The electrical power formula
Electrical power is the rate of energy use: P = V·I, voltage times current. A 120 V outlet supplying 0.5 A delivers 60 W (watts). Combining this with Ohm's law gives P = I²·R and P = V²/R. Compute the voltage first with the Ohm's law calculator.
Those alternative forms come straight from substituting V = I·R, so use whichever matches the values you already have.
How to use this calculator
Enter the voltage in volts and the current in amperes; the result is power in watts. Convert milliamps to amps before entering values. To get energy used, multiply power by time: a 60 W load running 1 hour uses 60 watt-hours.
A device drawing 500 mA is drawing 0.5 A, so entering 500 would overstate the power a thousandfold. For household estimates, remember that electricity bills are charged in kilowatt-hours: divide watts by 1,000, then multiply by the hours of use to get the kWh figure that appears on the meter. Power is work divided by time, so start with the work calculator if the energy figure is unknown.
Power as energy over time
Power can also be defined mechanically as P = W / t — energy (work) divided by time. One watt is one joule per second. So a device rated 60 W converts 60 joules of energy every second. This ties power to the energy you can compute with the kinetic energy calculator.
The two definitions describe the same quantity. Power tells you how fast energy moves; energy tells you how much moved in total. That distinction explains why a high-wattage appliance used briefly can cost less to run than a modest one left on all day.
Frequently asked questions
- How do I calculate electrical power?
- Multiply voltage by current: P = V·I. At 120 V and 0.5 A the power is 60 W.
- What is a watt?
- The SI unit of power, equal to one joule per second (1 W = 1 J/s).
- How do I find energy from power?
- Multiply power by time. A 60 W device running for one hour uses 60 watt-hours of energy.
- What is the P = W/t form?
- It defines power as work (energy) divided by time, the general definition of power in physics.
- Does this work for AC circuits?
- P = V·I gives true power only for direct current or purely resistive AC loads. For AC with motors or capacitors, the current lags or leads the voltage, and real power is V × I × power factor — the raw product gives apparent power in volt-amperes instead.