Electric Power
Calculate electric power from voltage and current, voltage and resistance, or current and resistance. Step-by-step circuit formulas for study and quick checks.
Key formula
P = IV, P = V^2/R, P = I^2R
How do you calculate electric power?
Use P = V × I when voltage and current are known. With Ohm’s law you can also write P = I²R or P = V²/R. Rearrange to find V or I when power is given. Pick the form that matches the two values you already have.
About this tool
Electric power is how fast electrical energy is transferred in a circuit. This page keeps P = IV, P = I²R, and P = V²/R visible so you can pick the form that matches the two values you already know.
How to use this calculator
- Choose the variable you want to solve for and confirm which values are already known.
- Enter inputs in consistent units before calculating so the formula is applied correctly.
- Check whether the result is physically reasonable by comparing it with the size of the inputs.
- Use the step-by-step breakdown to see how the equation was rearranged and evaluated.
Worked examples
Example 1: P = VI
V = 12 V, I = 2.0 A. P = (12)(2.0) = 24 W.
Example 2: P = I²R
I = 0.50 A through R = 40 Ω. P = (0.50)²(40) = 0.25 × 40 = 10 W.
Example 3: P = V²/R
V = 120 V across R = 240 Ω. P = (120)² / 240 = 14400 / 240 = 60 W.
Frequently asked questions
What is the electric power formula?
The basic form is P = V I. Equivalent forms are P = I²R and P = V²/R when the resistor obeys Ohm’s law. Power is in watts when V is in volts, I in amperes, and R in ohms.
Which power formula should I use?
Use P = VI if you know voltage and current. Use P = I²R if you know current and resistance. Use P = V²/R if you know voltage and resistance.
How is electric power different from mechanical power?
Both are measured in watts, but mechanical power is often P = W/t or P = Fv. Electric power comes from circuit quantities V, I, and R. Use the matching calculator for each context.
How do mA and kΩ affect power?
Convert first: 1 mA = 0.001 A and 1 kΩ = 1000 Ω. Leaving milli/kilo units unconverted makes P wrong by large factors.
How does electric power relate to Ohm’s law?
Ohm’s law V = IR lets you rewrite P = VI as P = I²R or P = V²/R. Find the missing circuit quantity with the Ohm’s law calculator, then compute power.