BUT WHAT POWER ARE WE TALKING ABOUT?
It has now become a habit, exclusive to the Italian UPS market, to create considerable confusion as to the real nominal power that can be delivered by the UPS.
We are thus witnessing the proliferation of terms such as computer power, computer power, switching power, etc., which have no technical basis and are based solely on the “fantasy” of the seller proposing the product.
Knowledge of the following parameters is of fundamental importance when choosing the sizing of a UPS.

INFORMATICS VAi
This is not a scientific value but a conventional estimate, born from the need to relate the power value actually absorbed by Personal Computers’ power supplies to the UPS power.
On PC power supplies, an oversized power expressed in VA (Volt-Ampere) or in W (Watt) is declared (normally at least double the power actually absorbed by the entire PC). This is due both to the intrinsic characteristics of switching power supplies (very low cos j of about 0.6) and to the increasingly low consumption of the components that make up the entire PC (hard disk, mother board, etc.).
In order not to excessively over-dimension the UPS with respect to the PC to be protected, it may be useful to indicate an estimated power, called computing and expressed in VAi (Volt-Ampere-Informatics), which is double the power in Watts actually delivered by the UPS.
Some UPS manufacturers, unfortunately, have excessively exploited these arguments to increase their economic competitiveness to the detriment of the User, declaring an IT power equal to 3 or more times the power expressed in Watts, even going so far as to attribute a power of 2,200 VAi to a UPS of just 487 Watts.

APPEARING POWER (VA, kVA)
This is defined as: P app = V x A
where V is the supply voltage of the load (Volts) and A is the current drawn by the load (Amps).
This expression indicates the power value when the current is in phase with the voltage.
In reality, in the most common circuits, the current is out of phase by a certain angle phi, either behind or ahead of the voltage. This phi angle is difficult to calculate, so the apparent power value is normally stated on the documents and/or nameplates of the loads.
The permanent apparent power of the UPS is defined relative to a permanent load in VA or KVA with specified cos phi (power factor) (usually between 0.6 and 0.8).

ACTIVE POWER (W, kW)
This is defined as: P att = Papp x cos phi
where cos phi is the power factor.
It is the power that actually acts in the circuit and is less (reactive loads: cos phi <1) or equal (resistive loads: cos phi =1) to the apparent power.
The active power and cos phi of loads are rarely indicated because they are difficult to calculate.
The Active Power of the UPS expressed in W or kW is the real power that can be delivered by the UPS.

Note:
The technical definitions given for apparent and active power are taken from the book “Static Uninterruptible Power Supplies – European Guide” by CEMEP (European Committee of Electrical and Power Electronics Manufacturers), published in 1999.

POWER FACTOR (cos phi, p.f.)
The power factor is the cosine value of the phase shift angle between voltage and current.
This value is equal to 1 in the case of zero phase shift (e.g. purely resistive load).
For computer loads, cos phi normally varies between 0.5 and 0.7.