous rate (W); energy is the amount accumulated over time (Wh).
- Skipping a unit: putting mA into the formula without converting to A throws the result off by 1000×.
Frequently Asked Questions
What is the power formula?+
What does a watt measure?+
What's the difference between power and energy?+
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