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Evaluation parameters

Apparent power (in VA or kVA)

It is defined as: PAPP =V x I

for single-phase load: PAPP = V x I x √3

for three-phase loads where V is the load voltage supply and I is the current absorbed by the load in normal load conditions. This information is normally shown on documents and/or load nameplates though it may be shown as an oversized value.

Active power (in W or kW)

It is defined as:
kW = kVA x cos ∏
(where cos ∏ means power factor PF).

The cos ∏ value of loads is very seldom indicated, therefore a correct UPS sizing requires measurements of kVA absorbed by loads. Experience, anyway, shows that typical loads of computer feature a PF between 0,65 and 0,8.

Considerations on the misleading concept of "computer power"

In the definition of the UPS rated power, the parameter values, defined as "computer power", "switching power", "actual power", power a particular temperature value, etc. Are sometimes indicated. Such arbitrary parameter values have no relation with apparent power and active power; they can be neither quantified nor defined and therefore must not be used for the correct sizing of the UPS.

Crest Factor

A linear load absorbs a sinewave current that shows an effective value
(IEFF usually measured and declared) and a peak value (IPK).
The Crest Factor value is defined as:

The nominal value for a linear load is CF = 1,41. Most loads applied to UPS’s are non- linear load: they absorb distorted current with a CF value greater than 1,41 and require therefore higher peak currents thus resulting in an increased distortion of
the output voltage than equivalent linear loads. Directive EN62040-3, indicates a typical non-linear load as CF=3, used for UPS testing, which may be used in the absence of other data.


Overloads are temporary requests from electrical equipment which exceed regular operation absorption. They are caused by current peaks which may occur when one or more users are switched on. In case the overload exceeds the admissible limits the UPS guarantees the energy supply via the automatic bypass line. For "On line" UPS’s the transfer is effected without any break in power (transfer time = 0 ms). The "bypass" is a safety device with independent protection and auxiliary supply able to supply the load independently from the UPS also when the UPS is switched off or broken.

Input current harmonics

The UPS battery charger rectifier absorbs a distorted current containing harmonics that are multiples to the reference 50Hz frequency. These harmonics may cause voltage distortion which may affect the normal operation of non privileged user’s.The harmonics level of AROS UPS's complies with the present directive. Howerver, in order to further reduce the presence of these harmonics variuos solutions are available such as the installation of Active Filetrs (Power Factor Controlled) or different Rectifier designs.


The batteries supplied with the UPS’s are valve regulated batteries (VRLA) known as sealed batteries with no electrolyte top-up, very low gas emission, suitable for installations in offices and public places with no needfor special precautions. Batteries are normally supplied with the UPS and can either be installed in the same cabinet or in additional ones. YSP guarantees the power supply runtimes specifying the apparent load power and the power factor.

Remarks about the misleading concept of "typycal autonomy"

In defing the runtime it is often used the concept "typical autonomy (or runtime)" which has nothing to do with the real runtime based on the 100% load value expressed in W or kW (active power). The runtime that YSP shows on all its documentation have been calculaterd at 100% of the load.


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