Many people do not know should be used in watt or UPS should be used to express the capacity. Many UPS manufacturers can not tell the difference between these two  concepts, even the W and VA two terms equated, which adds to the  understanding of the confusion.

Large-capacity UPS capacity is always expressed in VA; small-capacity UPS (less than 1000VA) with W said capacity.
Voltages in the 1KVA - 500KVA UPS with VA instead of W to represent the capacity. W with small capacity UPS may be due to small capacity, UPS users are more familiar with the concept of watt. However, with the VA can more accurately show the degree of matching  UPS and load capacity, because the most fundamental decision of the UPS  output capacity of the current value (A), so the natural VA that is more  appropriate.
The W value is always less than or equal to the VA value
Conversion relationship is as follows:
Watts = VA * Power Factor = Volts * Amps * Power Factor
Typical Volts = 120 or 230V
Amps = load current
Power Factor: The power factor, which is between 0 and 1
The power factor is between 0 and 1, which represents the percentage of useful current (Watts) of the load current.
Only  electric heaters and bulbs and other power factor of 1; for other  devices, some of the load current cycle in the load, there is no work  done. This part of the current is harmonic or reactive current, which is caused by the load characteristics. It is important to understand that VA is greater than W because of  this current, and W is considered to be a special case of VA when the  power factor is unity.
The Watt value of the computer is 60 to 70% of its VA value,
In fact all of today's modern computer chargers are capacitive, the power factor value of 0.6 - 0.7. The personal machine tends to 0.6, the mainframe tends to 0.7. Recently developed a power factor called a self-correction function of the new power supply, its power factor value of 1. The future is likely to be a wide range of use of this power, but the market is still rare to see this power.
For a computer load, the W value of the UPS is 60 to 70% of its VA value
Since all computers have a power factor of 0.6-0.7, the W value of the UPS for the computer load is 60-70% of the VA value.
Most UPS manufacturers use W for capacity, and in fact they refer to VA
When  the UPS manufacturer indicates the rated W value without the indication  of the power factor and the VA rating, the user can assume that this is  the W value (equal to the VA value) at the power factor of 1 and the  manufacturer actually refers to the VA rating of the UPS. The actual load on the computer W value of the marked value of 60 - 70%, so a rating of 100
W can drive a 100W bulb, but can only drive a 65W computer.
Most computer equipment use VA for capacity
Most  computer equipment capacity is represented by VA, and some computers  have recently begun to use W for capacity (most notably DEC and IBM). But in general with the VA or more. So VA VA with UPS capacity to better reflect the degree of matching and load. APC All UPSs provide both W and VA values. The model of the product includes the VA value, or it can be converted to a W value by multiplying the VA value by 0.65.
The UPS is rated VA to avoid confusion
When  a UPS is marked with a W value, its actual W capacity for the computer  load is 60--100% (typically 60%) of the marked value and the VA value is  a calibrated W value of 100-130%. And when a UPS rated VA value, it is the actual VA load on the  computer is equal to the marked value, W value of the marked value of 60  - 70%.

For example:
A standard Compaq386 machine with the following features: NEC Color  Display, 120M hard drive, one streaming tape drive, one Ethernet card,  and one Logitech
Bus mouse, measured in the 120VAC power supply under its W, current AVA values are as follows:
Total Watts = 230W
The total Amps = 3.04A
AC voltage = 120V
The total V-A = 365 VA
Power factor = 0.63
The computer power factor values at 230 VAC and other configurations are similar (see PC Magazine 9/16/1986)

To learn more about the power factor of non-linear loads, refer to the following:

IEEE GUIDE TO HARMONIC CONTROL AND REACTIVE
COMPESATION OF STATIC

POWER CONVWRTERS (IEEE std 519-1981)

The instiute of Electrcal and Electrcs Engineers, lnc., 345

E47th Street, New York, NY10017

GUIDELINE ON ELECTRICAL POWER FOR ADP INSTALLATIONS (FIP PUB
94 September 21, 1983) U.S. Dept. Of Commerce, National Technical information Service, 5285 Port
Royal Road, Springfield, VA 22161