How to Calculate Electrical Power
Electrical power measures the rate at which electrical energy is converted into another form — heat, light, or motion. The base definition is P = V × I (power in watts equals voltage in volts times current in amps). Combined with Ohm's Law (V = IR), this single relationship expands into two more equivalent forms, so you can solve for power from whichever two quantities you actually know.
The three equivalent power formulas
- P = V × I — use this when you know the voltage across a component and the current flowing through it.
- P = I² × R — use this when you know the current and the resistance (substitute V = IR into P = VI).
- P = V² / R — use this when you know the voltage and the resistance (substitute I = V/R into P = VI).
Because Ohm's Law ties voltage, current, and resistance together, knowing any two of the three is enough to find the power — the calculator above also backs out the missing third quantity for you.
Practical notes and common pitfalls
- This is a DC (or purely resistive) calculation. For AC circuits with reactive loads (motors, transformers, most appliances), real power is P = V × I × cos(φ), where cos(φ) is the power factor. Ignoring the power factor overstates the true power delivered.
- Watts vs. kilowatts: divide watts by 1000 to get kilowatts (kW) — useful when comparing to appliance nameplates or utility bills, which are usually rated in kW or billed in kWh.
- Power is not energy. Power (W) is an instantaneous rate; energy (Wh or kWh) is power sustained over time: Energy = Power × Time. A 100 W bulb run for 10 hours consumes 1,000 Wh = 1 kWh.
- Check component ratings. A resistor or wire rated below the calculated wattage will overheat — choose components rated comfortably above your calculated power, not just barely above it.
Frequently Asked Questions
What is the formula for electrical power?
Electrical power equals voltage times current: P = V × I, measured in watts. Using Ohm's Law (V = IR), this is equivalent to P = I²R and P = V²/R, so you can calculate power from any two of voltage, current, and resistance.
How do I find power from current and resistance?
Multiply the current squared by the resistance: P = I² × R. For example, 5 A flowing through a 24 Ω resistor dissipates 5² × 24 = 600 W.
Does this formula work for AC circuits?
It works directly for DC circuits and for AC circuits with a purely resistive load (power factor = 1). For AC circuits with motors, transformers, or other reactive loads, the real power is P = V × I × cos(φ), where cos(φ) is the power factor; the calculator above gives the apparent power (V × I), not necessarily the true power delivered.
What's the difference between power (watts) and energy (kWh)?
Power is the instantaneous rate of energy use, measured in watts. Energy is power sustained over time, measured in watt-hours or kilowatt-hours: Energy = Power × Time. A device rated at 600 W running for 2 hours uses 1,200 Wh, or 1.2 kWh.