In electrical circuits, power is measured in volts (V), as well as in amperes (A). A volt is the unit of electric potential difference. Another way to explain volt is the force that sends electrons through an electrical circuit to establish an electric current that's measured in amperes.

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If such circuits contain multiple devices, the power rating of these devices can be added linearly to know how much power the circuit requires. This linear addition is possible because the resistance in a DC circuit is pure resistance that's limited to the impedance (in ohms) of the conductor.

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The volt-ampere value of apparent power is used to simplify power ratings and calculations of current drawn in devices such as a UPS. This value helps determine what kind of power supply or circuit breaker is required for electrical or electronic devices, such as computers and peripherals. The power supply should have a higher rating than the equipment to avoid problems in case of an electrical surge.

A volt-ampere (VA) is a measurement of power in a direct current (DC) electrical circuit. The VA specification is also used in alternating current (AC) circuits, but in this case, it represents apparent power (represented in VA), which often differs from true power (represented in watts). In DC circuits, VA and watts (W) are equal.

Alternatively, multiplying the power supply's volt-ampere rating by 0.6 can also provide a good idea of its power-delivering capability in watts.

Like volt-amperes, watts also represent power. However, the meaning of volt-ampere vs. watt changes depending on whether the electrical circuit is AC or DC.

In AC circuits, power and volt-amperes mean the same thing only when there is no reactance. When a circuit contains an inductor or capacitor, reactance is introduced. Most AC circuits contain reactance, so volt-amperes are greater than the actual dissipated or delivered power in watts. That's why watts refer to real power, while volt-amperes refer to apparent power. This is also why a power supply rated at a certain volt-ampere value delivers actual power that is much lower.

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As a unit of measurement for electrical power, VA represents how much energy a device consumes or how much current it draws from the electrical circuit.

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In an AC circuit, the power calculation is not as straightforward. The power is the instantaneous power -- the power at a certain instance of time -- and is represented as the following:

In this circuit, the power P (in watts) is equal to the product of the voltage V (in volts) and the current I (in amperes).

Both volt-amperes and watts refer to the product of voltage multiplied by current. A device drawing 5 A at 120 V would be rated at 600 watts or 600 VA. Nonetheless, power sources, such as uninterruptible power supplies (UPSes), usually mention the volt-ampere rating but leave out the value in watts. When purchasing such devices for use with electronic equipment, including computers, monitors and other peripherals, it's important to ensure that the volt-ampere specification is used when determining the power supply's minimum ratings.

See also: ampere per meter, ampere per meter squared , henry, henry per meter, coulomb, coulomb per centimeter squared, volt per meter, magnetomotive force, faraday, tesla and siemens.

An ampere is the unit for electric current, which is the number of electrons flowing through a circuit. An ampere is the current produced by a force of 1 V acting through a resistance of 1 ohm (Ω). Put another way, a potential of 1 V appears across a resistance of 1 Ω when a current of 1 A flows through this resistance.

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A volt-ampere specification is also useful to measure reactive power, which is the power needed in capacitors and inductors to create electric and magnetic fields. It is measured in volt-ampere reactive and is required for power transmission lines.

Understanding volt-amperes, power ratings and power supply needs is important for IT professionals who are increasingly called on to apply a variety of technologies and ideas to help their organization be more energy-efficient.

The distinction between watts and volt-amperes can cause confusion when stating power supply specifications. For example, a supply might be rated at 600 VA. This does not mean it can deliver 600 W unless the equipment is reactance-free. In real life, the true wattage rating of a power supply is one-third to two-thirds of the volt-ampere rating. In fact, the volt-ampere rating is a warning to proceed with caution and to not assume the actual power is equivalent to the volt-ampere rating.

Instantaneous power changes over time. To calculate the average power value, multiple instantaneous values are integrated over a certain time period and then divided by the time period. The resultant value shows the device power in watts (not volt-amperes) in the AC circuit with voltage V(t) across it, current I(t) through it and for a particular time period.

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Suppose this circuit has three 120 V devices rated at 200 VA, 300 VA and 600 VA. Unlike DC circuits, the power rating of these devices cannot simply be added linearly to get 1,100 VAac. This is because the AC circuit also has inductive resistance, which means the total power rating of the three devices will be lower than 1,100 VA.

For example, if three 120 Vdc devices are calculated to work at 200 W, 300 W and 600 W, total power required by the DC circuit is the following: