Electrical Power Calculator
Calculate electrical power in watts from voltage and current readings. Used by electricians, engineers, and DIYers to size fuses, cables, and power supplies correctly.
About this calculator
Electrical power is the rate at which energy is transferred or consumed in a circuit. The fundamental formula is P = V × I, where P is power in watts (W), V is voltage in volts (V), and I is current in amperes (A). Combined with Ohm's Law, this expands to P = I² × R and P = V² / R, giving you three equivalent ways to compute power depending on which quantities you know. One watt equals one joule of energy per second. In practical terms, power determines how hot a resistor gets, how fast a motor spins, or how quickly a battery drains. Knowing the power consumption of a device is critical for selecting correctly rated wiring, fuses, transformers, and power supplies to ensure safe, efficient operation.
How to use
Imagine a household appliance connected to a 120 V outlet that draws 5 A of current. To find the power it consumes, use P = V × I. Substituting: P = 120 V × 5 A = 600 W. This means the appliance uses 600 watts, or 0.6 kilowatts. Running it for 10 hours would consume 6 kWh of energy. Enter your voltage and current values into the calculator to get the wattage instantly, then use that figure to size fuses or estimate electricity costs.
Frequently asked questions
How do I calculate electrical power consumption of a device?
Electrical power consumption is calculated with P = V × I, where V is the supply voltage in volts and I is the current draw in amperes. You can find the current on the device's nameplate or measure it with a clamp meter. Multiply the two values to get watts. For example, a device running at 230 V and drawing 3 A consumes 690 W. Over time, energy use in kilowatt-hours equals power (kW) multiplied by hours of operation.
What is the difference between power in watts and energy in kilowatt-hours?
Power (watts) measures the rate of energy use at any given moment, while energy (kilowatt-hours) measures the total amount of energy consumed over time. One kilowatt-hour equals 1,000 watts running for one hour. For example, a 500 W appliance running for 4 hours consumes 2 kWh of energy. Electricity bills charge for energy (kWh), not instantaneous power, so knowing your device's wattage helps you predict and manage costs.
Why is it important to know the power rating when choosing a fuse or cable?
Fuses and cables are rated for a maximum current they can carry safely before overheating or failing. Since P = V × I, knowing the power and voltage tells you the current: I = P / V. You then select a fuse rated slightly above that current to protect the circuit without nuisance tripping. Under-rated cables overheat and can cause fires, while correctly sized cables and fuses ensure long-term reliability and code compliance.