Power Calculator

About the Power Calculator

A power calculator computes electrical power using any two of the four fundamental electrical quantities — voltage, current, resistance, and power — applying all three forms of Ohm's Law simultaneously: P = V × I, P = I² × R, and P = V² / R. This universal calculator solves for the complete electrical picture from any two known inputs, making it the most versatile single tool in electronics and electrical engineering practice. Provide voltage and current, and it calculates resistance and power. Provide power and resistance, and it finds voltage and current. Provide any two values, and all four are computed using consistent cross-checks. Our calculator is designed for electronics engineers sizing resistors, troubleshooting circuits, and verifying component ratings; for electrical students learning Ohm's Law relationships; for technicians diagnosing motor and heater performance; and for anyone who needs to quickly navigate the four-variable electrical power equation. It confirms all three power law forms simultaneously, providing redundant verification that eliminates calculation errors.

How It Works

The four quantities V, I, R, P are related by: V = I × R (Ohm's Law); P = V × I; P = I² × R; P = V² / R. Any two values determine the other two. Given V and I: P = V × I; R = V / I. Given P and R: I = √(P/R); V = I × R. Given V and R: I = V / R; P = V² / R. Given I and R: V = I × R; P = I² × R. Example 1: V = 12V, R = 6Ω. I = 12/6 = 2A. P = 12 × 2 = 24W. All forms verify: P = I²R = 4 × 6 = 24W; P = V²/R = 144/6 = 24W ✓. Example 2 (resistor sizing): an LED driver needs to limit current to 20mA with a 5V supply and 2V LED forward voltage. Resistor voltage = 3V. R = V/I = 3/0.02 = 150Ω. Power = I²R = 0.0004 × 150 = 0.06W. Use ¼W (0.25W) resistor — provides 4× safety margin. Example 3 (motor): 240V motor drawing 5A. P = 240 × 5 = 1,200W. At PF 0.85: real power = 1,200 × 0.85 = 1,020W ≈ 1.37 HP.

Tips & Best Practices

• ✓Resistor wattage rating: always select a resistor rated for at least 2× the calculated power dissipation. Resistors run at 50-75°C above ambient at rated power — oversizing keeps temperatures lower and dramatically extends component life. Standard sizes: ¼W, ½W, 1W, 2W, 5W, 10W, 25W.
• ✓Joule heating is bidirectional: the same I²R formula that calculates resistor dissipation also calculates undesired heat in wires. A 20A circuit through 50 feet of 14 AWG wire (0.309Ω total): P_loss = 20² × 0.309 = 123.6W wasted in the wire — significant and a fire risk if wire is undersized for the current.
• ✓Motor power factor: when calculating motor power from V × I, multiply by PF to get real (shaft + losses) power: P_real = V × I × PF. The remainder (V × I × sin(θ)) is reactive power that circulates without doing work. This is why motor circuits must be sized for apparent power (VA) not just real power.
• ✓Power electronics safety: electrolytic capacitors store ½CV² joules. A 1,000μF capacitor at 400V stores 80J — enough to be lethal. Always discharge capacitors before working on power electronics. Calculate stored energy (½CV²) to assess hazard level: >1J poses burn risk; >10J poses cardiac risk.

Who Uses This Calculator

Electronics engineers sizing resistors for current limiting, bias networks, and voltage dividers. Students learning to apply Ohm's Law in complete circuit analysis. Technicians troubleshooting heater, motor, and resistive load circuits by comparing calculated vs measured values. Anyone designing or repairing electronic circuits who needs the complete V/I/R/P picture from two known values.