Power Calculator
Calculate electrical power using Ohm's Law and the Power Law. Enter any two known values (Power, Voltage, Current, or Resistance) and instantly find the missing values. Essential for electrical engineers, students, and DIY enthusiasts.
Understanding Electrical Power
Electrical power is the rate at which electrical energy is transferred by an electric circuit. It is measured in watts (W). Power is calculated using Ohm's Law and the Power Law, which relate power, voltage, current, and resistance.
The fundamental formulas are:
Power Law: P = V × I
Ohm's Law: V = I × R
Combined Formulas:
P = I² × R
P = V² / R
For AC circuits, the formulas include a power factor (PF):
- Single-Phase AC: P = V × I × PF
- Three-Phase AC: P = √3 × V × I × PF
This calculator automatically determines the missing values when you enter any two known quantities.
Common Power Values
- Household Appliance: LED bulb (5-15W), Laptop (50-100W), Microwave (800-1500W)
- Electrical System: US Household (120V, 15A, 1800W), European Household (230V, 16A, 3680W)
- Industrial: Motor (1-1000 kW), Generator (100 kW - 1 GW)
❓ Power Calculator FAQ
What is the formula for electrical power?
The fundamental formula is P = V × I (Power = Voltage × Current). Other formulas include P = I² × R and P = V² / R, derived from Ohm's Law.
What is Ohm's Law?
Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points. The formula is V = I × R (Voltage = Current × Resistance).
What is the difference between DC and AC power?
DC (Direct Current) power flows in one direction and is calculated as P = V × I. AC (Alternating Current) power changes direction periodically and is calculated as P = V × I × PF for single-phase, or P = √3 × V × I × PF for three-phase.
What is power factor (PF)?
Power factor is the ratio of real power to apparent power in an AC circuit. It ranges from 0 to 1. A power factor of 1 means all power is used effectively (resistive loads). Lower power factors indicate reactive loads (motors, transformers).
How do I calculate power from voltage and resistance?
Use the formula P = V² / R. For example, if V = 120V and R = 60Ω, P = 120² / 60 = 240W.
How do I calculate power from current and resistance?
Use the formula P = I² × R. For example, if I = 2A and R = 50Ω, P = 2² × 50 = 200W.
What is the unit of power?
The SI unit of power is the watt (W). Other common units include kilowatts (kW, 1,000 W), megawatts (MW, 1,000,000 W), and horsepower (hp, approximately 746 W).
How many watts are in a kilowatt?
1 kilowatt (kW) = 1,000 watts (W).
What is the difference between power and energy?
Power is the rate at which energy is used (P = E / t). Energy is the total amount of work done. For example, a 100W bulb uses 100 joules of energy per second (100W).
How do I calculate the power of a three-phase motor?
Use the formula P = √3 × V × I × PF. For example, if V = 480V, I = 10A, and PF = 0.85, P = 1.732 × 480 × 10 × 0.85 = 7,067W (7.07 kW).
What is the voltage in a standard US household outlet?
A standard US household outlet provides 120V. Larger appliances (dryers, ovens) use 240V. The calculator defaults to 120V for standard mode.
What is the voltage in a standard European household outlet?
A standard European household outlet provides 230V. The calculator's metric mode can display values in kV for higher voltages.
How do I calculate the current draw of an appliance?
Use the formula I = P / V. For example, if an appliance uses 1200W and is connected to 120V, I = 1200 / 120 = 10A.
What is the resistance of a 60W light bulb at 120V?
R = V² / P = 120² / 60 = 240Ω. This assumes the bulb is an ideal resistor (which it approximates when operating at nominal voltage).
What is the difference between apparent power and real power?
Apparent power (VA) is the product of voltage and current (S = V × I). Real power (W) is the actual power consumed. The ratio is the power factor: PF = P / S. Apparent power is used in AC circuits to size equipment.
How do I calculate the power factor of a circuit?
Power factor = Real Power / Apparent Power = P / (V × I). This calculator includes a power factor input for AC calculations.
What is a kilowatt-hour (kWh)?
A kilowatt-hour (kWh) is a unit of energy equal to 1,000 watt-hours. It is the standard unit used on electricity bills. 1 kWh = 1,000 W × 1 hour = 3,600,000 joules.
How do I calculate the cost of running an appliance?
First calculate the energy used: Energy (kWh) = Power (W) × Hours / 1,000. Then multiply by the cost per kWh. For example, a 100W bulb running for 10 hours uses 1 kWh. At $0.15/kWh, the cost is $0.15.