Difference between VA and Watts

In UPS (Uninterruptible power supply) and some of the power generators, they mention the capacity in VA, why is that ? Why did they not mention it in Watts ?

The power drawn by computing equipment is expressed in Watts or Volt-Amps (VA). The power in Watts is the real power drawn by the equipment.

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.

Both Watt and VA ratings have a use and purpose. The Watt rating determines the actual power purchased from the utility company and the heat loading generated 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" and is expressed either as a number (i.e. 0.7) or a percentage (i.e. 70%).

VA is total power, Watts is real power

real power in your circuit performs WORK (i.e. useful output). some of your supply's input power does this work (hopefully most of it) but some goes off to perform other functions, such as "establishing the field" and so forth.

total power = real power + reactive power
S = P + jQ

where the units are:
VA = Watt + VAR

VA is the power delivered if the load is purely resistive.... if the load has capacitance or inductance in it, this power gets divided into two parts, real (Watt) and reactive (VAR)..... where real part is the one that does the actual work, while reactive is used in energizing and establishing field, the relationship is not simple algebraic but complex

VA = voltage × current;
watts = Voltage × Current × Power factor

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Antonyb and hameeds01 thanks for your explanations and it helps me to solve my problem.

The power in watts be real power and power in volt-amperes are called the apparent power, UPS has both watt and VA rating and one most important thing is that the watt rating is never equal to volt-amperes rating.

VA ratings tell the maximum power that a system can handle....
watts represent the actual power supplied to the system....
watts = VA cos(fie)
where cos(fie) is power factor...