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Calculate electrical power (watts) from voltage and current, or find any missing value. Works for both DC and AC circuits. Includes power factor calculation for AC loads, energy cost estimation, and common appliance reference.
Electrical power calculations underpin every aspect of electrical engineering โ from sizing residential circuit breakers and selecting motor starters to designing data center power distribution and renewable energy systems. A solid understanding of power relationships prevents dangerous undersizing and costly oversizing of electrical equipment.
Electrical power is the rate at which electrical energy is consumed or generated. Measured in watts (W), it is the product of voltage and current. Understanding power is essential for selecting the right fuses, circuit breakers, wire gauges, and power supplies for any electrical installation.
In DC circuits, power is simply P = V ร I. In AC circuits, a power factor (cos ฯ) must be applied: P = V ร I ร cos(ฯ). For resistive loads (heaters, incandescent bulbs), power factor = 1. For inductive loads (motors, transformers), power factor is typically 0.7-0.95.
Real power (W) = actual energy consumed. Apparent power (VA) = V ร I without power factor. Reactive power (VAR) = power stored and released by inductors/capacitors. Power factor = Real/Apparent. Utilities charge for real power but must supply apparent power.
| Appliance | Power | Voltage | Current |
|---|---|---|---|
| LED Bulb | 10W | 120V | 0.083A |
| Laptop | 65W | 19V | 3.4A |
| Microwave | 1,200W | 120V | 10A |
| Air Conditioner | 2,000W | 240V | 8.3A |
| Electric Oven | 5,000W | 240V | 20.8A |
| EV Charger (Level 2) | 7,200W | 240V | 30A |
Understanding electrical power calculations is essential for safe system design. Every component โ wire, switch, fuse, connector โ must be rated to handle the power flowing through it. Undersized components cause failures ranging from nuisance trips to dangerous fires.
Circuit breakers are sized based on current, not power. I = P/V. A 2,400W 120V circuit draws 20A โ use a 20A breaker with 12 AWG wire. A 4,800W 240V circuit draws 20A โ same breaker but can use thinner wire due to lower current per conductor. Always derate to 80% for continuous loads: 20A breaker โ max 16A continuous.
Industrial facilities with large motor loads often have poor power factor (0.7-0.85). Utilities charge for apparent power (kVA) but customers only use real power (kW). Installing capacitor banks raises PF toward 1.0, reducing reactive current, lowering electricity bills, and reducing equipment heating. A PF improvement from 0.8 to 0.95 reduces current by 16%.
Three-phase power delivers โ3 โ 1.732ร more power than single-phase at the same voltage and current. A 240V single-phase 30A circuit delivers 7,200W. A 240V three-phase 30A circuit delivers 12,470W. This is why large industrial equipment (motors over 5 HP, commercial HVAC) always uses three-phase.