Electrical Power Formula (P = IV)
Calculate electrical power with P = IV, P = I squared R, and P = V squared / R.
Worked examples for circuits and components.
The Formula
Electrical power measures the rate at which electrical energy is converted into another form of energy (heat, light, motion, etc.). The basic formula multiplies current by voltage to give power in watts.
By substituting Ohm's Law (V = IR), you can derive two additional forms that are useful when you only know two of the three quantities.
Variables
| Symbol | Meaning | Unit |
|---|---|---|
| P | Electrical power | watts (W) |
| I | Current flowing through the component | amperes (A) |
| V | Voltage across the component | volts (V) |
| R | Resistance of the component | ohms (omega) |
Alternative Forms
P = I² x R
P = V² / R
Example 1 — Light Bulb Power
A light bulb draws 0.5 A of current from a 120 V outlet. What is its power consumption?
P = I x V = 0.5 x 120
P = 60 W
Example 2 — Resistor Heat Dissipation
A 100 ohm resistor carries 0.3 A of current. How much power does it dissipate as heat?
P = I² x R = (0.3)² x 100 = 0.09 x 100
P = 9 W
Example 3 — Finding Current from Power
A 1500 W space heater runs on a 120 V circuit. How much current does it draw?
Rearrange P = IV to get I = P / V
I = 1500 / 120
I = 12.5 A
When to Use It
Use the electrical power formula when:
- Sizing circuit breakers and fuses for appliances
- Calculating energy consumption and electricity costs
- Choosing the right resistor wattage rating to avoid overheating
- Designing power supplies for electronic circuits
- Comparing efficiency of electrical devices