What should be used as a performance factor?

For purely resistive loads such as heaters and light bulbs use 1. The power factor of motors and transformers is usually between 0.8 and 0.9. If the power factor is given on the nameplate then that value should be used.

AC Wattage Calculator

Your circuit

Current type

Direct current (DC)

AC single phase

AC three phase

Voltage

Current

Power factor

Wattage (real power)

Fill in any three of voltage, current, power factor, and wattage. The calculator solves for the fourth automatically, so it works as an amps-to-watts or a watts-to-amps tool.

Please input values into the fields

Phase angle: deg (°)

Wattage is the actual power consumed by an electrical device. In AC circuits, this cannot be simply calculated by multiplying voltage and current together. This tool takes into account the relationships between voltage, current, power factor, and wattage to calculate unknown values. By entering any three known values, you can find the fourth value, making it a useful converter from watts to amps or vice versa.

What is AC Power?

A watt (W) is the unit of power and indicates how quickly a device converts electrical energy into useful work, heat or light. One watt equals one joule per second.

Calculating power in DC circuits is simple; you just multiply voltage by current. AC power is more complex because both the voltage and the current are varying periodically many times per second, and they do not necessarily reach their maximum values at the same time. The power factor describes this phase shift, which can cause an AC device to actually draw more current than would be estimated from its wattage rating alone.

The formula for calculating AC power is:

In single-phase alternating current, active power is the product of power factor, current and voltage.

P = pf \times I \times V

In three-phase alternating current, additional geometric factors must be considered due to the method of measuring voltage levels. If line-to-line voltage is used, it must be multiplied by the square root of three; if phase conductor voltage (between conductor and neutral) is used, it must be multiplied by three.

P_{\text{line-line}} = \sqrt{3} \times pf \times I \times V_{LL}

P_{\text{line-neutral}} = 3 \times pf \times I \times V_{LN}

For DC power the formula can be simplified to P = V * I since the power factor is assumed to be one and there are no phase coefficients. The following table summarizes the four cases that this tool covers.

Current type	Power formula	Power factor
Direct current (DC)	P = V x I	always 1
AC single phase	P = pf x I x V	0 to 1
AC three phase (line-to-line)	P = sqrt(3) x pf x I x V	0 to 1
AC three phase (line-to-neutral)	P = 3 x pf x I x V	0 to 1

How to use this calculator:

First select the type of electricity. If you choose AC, specify whether the voltage entered is between lines (line-line voltage) or line-neutral voltage. Then enter the known values.

It is not necessary to enter the values in top down order. If the field for voltage is left blank then the tool will calculate the voltage; if the field for current is left blank it will calculate the current (amps); and if the field for watts is left blank it will calculate the power. The calculator will automatically calculate one of the fields that are left blank.

Examples of calculations:

Assume a single-phase motor has an operating voltage of 110V, current of 9A and power factor of 0.85. The real power is then:

P = 0.85 \times 9 \times 110 \approx 841.5 \text{ W}

The apparent power is the capacity that the power supply system actually has to handle. The calculation does not take into account the power factor; it simply multiplies voltage by current: 9 x 110 = 990 VA. The difference between 990 VA and 841.5 W is due to the reactive power. This energy oscillates between the windings of the motor and the source of electricity, but does not produce any useful work.

Conversely this tool can also be used to convert watts into amps. For example if a heater with an output of 2000W is connected to a single phase supply at 230V and the power factor is 1, then the current will be 2000/230 which equals approximately 8.7A. This value must be taken into account when selecting fuses or cable standards.

Difference between power factor, real power and apparent power:

The power factor is the ratio of real power to apparent power. Pure resistive loads such as heaters or incandescent light bulbs have a power factor of one. Every volt-ampere drawn is used completely for useful work. Motors, transformers and fluorescent lights have a power factor less than one because they store energy in a magnetic field or radiate it out, causing the current and voltage to be out of phase.

A low power factor means that the current drawn by a device is larger than it needs to be for its wattage. This extra current causes cables to heat up, strains the capacity of the electricity supply system and can lead to commercial consumers being charged extra fees. The three power values can be represented in a right triangle: real power is the base, reactive power is the vertical side and apparent power is the hypotenuse.

S = \sqrt{P^2 + Q^2}, \qquad pf = \cos(\varphi) = \frac{P}{S}

Device	Typical power factor
Resistive heater or oven	1.0
Incandescent lamp	1.0
Fluorescent lamp	0.95
Synchronous motor	0.9
Induction motor, full load	0.85
Induction motor, no load	0.35

Fields of application:

This tool can be used for various purposes such as selecting the rating of circuit breakers and wire gauge based on an appliance's power consumption, checking if a circuit is capable of handling the load requirements of new appliances, calculating the load of lighting fixtures or motors, and calculating current in three phase power supply system for workshops or factories. If two out of three values are given on an appliance's nameplate and you want to find the third value, then this tool can be used to calculate it.

