Electrical Power Calculator
Calculates the electrical power consumed or produced using voltage and current. Essential for sizing circuits, choosing fuses, and estimating energy costs for any electrical device.
About this calculator
Electric power is the rate at which electrical energy is transferred or consumed in a circuit. The fundamental formula is P = V × I, where P is power in watts (W), V is voltage in volts (V), and I is current in amperes (A). This relationship, derived from the definition of power as energy per unit time and Ohm's law, underpins all electrical engineering. Combined with Ohm's law (V = I × R), the formula can also be expressed as P = I² × R or P = V² / R, giving three equivalent ways to calculate power depending on which quantities are known. Understanding power consumption is critical for sizing wiring, selecting fuses, calculating electricity bills, and preventing overloads.
How to use
A household kettle operates at 230 V mains voltage and draws 8.7 A of current. Enter voltage = 230 V and current = 8.7 A. The calculator computes: P = 230 × 8.7 = 2,001 W ≈ 2 kW. Running this kettle for 5 minutes (1/12 of an hour) consumes 2 kW × (5/60) h ≈ 0.167 kWh of electrical energy. At a rate of $0.30/kWh, that costs roughly $0.05 per boil — useful for estimating your electricity bill.
Frequently asked questions
What is the formula for electric power and when should I use P = V × I versus P = I² × R?
All three power formulas — P = V × I, P = I² × R, and P = V² / R — are equivalent and derived by combining P = V × I with Ohm's law (V = I × R). Use P = V × I when you know both voltage and current directly, which is the most common case. Use P = I² × R when you know current and resistance but not voltage, such as calculating heat loss in a wire. Use P = V² / R when you know voltage and resistance but not current, which is handy for fixed-voltage circuits. Choose whichever form matches your known quantities.
How do I calculate monthly electricity cost from power consumption?
First find the power in watts using P = V × I, then convert to kilowatts by dividing by 1,000. Multiply kilowatts by the daily usage hours to get kWh per day, then multiply by 30 for a monthly figure. Finally, multiply by your electricity tariff in $/kWh. For example, a 100 W bulb running 5 hours a day uses 0.1 × 5 × 30 = 15 kWh per month; at $0.25/kWh that costs $3.75. This calculation helps identify energy-hungry appliances and prioritise efficiency upgrades.
What is the difference between watts and volt-amperes in electrical power?
Watts (W) measure real power — the actual energy converted to useful work or heat. Volt-amperes (VA) measure apparent power, which is the product of RMS voltage and RMS current regardless of phase. In purely resistive circuits (like heaters and incandescent bulbs) they are equal. However, in circuits with capacitors or inductors — motors, transformers, and most electronics — current and voltage are out of phase, so real power is less than apparent power. The ratio of real to apparent power is the power factor (0 to 1). This distinction matters when sizing generators and uninterruptible power supplies, which must handle apparent power.