6.1 Types of Loads :While sizing the UPS rating, it is necessary to determine load-characteristics. Various loads may be classified as the following:
» Heavy in-rush current loads (current required for smaller duration, typically 100-200 ms)
e.g Disk-Drives, Monitors, SMPS based computer loads, etc. » Higher starting current loads (current required for longer duration, typically 15-30 seconds) eg. Motors, Fans, Laser-Printers.
» Loads needing rated running current on being energised (current required continuously) eg Dot-Matrix printers, lamps, etc.
6.2 Power Factor :The active power required by any load is a percentage of the total power required from the mains supply. This ratio is called the power factor, which should be considered while sizing a UPS. eg. Typically a computer of 200 VA connected load consumes active power of 140 watts, at 0.7 power factor.
Computer Load (in watts)
Computer Power required (VA)
= 0.7Any UPS should typically feed loads with power factor varying between 0.6 (lag) to 1.0 (unity), without exceeding its VA/watt capacity cumulatively.
6.3 Load Estimation» Identify all critical loads to be connected to the UPS system » Determine the watt /VA Rating of each load for Starting and Running conditions. (Many times UPS are wrongly over-sized considering the loads for their starting current cummulatively. However in practice, most loads are switched on different times).
» Add the loads for determining the Watt/VA Rating of the UPS (Note : The cumulative watt/VA rating of the loads based on running currents should not exceed the watt/VA rating of the UPS, respectively).
6.4 DIVERSITY FACTOR :In sizing the UPS capacity, consider the diversity factor which means taking into account the usage of maximum member of loads at any given point in time. Typically most IT applications have a diversity factor varying between 0,6 to 0,9.