⚡ Watt Calculator
Calculate electrical power in watts
How to Use This Calculator
Select Calculation Method
Choose to calculate power from: (1) Voltage and Current (P = V × I), (2) Current and Resistance (P = I² × R), or (3) Voltage and Resistance (P = V² / R).
Enter Values
Input the required values based on your selected method. Enter voltage in Volts (V), current in Amperes (A), and resistance in Ohms (Ω).
Calculate
Click "Calculate Power" to get the power in watts (W). For power ≥ 1000 W, the result is also displayed in kilowatts (kW).
Formula
P = V × I
Power from Voltage and Current
P = I² × R
Power from Current and Resistance
P = V² / R
Power from Voltage and Resistance
Where:
- P = Power (Watts, W)
- V = Voltage (Volts, V)
- I = Current (Amperes, A)
- R = Resistance (Ohms, Ω)
Example Calculation 1:
For V = 120 V, I = 10 A:
P = V × I = 120 × 10 = 1,200 W = 1.2 kW
A device operating at 120V with 10A current consumes 1,200 watts of power.
Example Calculation 2:
For I = 5 A, R = 24 Ω:
P = I² × R = 5² × 24 = 25 × 24 = 600 W
A resistor with 24Ω resistance carrying 5A current dissipates 600 watts.
Example Calculation 3:
For V = 120 V, R = 144 Ω:
P = V² / R = 120² / 144 = 14,400 / 144 = 100 W
A 144Ω resistor connected to 120V dissipates 100 watts of power.
About Watt Calculator
The Watt Calculator determines electrical power consumption or generation in watts. Power represents the rate at which energy is consumed or produced. This calculator supports three methods: calculating from voltage and current (P = VI), current and resistance (P = I²R), or voltage and resistance (P = V²/R). All three formulas are equivalent and can be derived from Ohm's law (V = IR).
When to Use This Calculator
- Circuit Design: Calculate power consumption in electrical circuits
- Device Selection: Determine appropriate power ratings for components
- Energy Estimation: Estimate power requirements for electrical loads
- Safety Analysis: Verify components can handle calculated power levels
- Educational: Learn power calculations in electrical engineering
Why Use Our Calculator?
- ✅ Multiple Methods: Three different calculation approaches
- ✅ Quick Calculation: Instantly determine power consumption
- ✅ Versatile: Works with any combination of voltage, current, resistance
- ✅ Free Tool: No registration or payment required
- ✅ Educational: Learn power formulas and Ohm's law relationships
Common Applications
Household Appliances: Calculate power consumption of household devices like heaters, air conditioners, and lighting. For example, a 120V heater drawing 12.5A consumes P = 120 × 12.5 = 1,500 W = 1.5 kW. This helps estimate energy costs and select appropriate circuit breakers.
Electronic Components: Determine power dissipation in resistors, transistors, and other components to ensure they operate within safe limits. A resistor's power rating must exceed calculated power to prevent overheating. For example, a 100Ω resistor at 10V dissipates P = 10² / 100 = 1 W, requiring at least a 1W rated resistor.
Power Supply Design: Size power supplies based on load power requirements. If a circuit needs 500W at 12V, the current requirement is I = P / V = 500 / 12 = 41.7A. This ensures the power supply can deliver sufficient current.
Tips for Best Results
- Use the formula that matches your known values
- For AC circuits, use RMS values for voltage and current
- Remember: 1 kW = 1,000 W (kilowatts)
- Power ratings should exceed calculated power for safety margin
- Consider power factor for AC reactive loads (PF = P / S)
Frequently Asked Questions
What is a watt?
A watt (W) is the SI unit of power, representing one joule per second. Power measures the rate of energy transfer or consumption. 1 W = 1 J/s. For electrical power: 1 W = 1 V × 1 A. Common multiples: 1 kW = 1,000 W, 1 MW = 1,000,000 W.
Which formula should I use?
Use the formula that matches your known values: P = VI if you know voltage and current, P = I²R if you know current and resistance, P = V²/R if you know voltage and resistance. All three are equivalent (via V = IR), so choose based on available measurements.
Is this calculator for AC or DC?
The formulas work for both AC and DC. For AC circuits, use RMS (root mean square) values for voltage and current. For DC, use actual values. Note: For AC with reactive loads, P = VI applies to real power, not apparent power (S = VI).
How is power different from energy?
Power (watts) is the rate of energy consumption. Energy (watt-hours or joules) is power × time. For example, a 100W device running for 10 hours consumes 1,000 Wh = 1 kWh of energy. Power is instantaneous; energy is accumulated over time.
What is the difference between real power and apparent power?
Real power (P, in watts) is actual power consumed. Apparent power (S, in VA) is voltage × current. For resistive loads, P = S. For reactive loads (inductors, capacitors), P = S × power factor. Real power does useful work; apparent power includes reactive components.