What Is Electrical Power?
Electrical power is the rate at which electrical energy is transferred or consumed. It is measured in Watts (W) or kilowatts (kW). Understanding power calculations is essential for sizing electrical systems, estimating running costs, and designing safe installations.
The Power Formula Triangle
The three core power formulas relate Power (P), Voltage (V), and Current (I):
| Formula | Use When You Know |
|---|---|
| P = V × I | Voltage and Current |
| P = I² × R | Current and Resistance |
| P = V² ÷ R | Voltage and Resistance |
Worked Examples
Example 1: Calculating Power from V and I
A kettle operates at 230 V and draws 8 A. What is its power rating?
P = V × I = 230 × 8 = 1,840 W = 1.84 kW
Example 2: Calculating Power from Current and Resistance
A current of 5 A flows through a 20 Ω heating element. What power does it dissipate?
P = I² × R = 5² × 20 = 25 × 20 = 500 W
Example 3: Calculating Power from Voltage and Resistance
A 100 V source drives a 50 Ω load. What is the power consumed?
P = V² ÷ R = 100² ÷ 50 = 10,000 ÷ 50 = 200 W
From Watts to Energy: kWh Explained
Power tells you the rate of energy use. To find total energy consumed, multiply power by time:
Energy (kWh) = Power (kW) × Time (hours)
Energy Consumption Example
A 2 kW air conditioner runs for 6 hours. How much energy is consumed?
Energy = 2 kW × 6 h = 12 kWh
If electricity costs ¥25 per kWh, the running cost is: 12 × ¥25 = ¥300.
AC Power: Real, Reactive, and Apparent Power
In AC circuits, power has three components:
- Real Power (P) — measured in Watts (W). Actually does useful work.
- Reactive Power (Q) — measured in VAR. Stored and released by inductors/capacitors.
- Apparent Power (S) — measured in VA. The vector sum: S = √(P² + Q²).
Power Factor (力率)
Power Factor (PF) indicates how efficiently power is being used in an AC circuit:
PF = P ÷ S = Real Power ÷ Apparent Power
A power factor of 1.0 (unity) is ideal. Industrial equipment often has a PF of 0.7–0.9, meaning not all apparent power is doing useful work. Improving power factor reduces energy waste and lowers electricity bills.
Common Appliance Power Ratings (Reference)
- LED light bulb: 8–15 W
- Desktop computer: 150–300 W
- Microwave oven: 800–1,200 W
- Electric water heater: 1,500–3,000 W
- Air conditioner (room): 1,000–2,500 W
Key Takeaways
- Use P = V × I as your primary power formula.
- Convert to kW (divide by 1,000) for practical energy calculations.
- Multiply kW × hours = kWh to find energy consumption.
- In AC systems, always consider power factor for accurate calculations.