kW vs kVA

Do you know it is possible to appear to have correctly sized a UPS but still overload the UPS? This is because nameplate rating of some UPS(usually in VA or Watt) could be confusing when the actual usable power is not considered for the prospective load.

Example:
A 1000VA UPS. The user wants to power a 900VA server with the UPS. The server has a Power Factor of 1, so has a Watt rating of 900W and a VA rating of 900VA. Although the VA rating of the load is 900VA, which is within the VA rating of the UPS, the UPS will not power this load. That is because the 900W rating of the load exceeds the Watt rating of the UPS, which is 60% of 1000VA or around 600W which has a power factor of 0.6 which was not stated on the UPS name plate.

So what is the real power obtainable from a UPS?

The power in Watts is the real or usable power drawn by the equipment. It is the resistive component of power supplied to the equipment and is the portion that actually does useful work. Volt-Amp is called the "apparent power" and is the product of the voltage applied to the equipment and the current drawn by the equipment. the VA power is a sum of the resistive and the reactive components of power.

kVA = kW + jkVA(r)

As seen above, the smaller the reactive component of any equipment, the closer the useful power is to the apparent power.

Resistive loads e.g incandescent lamps and heaters have identical VA and Watt ratings which implies that all the power delivered to such an appliance is converted to useful power. However, for computer equipment and other partly reactive loads, the Watt and VA ratings can differ significantly, with the VA rating as seen from the expression iniial above. 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%).

Power factor = kW / kVA

Significance of kW and kVA
The Watt rating determines the actual power purchased from the utility company or the heat loading generated by the equipment.
The VA rating NOT kW is used for sizing wiring, battery capacity and circuit breakers.

Different types of switch mode power supply
1. Power Factor Corrected supplies
2. Capacitor Input supplies.

It should be noted that it is usually not possible to tell which kind of power supply is used by physical inspection of the supply.

For PFC supplies, the Watt and VA ratings are equal (power factor of 0.99 to 1.0). Capacitor Input supplies, however, have the characteristic that the Watt rating is in the range of 0.55 to 0.75 times the VA rating (power factor of 0.55 to 0.75). This is because the capacitor introduces a reactive component into the supply.

Large computing equipment such as routers, switches, drive arrays, and servers typically use the Power Factor Corrected supply and consequently for this type of equipment the power factor is 1. Personal computers, small hubs, and personal computer accessories typically have Capacitor Input supplies and consequently for this type of equipment the power factor is less than one, and is ordinarily in the range of 0.65.