Volt-amperes (VA) are a measure of apparent power, calculated by multiplying voltage and current. They are used to determine the power needs of electrical equipment and to select appropriate power supplies. In AC circuits, VA values are typically higher than actual power due to reactance from inductors and capacitors. Reactive power is measured in volt-ampere-reactive and is important for power transmission lines. Computer power supplies often have a power factor correction system that makes watts and VA equal.
Electrical horsepower is often measured in volts, as well as amperes which are a measure of current. Volt-ampere (VA), sometimes referred to as apparent power, is a value determined by multiplying the voltage of an electrical appliance by the current it draws. These units can help determine what type of power or switch is needed for electrical equipment such as a computer. Watts, on the other hand, represent the actual power drawn by the system. Typically used with direct current (DC) circuits, volt-amperes are sometimes calculated for alternating current (AC) systems as well.
Volt-amperes generally reflect the power used by a DC circuit. In AC circuits, inductors and capacitors alter electric current by producing reactance, which usually results in a higher VA value than power. Power expressed in watts is typically one-half to two-thirds of the volt-ampere rating in an AC system. Selecting your equipment and accompanying universal power supplies often requires this knowledge to find products with the correct specifications.
The volt-ampere rating can typically be calculated by multiplying the wattage of a system by 1.67. This formula is often useful for determining the power needs of computers and related equipment, such as monitors, printers, and various other power-hungry peripherals. The capacity of the power supply can be derived by multiplying its VA rating by 60 percent, so that both the power supply and the equipment can be accurately rated. Power supplies should generally be rated higher than the equipment, which can help avoid problems in the event of a power surge.
Another use of VA specifications is often the measurement of reactive power. Reactive power is typically needed in capacitors and inductors in circuits to create electric and magnetic fields. Measured in volt-ampere-reactive, it is often important for power transmission lines. High current flows with reactive power can cause electrical losses, which are often annoying for power transmission systems.
For an incandescent light bulb, volt-amperes and watts usually mean the same thing. The two rating systems, however, are different for most other electrical devices. Computer power supplies often have a power factor correction system that makes watts and VA equal. The closeness between the two evaluations is determined by a number less than or equal to one. A power factor of one generally means that the ratings for wattage and volt-amperes are the same; metering is often used for servers, switches, and routers.
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