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# How to determine the specifications of a new uninterruptible power supply unit

Mar 25, 2021

How to determine the specifications of a new uninterruptible power supply unit

1. kW and kVA mean kilowatts and kilovolt-amperes respectively-"thousand" is often used as a prefix to describe larger numbers.

2. According to the basic laws of physics, in a direct current (DC) circuit, "watt=volt×ampere". Usually, alternating current (AC) is used in our buildings and equipment. Because for power companies, AC power transmission is more efficient and there are fewer losses. But when the alternating current reaches the transformer of the device, it often produces a characteristic of reactance (the obstructive effect of capacitance and inductance on the alternating current in the circuit).

3. From the perspective of apparent power (volt-amperes), reactance will reduce the value of available power (watts). We call the ratio of these two data the power factor (PF). Therefore, the actual power formula of an AC circuit is "watt = volt x ampere x power factor". Unfortunately, although the power factor of most electric devices is always stable, it is usually 1.0 or less. As far as I know, the only devices that can maintain a power factor of 1.0 are light bulbs.

For many years, the design of large UPS systems has been based on 0.8 power factor, which means that a 100 kVA UPS power supply can actually only support a power load of 80 kW. Nowadays, most UPS systems continue to be designed according to this specification, even though most technologies now can make the power factor of the equipment reach 0.95-0.98.

Regarding UPS power supply, whether it is measured in kilowatts or in kilovolt-amperes, it cannot exceed its rated power supply capacity. However, there are also some UPS systems on the market whose PF values have been further revised, which allows us to treat kilowatts and kilovolt-amperes equally.

Data on the nameplate of the UPS system

The biggest problem in determining the specifications of a UPS unit is how to determine its actual load. The power data provided by many data hardware manufacturers on the device is inconsistent with the facts, and some are even completely wrong. Large manufacturers usually set up a link or configure an evaluation device on their website. This allows them to provide fairly accurate information.

Be careful to use the nameplate of the device. This is a statutory quota mark, but generally speaking, its marked quota is much higher than the actual power that the equipment can provide. For example, if a UPS unit is marked on the nameplate that it can provide 4 to 8 amperes of current under the voltage standard of 90 to 240 volts, its actual power may be only 500 watts.

First of all, these data may shrink. The higher the current, the lower the voltage. If the voltage is 120 volts and the current is 8 amps, then the power you can get is 960 volts. With a power factor of 0.95, the power it can provide is 912 watts. The efficiency of any power supply is never that low, and the power supply never runs at full load. Therefore, the power of this UPS unit may never exceed 500 watts, but if you are really conservative, based on the power factor of 1.1, the input power specification of the power supply should also be around 550 watts.

Also, don't be fooled by dual-corded devices. The power supply is to share the load task, among which, each single power supply is required to support full-load operation. Therefore, a UPS unit with two 500-watt power supplies should also be considered the same as a 500-watt power supply.