Electrical Power Calculator
Answer
P = V × I = I² R = V² / R. Enter any two of voltage (V), current (A) and resistance (Ω). The calculator fills the third and reports power in watts.
Calculator
How it works
Resistor power rating is the silent failure mode on LED strings and divider bleeds. You already know two of V, I and R from a measurement or another calculator on this site — this page completes the triangle and flags wattages above a quarter watt.
Fill any two fields. Power, voltage, current and resistance update together so the three formulas agree. Use the LED tool first when a diode drop shares the supply voltage.
Parameters used on this page
| Parameter | Value | Source |
|---|---|---|
| Power from V and I | P = V I | Definition of electrical power |
| Power from I and R | P = I² R | Substitute V = I R |
| Power from V and R | P = V² / R | Substitute I = V / R |
Worked example
3 V at 20 mA dissipates 60 mW in the series resistor. 12 V across 1 kΩ is 144 mW. 2 A through 6 Ω is 24 W — the dissipation check nudges you toward a heatsink.
Pitfalls
- Using peak current on a PWM pin without averaging — power follows the RMS or time-averaged I.
- Sizing a resistor for average power when the waveform is pulsed and peak exceeds the part rating.
- Ignoring that the load might not be purely resistive on AC circuits.
FAQ
How do I calculate power from voltage and current?
Multiply them in SI units — 5 V × 0.1 A = 0.5 W.
What if I know resistance and current?
P = I² R. The calculator does this when you leave voltage blank.
How much power in a 10 kΩ at 5 V?
I = 0.5 mA, P = 2.5 mW — well within a 0603 rating.
Is this for AC apparent power?
No. This is resistive DC (or magnitude-only) power. Reactive loads need a different model.
Related
Sources
- Joule's law and Ohm's law combinations for resistive loads.