This tool is a reference for planning and learning. When designing electrical circuits, selecting circuit breaker standards, and all work related to safety must comply with local electrical codes, and the work should be inspected by a licensed electrician.

FAQs:

How to convert watts to amperes?

Divide the power by the voltage and divide by the power factor in case of AC. The current for a single-phase AC is P/(power factor x V), while that for a three-phase system is P/(root(3) x power factor x V). Enter the power and voltage here, leaving the "current" field blank to perform the calculation automatically.

How to convert amperes to watts?

Multiply the voltage by the current. For AC power, you must also take into account the power factor. The wattage for single phase AC is pf x I x V while for a three-phase system it's sqrt(3) x pf x I x V. If the field for watts is left blank, the calculator will automatically fill in the result.

What power factor should be used?

For purely resistive loads such as heaters, kettles and light bulbs use 1. Motors and transformers usually have a power factor between 0.8 and 0.9 but can be much lower for lightly loaded motors. If the power factor is given on the nameplate then use that value.

What is the difference between watts and volt-amps?

Watts measure the real power, or the portion of the power that is actually used for useful work. Volt-amps measure the apparent power, which is the total amount of current capacity that must be provided by the electrical system. Only when the power factor is equal to 1 are watts and volt-amps the same. Otherwise, the value for volt-amps will be higher, and the ratio of volt-amps to watts gives the power factor.

Why is the square root of three used in 3-phase power?

In a symmetrical three-phase system the three voltages between the conductors are shifted by 120 degrees against each other. When calculating with the voltage between the conductors, a factor of root three is introduced to add these phase-identical voltage components. If the voltage between one conductor and the neutral conductor is used, the corresponding factor is three.

Frequently asked questions

How to convert watts to amperes?: Divide the watts by the voltage and for AC also divide by the power factor. The current for single phase AC is P/(power factor x V), while the current for a three-phase system with line voltage is P/(root(3) x power factor x V). Enter the watts and the voltage here, leaving the field for the current blank to perform the calculation.
How to convert amperes to watts?: Multiply the current by the voltage and in AC additionally by the power factor. The wattage for single phase AC is Power Factor x I x V while the wattage for a three phase system with line to line voltage is Root(3) x Power Factor x I x V. Leave the field for the wattage blank, then the calculator will automatically fill in the result.
What should be used as a performance factor?: For purely resistive loads such as heaters and light bulbs use 1. The power factor of motors and transformers is usually between 0.8 and 0.9. If the power factor is given on the nameplate then that value should be used.
What is the difference between watts and volt-amps?: Watts measures the real power, or the portion of the power that is actually used for useful work. Volt-amperes measure the apparent power, which is the total amount of power that must be supplied by the electrical system. Only when the power factor is one are these two values equal; otherwise, the volt-ampere value will be larger and the ratio of the two values gives the power factor.
Why is the square root of three used in 3-phase power?: In a symmetrical three-phase system the voltages between the lines shift by 120 degrees relative to each other. When calculating with line voltages, a factor of root three is introduced in order to add these voltages. If the calculation is done using the voltage between conductor and neutral wire, then the corresponding factor is three.

Related calculators

Appliance Wattage CalculatorFind an appliance's wattage and what it costs to run. Enter watts, or volts and amps, or pick a common appliance, then estimate kWh and cost per day, month, and year.

3-Phase Motor Amperage CalculatorCalculate the full-load current of a 3-phase (or single-phase) motor from its power, voltage, power factor, and efficiency, with apparent power, starting current, and NEC circuit sizing. Solve for any field.

Amp Hour CalculatorSolve the amp-hour relationship between battery capacity, current, and time, convert amp-hours to watt-hours, and size banks with depth of discharge and C-rate.

Breaker Size CalculatorWork out the circuit breaker amp rating a load needs. Enter amps, watts, kW or kVA with the supply voltage and phase, apply the continuous or motor duty factor, and get the next standard breaker frame plus its conductor pairing.

Battery Capacity CalculatorConvert battery capacity between amp-hours and watt-hours, and estimate runtime, charge time, and discharge current from the C-rate.

Bridge Rectifier CalculatorWork out the DC output, ripple voltage, ripple factor and diode ratings of a full-wave bridge rectifier, with or without a smoothing capacitor.

Disclaimer: This calculator is provided for general informational and educational purposes only. Our calculators are under active development, and results may be inaccurate, incomplete, or unsuitable for your situation. Always verify the figures independently and seek advice from a qualified professional before relying on them. We make no warranties and accept no liability for any loss or decision arising from use of this tool.

References

NIST Special Publication 811: Guide for the Use of the International System of Units
Standard definitions for the watt, volt, ampere, and the electrical derived units.
Wikipedia: AC power
Real, reactive, and apparent power in alternating-current circuits, and the role of the power factor.