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Volt-Amps are called the "apparent power" and are the product of the voltage applied to the equipment times the current drawn by the equipment. The VA rating is used for sizing wiring and circuit breakers. The VA and Watt ratings for some types of electrical loads, like incandescent light bulbs, are identical. However, for computer equipment the Watt and VA ratings can differ significantly, with the VA rating always being equal to or larger than the Watt rating. The ratio of the Watt to VA rating is called the "Power Factor" (PF) and is expressed either as a number (i.e. 0.7) or a percentage (i.e. 70%). Unfortunately, PF value is practically never stated in appliance's spec sheet. Old computers used to have PF = 0.6-0.65. Normally, single-phase generators for homes are usually rated for loads with PF = 1, so their wattage and VA ratings are the same. Since typical appliances have PF=0.6-0.8, their VA power consumption is 25-60% greater than their wattage.[1]

Power Factor

PF value measures how effectively electricity is being utilized. We know from physics that when an object is moved by a force, mechanical work is done only by the component of the force in the direction of the motion. At a given force, maximum work is done when the force and the motion are in the same direction. If the force is perpendicular to the direction of motion, no energy is transferred by this force. Similarly, in electrical circuits, the real (working) power is transferred by the components of voltage and current which have the same frequency. At given voltage and current values, the maximum wattage transfers when they are in phase. If sinusoidal voltage and current have 90o phase shift, the wattage is zero. In some regions utilities already installed digital power meters ("smart meters") at the residential level, which compute W, VAR, and PF. They may surcharge you for VAR. However, so far most US residential meters are still rotating-disc devices that measure only real watts, so PF of your appliances does not affect your cost of electricity. In these applications using power factor correcting (PFC) devices will not reduce your electric bills. Nevertheless, PF of the appliances should be taken into account when sizing a backup energy system, such as a home generator or an UPS (see: selecting an uninterruptible (UPS) power supply). Also, lower PF will cause larger current in utility lines and additional voltage drop in the wiring. In an extreme case, reduced voltage in the electrical system can cause overheating and premature failure of motors and other inductive equipment. Unlike residential customers, for commercial and industrial electrical customers, an electric utility company may assess a penalty for low power factor and collect additional charge when PF drops below 0.95.[2]

Wat to Amp Conversions

Once watts have been calculated using the formula:

W = VA * PF (<1)

Next, we use the formula P = IE to solve current. The folmula is:

I = P / E

For example, lets imagine w have a low leakage transformer that is 120VAC and has a VA power rating of 1500 volt-amps.

Step 1: convert to watts. Lets use the numbers in our example to solve this.

1500 = (1500 * 1)

Step 2: convert watts to amps. Again using the same numbers as before.

12.5A = (1500 / 120)

Lastly, our conversion answer is 12.5 Amps

Reference

  1. Power Solutions. accessdate = 2011. [1]
  2. Rozenblat,Lazar. AC POWER and ENERGY BASICS: WATT, KILOWATT, VA, and POWER FACTOR. 2010. [2]

Links

See also

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