Electrical Power Calculator (P = VI)

This calculator uses the fundamental electrical formula P = VI to work out power, voltage or current when you know the other two values. Start by choosing what you want to solve for: power in watts, voltage in volts, or current in amps. Enter the two known values, for example voltage and current, and the calculator instantly solves the third using P = V x I, V = P / I or I = P / V depending on your selection. Your results show the solved value highlighted alongside power in kilowatts, the formula and figures used, and a full step-by-step working panel with the exact calculation performed. A separate panel converts the power reading into watts, kilowatts, milliwatts, horsepower and BTU per hour, and a plain-English summary explains what the numbers mean for your circuit. It is useful for checking appliance power draw, sizing a circuit, working out what current a device pulls at a given voltage, or converting between power units for New Zealand's 230 V mains supply. The formula is exact for DC circuits and for resistive AC loads such as heaters and incandescent lighting, where the power factor is 1. For AC circuits with motors, transformers or other reactive loads, multiply the result by the load's power factor to get true power, since apparent power alone will overstate what the appliance actually consumes.

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Standard Formula  Based on Ohm's Law and the definition of electrical power (P = VI). Valid for DC circuits and resistive AC loads.
P = V × I
P = Power (watts)  |  V = Voltage (volts)  |  I = Current (amperes)

1. What Do You Want to Find?

2. Enter Known Values

V
A
Please enter valid positive numbers for both known values.

Result

Power
1,200 W
Watts
Power (kW)
1.2
Kilowatts
Formula Used
P = V × I
240 V × 5 A

Step-by-Step Working

FormulaP = V × I
Known: Voltage240 V
Known: Current5 A
Calculation240 × 5
Result1,200 W

Power in Other Units

Watts (W)1,200 W
Kilowatts (kW)1.2 kW
Milliwatts (mW)1,200,000 mW
Horsepower (hp)1.6092 hp
BTU/hour4,094.57 BTU/hr
Summary: With a voltage of 240 V and a current of 5 A, the circuit consumes 1,200 W (1.2 kW) of power.

How the P = VI Formula Works

The relationship between electrical power, voltage, and current is one of the most fundamental in physics and electrical engineering. Power (P), measured in watts, is the rate at which electrical energy is transferred or consumed. Voltage (V), measured in volts, is the electrical potential difference that drives current through a circuit. Current (I), measured in amperes (amps), is the rate at which electric charge flows past a point in the circuit.

The three forms of the formula let you solve for any unknown when two values are known:

FindFormulaExample
Power (W)P = V × I240 V × 5 A = 1,200 W
Voltage (V)V = P ÷ I1,200 W ÷ 5 A = 240 V
Current (A)I = P ÷ V1,200 W ÷ 240 V = 5 A

Worked Example

A household electric kettle is connected to a 240 V mains supply and draws 5 A of current. What is its power consumption?

Using P = V × I:

P = 240 V × 5 A = 1,200 W (1.2 kW)

This matches the calculator's default output above. The kettle consumes 1.2 kW. If it runs for one hour, it uses 1.2 kWh of electricity. At a typical New Zealand electricity rate of around 30 cents per kWh, that costs approximately 36 cents per hour to run.

Common Electrical Values in New Zealand

ApplianceVoltage (V)Typical Current (A)Power (W)
Electric kettle2308.7 A2,000 W
Microwave oven2304.3 A1,000 W
Toaster2303.5 A800 W
LED light bulb2300.04 A9 W
Laptop charger193.2 A60 W
EV home charger (7 kW)23030.4 A7,000 W

Note: New Zealand's standard mains voltage is 230 V at 50 Hz. Nominal 240 V is commonly quoted but the standard has been 230 V since 2001 (to align with the European standard). In practice, supply can vary between 216 V and 253 V.

AC vs DC Power

For direct current (DC) circuits, P = VI is exact. For alternating current (AC) circuits (such as New Zealand's 230 V mains), the formula gives apparent power in volt-amperes (VA). The actual power consumed (true power, in watts) is:

P = V × I × power factor (PF)

For purely resistive loads such as heaters, incandescent bulbs, and electric kettles, the power factor is 1, so P = VI applies directly. For inductive loads such as motors, air conditioners, and fluorescent lights, the power factor is less than 1, meaning less real power is consumed than apparent power suggests. This calculator uses the simple P = VI relationship and is suitable for DC circuits and resistive AC loads. For motor or reactive load calculations, multiply the result by your load's power factor.

Related Calculators

Sources and method: Formula P = VI is the standard definition of electrical power as used in IEC 60050 (International Electrotechnical Vocabulary). New Zealand standard voltage reference: Electricity (Safety) Regulations 2010 and AS/NZS 61000.3.100. Unit conversions: 1 hp = 745.7 W; 1 BTU/hr = 0.29307 W.

This calculator uses the basic P = VI relationship for DC circuits and resistive AC loads. For reactive AC loads (motors, transformers, capacitive loads), multiply power by the appropriate power factor. Results are for educational and planning purposes.

Frequently asked questions

What does P = VI mean?

P = VI is the fundamental formula for electrical power. P stands for power measured in watts (W), V stands for voltage measured in volts (V), and I stands for current measured in amperes (A). The formula states that power equals voltage multiplied by current. For example, a 240 V appliance drawing 5 A of current consumes 1,200 W (1.2 kW) of power. The formula can be rearranged to find any unknown: V = P / I and I = P / V.

How do I convert watts to kilowatts?

Divide watts by 1,000 to get kilowatts. For example, 1,200 W equals 1.2 kW. Kilowatts are more commonly used for appliances and electricity billing. New Zealand power bills charge per kilowatt-hour (kWh), which is the energy consumed by a 1 kW device running for one hour. To find how much it costs to run an appliance, multiply its power in kW by the hours it runs and then by your electricity rate per kWh.

What is the difference between AC and DC power calculations?

For direct current (DC) circuits, P = VI is exact and straightforward. For alternating current (AC) circuits, the formula gives apparent power in volt-amperes (VA) when you use the peak or RMS values of voltage and current directly. True power in watts for AC circuits is P = VI x power factor (PF), where the power factor accounts for the phase difference between voltage and current caused by reactive loads (motors, transformers). For resistive loads such as heaters and incandescent lights, the power factor is 1, so P = VI applies directly. This calculator uses the simple P = VI relationship, which is suitable for DC circuits and resistive AC loads.

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