Watt Calculator

Calculate electrical power using any combination of voltage, current, and resistance. Choose your input method and get instant results.

Power Formulas: P = V × I (Power = Voltage × Current) P = I² × R (Power = Current² × Resistance) P = V² / R (Power = Voltage² / Resistance) Where: • P = Power (watts) • V = Voltage (volts) • I = Current (amperes) • R = Resistance (ohms) Related formulas: • Energy (Wh) = Power (W) × Time (h) • 1 kWh = 1000 Wh
Example 1 (Voltage & Current): V = 120 V, I = 10 A P = 120 × 10 = 1200 W (1.2 kW) Use: Household space heater Example 2 (Current & Resistance): I = 5 A, R = 8 Ω P = 5² × 8 = 25 × 8 = 200 W V = 5 × 8 = 40 V Use: Electric motor Example 3 (Voltage & Resistance): V = 12 V, R = 6 Ω P = 12² / 6 = 144 / 6 = 24 W I = 12 / 6 = 2 A Use: Car light bulb Example 4 (LED strip): V = 5 V, I = 0.5 A P = 5 × 0.5 = 2.5 W Daily energy = 2.5 W × 8 h = 20 Wh = 0.02 kWh Example 5 (Cost calculation): 1500 W heater, 6 hours/day, $0.12/kWh Daily: 1.5 kW × 6 h = 9 kWh = $1.08 Monthly: 9 kWh × 30 = 270 kWh = $32.40

What is a watt?

A watt (W) is the SI unit of power, measuring the rate of energy transfer. One watt equals one joule per second. It represents how much electrical energy is consumed or produced per unit time.

What is the difference between watts and watt-hours?

Watts measure power (rate of energy use), while watt-hours measure energy (total amount used). A 100W bulb running for 10 hours consumes 1000 Wh (1 kWh) of energy. Power × Time = Energy.

How do I calculate power with voltage and current?

Use P = V × I, where P is power in watts, V is voltage in volts, and I is current in amperes. For example, 12V at 5A = 60W.

How do I calculate power with current and resistance?

Use P = I² × R, where I is current in amperes and R is resistance in ohms. For example, 3A through 4Ω = 9 × 4 = 36W.

How do I calculate power with voltage and resistance?

Use P = V² / R, where V is voltage in volts and R is resistance in ohms. For example, 12V across 6Ω = 144 / 6 = 24W.

What is power efficiency?

Efficiency = (Output Power / Input Power) × 100%. Real devices lose power as heat. A 90% efficient power supply drawing 100W delivers 90W to the load and wastes 10W as heat.

How much does electricity cost?

Multiply power (kW) by time (hours) and rate ($/kWh). Example: A 1500W heater running 5 hours at $0.12/kWh costs 1.5 kW × 5 h × $0.12 = $0.90.

What is the power rating of my appliance?

Check the nameplate or label. Common examples: LED bulb (8-15W), laptop (45-90W), microwave (600-1200W), electric heater (1000-1500W), air conditioner (1000-3500W).

Can I use these formulas for AC power?

For resistive AC loads, yes. For reactive loads (motors, transformers), use P = V × I × cos(φ), where φ is the phase angle. This calculator assumes resistive loads or DC circuits.

Why is my circuit breaker rated in amps, not watts?

Breakers protect against overcurrent. To find maximum watts, multiply breaker amps by voltage. A 15A breaker at 120V handles 1800W, but continuous loads should be limited to 80% (1440W).

What happens if I exceed the power rating?

Exceeding power ratings causes overheating, potential fire hazards, shortened component life, or immediate failure. Always use components rated above your calculated power requirements, typically with 20-50% safety margin.

How do I convert between horsepower and watts?

1 horsepower (HP) = 746 watts. To convert HP to watts, multiply by 746. To convert watts to HP, divide by 746. Example: A 2 HP motor = 1492 watts.