When considering Uninterruptible Power Supply (UPS), for a data center application, UPS VA is generally used to appraise UPS Size. New computers however, designed to comply with recent green oriented server standards, feature input power factor correction which requires considering UPS Watt capacity when sizing a UPS.

Two figures Volt – Ampere (VA) and Watt (W) define Uninterruptible Power Supply capacity. Both should be above corresponding load requirements.

VA represents the Total (sometimes called apparent) Power, derived simply by multiplying Main's Voltage by consumer's Current (Amperes).

Watt figure represents the True power, required by the computer power supplies, derived by multiplying Main's Voltage by the value of Main's frequency Current component. The ratio of Watt to VA is defined as Power factor (PF).

Normally, Computer Power Supply's input current has a pulse shaped waveform. This current comprises basic mains frequency sinusoidal component of about 0.7 (70 %) of total current and also parasitic higher harmonics, which increase input current but do not contribute to the power which the Computer Power Supply can deliver.

Thus, a 120V server drawing, for instance, 2.5A from the mains, has input power rating of 300 VA (120V x 2.5 A) and 210W (300VA x 0.7), and requires at least 300VA/ 210W UPS.

Recent movement towards a green world affects also legislation of server power supplies, demanding to increase server's efficiency and decrease pollution of grid by elimination of unwanted current harmonics. New computer power supplies, which implement power factor correction to achieve sinusoidal input current with almost unity power factor, are the common trend.

Latest regulations, such as Energy Star, require server input power factor figure to be above 0.95 at full load. Thus, a new 120V server drawing 2.5A from mains may require 300VA and 285Watt (300VA x 0.95 PF).

The right UPS rating, to drive such server, should therefore be at least 300VA/ 285Watt. Most 300VA UPS systems are designed to supply maximum 210 Watt loads, and will not be able to feed a load that requires 285 Watt.Before buying a new UPS its Wattage capacity should be verified, to ascertain that it meets not only the VA but also the Watt requirements.

If the UPS feeds more than a single server, both total UPS Volt Ampere and Watt ratings should be above the respective VA and Watt sums of the individual servers.

Connecting to a UPS consumers that require real power Wattage, above the rating which the UPS supports, might damage UPS's Inverter semiconductors due to overheating, cause complete battery destruction, and expose the UPS and the critical load to various, severe phenomena harming the UPS as well as the load which it aims to protect

Load Volt Ampere and Watt figures are needed for sizing a proper power backup. VA rating is easily calculated by multiplying Mains (Line) voltage by total consumer's current, generally presented on each consumer's label.

Wattage rating is not always displayed on consumer labels or associated documents. In such case, it should be measured by a Watt meter, or obtained from the consumer's supplier.

An alternative approach is to assume that the load has a unity power factor and to look for a UPS that has a wattage rating, which is equal to the calculated VA rating. This attitude, although more expensive, might be the best approach, as it covers also the possibility of future server replacement, which will probably have unity PF rating.

Both UPS's VA and Watt ratings should exceed the respective load ratings with a reasonable safety margin. Unfortunately we may confront new hurdle when looking for a proper UPS. Frequently, only UPS's VA figures are provided by sellers or on manufacturer's sales information. The Specific UPS technical specifications should be consulted to get the relevant Wattage rating which the UPS supports.

To help solving the problem, UPSonNet displays on its website, www.upsonnet.com, a directory of Rack Mounted UPS systems, which enables to see at glance available systems from main manufacturers, including VA and Watt ratings. In addition, technical information, links to system suppliers,and relevant average system prices are also presented.