Electrical Power Calculator
Calculate electrical power using P = VI, P = I²R, or P = V²/R.
📖 What is Electrical Power?
Electrical power is the rate at which electrical energy is consumed or transferred. It is measured in watts (W) - one watt equals one joule of energy used per second. Power is the product of voltage (the "electrical pressure" driving current through a circuit) and current (the flow of charge).
Understanding electrical power is essential in virtually every engineering context: designing circuits, sizing wires and fuses, calculating energy bills, specifying motors and heaters, and understanding why undersized cables overheat.
Ohm's law (V = IR) combined with the power formula (P = VI) gives us three equivalent expressions for power, each useful when different quantities are known. These are sometimes called the "power wheel" or "power triangle" relationships.
📐 Formula
P = V × I (Power from voltage and current)
P = I² × R (Power from current and resistance)
P = V² / R (Power from voltage and resistance)
P = Power (watts, W)
V = Voltage (volts, V)
I = Current (amperes, A)
R = Resistance (ohms, Ω)
Energy (kWh) = Power (kW) × Time (hours)
📖 How to Use This Calculator
1
Select which two values you know: V and I, I and R, or V and R.
2
Enter the known values.
3
Click Calculate - power in watts and kilowatts, plus all related values, are shown.
💡 Example Calculations
Example 1 - Home appliance
1
A hair dryer draws 8 A from a 230 V outlet:
2
P = 230 × 8 = 1,840 W = 1.84 kW
3
R = 230/8 = 28.75 Ω
4
Running 30 minutes = 1.84 × 0.5 = 0.92 kWh
Example 2 - Resistor power dissipation
1
A 100 Ω resistor carries 0.5 A:
2
P = 0.5² × 100 = 0.25 × 100 = 25 W
3
Must use a resistor rated at least 25W.
Frequently Asked Questions
What is electrical power?
Electrical power is the rate at which electrical energy is transferred or converted. It is measured in watts (W), where 1 watt = 1 joule per second. Power can be calculated from any two of the three fundamental electrical quantities: voltage (V), current (I), and resistance (R).
What is the difference between watts and volt-amperes?
Watts (W) measure real power - the actual energy consumed. Volt-amperes (VA) measure apparent power - the product of voltage and current. In purely resistive DC circuits, W = VA. In AC circuits with reactive loads (motors, capacitors), they differ by a power factor: W = VA × cos(φ).
How do I calculate my electricity bill?
Electricity Bill = Power (kW) × Hours Used × Electricity Rate (₹ per kWh). A 100W bulb running for 10 hours uses 1 kWh (1 unit). At ₹8 per unit, it costs ₹8. A 2 kW AC running 8 hours/day for 30 days uses 480 kWh = ₹3,840 at ₹8/kWh.
What is the power triangle?
The power triangle shows the relationship between: Real power (P, in watts) = V × I × cos(φ), Reactive power (Q, in VAR) = V × I × sin(φ), and Apparent power (S, in VA) = V × I. They relate as: S² = P² + Q².
Why does doubling current quadruple power?
Because P = I²R. Power is proportional to the square of current. If current doubles from I to 2I, power changes from I²R to (2I)²R = 4I²R - four times more. This is why high current causes much more heating than high voltage at the same power.
How do I calculate electrical power consumption in kWh?
Power consumption in kilowatt-hours (kWh) = Power (kW) x Time (hours). First convert watts to kilowatts: divide by 1000. Example: a 1500W heater running for 3 hours uses: 1.5 kW x 3 h = 4.5 kWh. Monthly consumption (running 2h/day): 4.5 kWh/day x 30 days = 135 kWh. At Rs 8/kWh, monthly cost = 135 x 8 = Rs 1,080. Electricity bills are in kWh units - this calculation estimates the monthly cost of running any appliance.
What is the difference between AC power (apparent, real, reactive)?
In AC circuits, power has three components: Real power (P, measured in Watts) is the actual power consumed and converted to work/heat. Reactive power (Q, measured in VAR) is the power oscillating between source and reactive components (inductors, capacitors) - it does no net work. Apparent power (S, measured in VA) = sqrt(P^2 + Q^2). Power factor = P/S (ranges from 0 to 1). Industrial loads with motors and transformers have power factors below 1 - utilities charge for low power factor because it increases current in transmission lines.
What is power factor and how does it affect real power?
Power factor (PF) is the ratio of real power (watts) to apparent power (volt-amperes) in an AC circuit: PF = Real Power / Apparent Power. A PF of 1.0 means all supplied power does useful work. Inductive loads (motors, transformers) cause PF to drop below 1.0, meaning more current must flow to deliver the same real power. For example, a 1000 VA load at PF 0.8 draws only 800 W of real power. Utilities charge commercial customers for low power factor. Power factor correction using capacitor banks can bring PF close to 1.0 and reduce energy bills.
What is apparent power, real power, and reactive power?
Real power (P, watts) does actual work. Reactive power (Q, VAR) is stored and returned by inductors and capacitors - it causes current flow without doing work. Apparent power (S, VA) = sqrt(P squared + Q squared) is the product of RMS voltage and current. Power factor = P/S. Most household devices have power factor close to 1; motors and transformers can be 0.7-0.9.