Electrical Power Calculator Online | P=VI, P=I²R, P=V²/R
Calculate electrical power using P=VI, P=I²R, or P=V²/R. Enter any two known values to find power, voltage, current, or resistance.
How to Use
- Select the formula you want to use: P=VI, P=I²R, or P=V²/R.
- Enter the two known values for the selected formula.
- Click 'Calculate' to compute power and any other derivable values.
- All three power formulas are derived from Ohm's Law — results are consistent.
- Use P=VI when you have voltage and current measurements (e.g. from a multimeter).
- Use P=I²R when you know the current through a resistor.
- Use P=V²/R when you know the voltage across a load resistance.
About Electrical Power
Three Power Formulas
Electrical power can be calculated in three equivalent ways: P = V × I (Watts = Volts × Amps), P = I² × R (power from current and resistance), and P = V² / R (power from voltage and resistance). All three formulas are mathematically equivalent when combined with Ohm's Law (V = IR), so choose whichever two values you know.
What is Electrical Power?
Electrical power (measured in Watts, W) is the rate at which electrical energy is transferred or consumed. A 100W light bulb consumes 100 joules of energy per second. Power equals the product of voltage (the driving force) and current (the flow of charge). Understanding power is essential for sizing wires, fuses, transformers, and power supplies correctly.
Power Dissipation in Resistors
When current flows through a resistor, electrical energy is converted to heat. The power dissipated is P = I² × R. This is why resistors have power ratings (e.g., 0.25W, 0.5W, 1W). If the actual power exceeds the rating, the component may burn out. Always select resistors with a power rating at least twice the calculated dissipation for safety margin.
AC vs DC Power
In DC circuits, power is simply P = V × I. In AC circuits, the calculation involves the power factor (PF): real power P = V × I × PF (in watts), apparent power S = V × I (in volt-amperes, VA), and reactive power Q (in VAR). For purely resistive loads, PF = 1 and P = V × I. For inductive/capacitive loads, PF < 1 and some power is reactive.
Key Features
- Supports three power calculation formulas: P=VI, P=I²R, P=V²/R
- Automatically derives all four quantities (P, V, I, R) from two inputs
- Instant calculation with clear labeled results
- Covers both DC circuit analysis and component sizing
Common Applications
- Calculating heat dissipation in resistors and components
- Sizing power supplies for electronic projects
- Determining electrical energy consumption of appliances
- Designing motor drives and power converters
- Verifying component ratings before building a circuit