Electrical Power Calculator
Calculate voltage, current, resistance, or power using Ohm's law. Enter any two values.
Input — Enter Any Two Values
Complete Results
Ohm's Law Formulas
Given V and I: R = V/I, P = V×I
Given V and R: I = V/R, P = V²/R
Given I and R: V = I×R, P = I²×R
Given V and P: I = P/V, R = V²/P
Given I and P: V = P/I, R = P/I²
Given R and P: V = √(P×R), I = √(P/R)
Ohm's Law and Electrical Power
Ohm's Law and the power equation are the two most fundamental relationships in electrical engineering. Together, they allow you to calculate any electrical quantity — voltage, current, resistance, or power — from any two known values.
Core Formulas
Ohm's Law states that voltage equals current times resistance:
Electrical power is voltage times current:
By combining these two equations, we derive the complete set of power formulas:
These three forms of the power equation allow calculation of any parameter from any two known values.
Derivation Table
- Given V and I: R = V/I, P = V×I
- Given V and R: I = V/R, P = V²/R
- Given I and R: V = I×R, P = I²×R
- Given V and P: I = P/V, R = V²/P
- Given I and P: V = P/I, R = P/I²
- Given R and P: V = √(P×R), I = √(P/R)
Engineering Applications
- Resistor power rating: Determine the minimum wattage rating for a resistor in a circuit. Always choose a resistor rated for at least 2× the calculated power dissipation.
- Power supply design: Calculate the current draw and power requirements for power supply selection.
- Heater element design: Determine the resistance needed for a heating element to produce a specific power output at a given voltage.
- Battery life estimation: Calculate power consumption to estimate how long a battery will last (battery capacity in Wh ÷ power consumption in W = hours).
- LED resistor calculation: Find the correct resistor to limit current and power through an LED circuit.
Frequently Asked Questions
What is the difference between watts and watt-hours?
Watts (W) measure power — the rate of energy consumption at a specific moment. Watt-hours (Wh) measure energy — the total amount of energy consumed over time. A 100W light bulb running for 2 hours uses 200 Wh of energy. This calculator shows both the instantaneous power (W) and energy per hour (Wh).
Why should I use a resistor with 2× the power rating?
Running a resistor at its maximum rated power causes it to run very hot (often 70–100°C above ambient), which shortens its life and can damage nearby components. Using a resistor rated for 2× the expected dissipation keeps it cool and reliable. For high-reliability applications, a 3× or even 5× margin is recommended.
Does this calculator work for AC circuits?
This calculator uses Ohm's Law, which applies directly to DC circuits and resistive AC loads. For AC circuits with reactive components (inductors, capacitors), you need to account for impedance and power factor. The real power in AC circuits is P = V × I × cos(φ), where φ is the phase angle between voltage and current.
What happens if I enter negative values?
In practice, voltage, current, resistance, and power are positive values in most circuits. The calculator will compute results for any numerical input, but negative resistance values are not physically meaningful for passive components. Negative power indicates power being generated (like a battery) rather than consumed.