Electrical Power Calculator
Calculate watts, voltage, current, or resistance
Required Parameters
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Quick Answer
Power (P) in watts equals Voltage × Current (P = V × I). Equivalently: P = I²R or P = V²/R.
Design Notes
Always derate components by at least 50%. A resistor rated for 0.25W should not exceed 0.125W in continuous operation for reliability. Power dissipation directly causes temperature rise: ΔT = P × θ_JA (thermal resistance).
Common Mistakes
- 1
Confusing VA (apparent power) with Watts (real power) in AC circuits.
- 2
Ignoring duty cycle — pulsed loads dissipate less average power than continuous.
- 3
Forgetting that power scales with the SQUARE of voltage or current changes.
Knowledge Base
What is the formula for electrical power?
Power (P) = Voltage (V) × Current (I), measured in watts (W). Equivalently: P = I²R (current squared times resistance) or P = V²/R (voltage squared divided by resistance). These three forms are derived by substituting Ohm's Law into the basic power equation.
How do I calculate wattage from voltage and current?
Multiply voltage by current: P = V × I. For example, a 12V supply delivering 2A provides 24 watts. For AC circuits with reactive loads, multiply by the power factor: P = V × I × PF. A power factor of 0.8 means only 80% of the apparent power does useful work.
What is the difference between watts, volt-amps, and var?
Watts (W) = real power that does actual work (heat, motion, light). Volt-Amps (VA) = apparent power, the product of RMS voltage and current. VAR = reactive power stored/released by capacitors and inductors. Relationship: VA² = W² + VAR². For DC circuits, watts and VA are identical.
How do I size a resistor's power rating?
Calculate P = I²R or P = V²/R, then select a resistor rated for at least 2× the calculated value. Standard ratings: 1/8W (0402 SMD), 1/4W (0603/through-hole), 1/2W (0805), 1W+ (larger packages). Operating at 50% of rated power roughly doubles the component lifetime.
How does power relate to heat?
All electrical power dissipated in a component converts to heat. Temperature rise ΔT = Power × Thermal Resistance (θ). For example, a TO-220 transistor dissipating 5W with θJA = 60°C/W will be 300°C above ambient without a heatsink — hence heatsinks are essential for power electronics.
What is RMS power vs peak power?
RMS (Root Mean Square) power is the average continuous power. Peak power is the instantaneous maximum. For a sine wave, P_peak = 2 × P_RMS. Audio amplifiers are often rated in peak or PMPO (Peak Music Power Output), which can be 4-8× higher than the true RMS rating.