Table of Contents
Electrical Power
Electrical power is the rate at which electrical energy is converted into other forms (heat, light, motion, etc.). Measured in Watts (W), power is the product of voltage and current. In resistive circuits, Ohm's law provides three equivalent formulas for power, each useful depending on which quantities are known.
Power dissipation is a critical design parameter for electronic components. Resistors, transistors, and ICs all have maximum power ratings. Exceeding these ratings causes overheating and component failure. Understanding power also enables energy cost calculations and efficiency analysis.
Power Formulas
All three forms are mathematically equivalent for resistive (DC) circuits. For AC circuits, real power also involves the power factor: P = VI cos(φ).
Common Power Ratings
| Component | Typical Power Rating |
|---|---|
| SMD resistor (0402) | 0.063 W |
| Through-hole resistor | 0.25-2 W |
| Small signal transistor | 0.5-1 W |
| Power MOSFET | 50-300 W |
Frequently Asked Questions
What is the difference between Watts and Volt-Amperes?
Watts measure real (active) power that does actual work. Volt-Amperes (VA) measure apparent power in AC circuits. They are equal for purely resistive loads but differ when reactive components (capacitors, inductors) are present. The ratio is the power factor: PF = W/VA.
How do I calculate energy consumption?
Energy = Power × Time. A 100W light bulb running for 10 hours uses 1000 Wh = 1 kWh. At $0.12/kWh, that costs $0.12. This is why energy-efficient appliances save money despite sometimes costing more upfront.