Choosing the Correct UPS Backup (Part Two)

2nd Sep 2014

Choosing a UPS unit is a responsible task that might seem a little overwhelming, especially if you are not quite sure what you are looking for. In order to choose the right UPS, there are a few things you need to bear in mind. In “Choosing the right UPS (Part 1)” we touched upon some of them but there is more to be covered. In “Choosing the right UPS (Part 2)” we will take a closer look at some of the technical indicators you need to pay special attention to when choosing a UPS unit.

UPS units

Estimating UPS Requirements

Nowadays, most mainstream domestic and industrial UPS units utilize Voltage-Ampere ratings (VA) to indicate their load capacity. The only difference between Watts and Volt-Amperes is that Watts accounts for an inefficiency called Power Factor (PF) and VA does not. It is important to note that Watt ratings are more precise than VA ratings. In comparison to Watt ratings, VA ratings are always higher. VA ratings could sometimes be misleading and make the UPS unit seam more efficient than it actually is, as they don’t take PF loss into account.

The formula that is used to determine the UPS protection needed for a particular equipment, is given below:

Where “Amps” is the current pull of the system. Voltage is a constant of 120 in North America Watts or Wattage for Alternating Current:

Power factor (PF) is a number between 0-1. It represents the Watts after reactance loss. Volt-Amperes or "VA" is Watts divided by PF or Watts without Power Factor loss taken into account

Older personal computers used to have power supplies with a Power factor of approximately 0.8. Modern day PC power supplies, however, have a higher efficiency level, with PF ratings of about 0.95 and higher.

Backup Time

Backup time is the the time your equipment would run off UPS power during an electrical outage. The backup time is inversely proportional to the amount of load connected to the UPS i.e. the more devices you connect to your UPS, the shorter its back up time would be.

For example, a UPS with VA ratings of 1500VA has a battery run off time of approximately 15 minutes at half load i.e. 750VA, while at full load the run off time is reduced to 5-7 minutes (Tripp Lite OMNIVS1500XL line-interactive backup unit was referenced for these estimations).

A full load VA rating with a PF of .8 is considered safe by many manufacturers, however most modern power supplies have a power factor higher than 0.9.

There is a certain point over which a UPS unit would be regarded as overloaded. To keep it simple, a UPS unit is considered overloaded when VA ratings (or Watts power ratings) is exceeded. Although mostly UPS utilize VA ratings, it is safer to be aware of both Watt and VA load when choosing a UPS. If you know the Power Factor of a particular UPS, you can use the given Watt ratings to calculate its efficiency and compare it to the efficiency of other UPS products. These calculations would also help you determinate the “real power” of a UPS (after all the losses) and the maximum load that could be connected to it.

Power Factor

In order to calculate Watts on a Direct Current (DC), the only thing you need to do is calculate the product of Volts and Amps, as there is no Power Factor on direct current. Alternating current has the capacity of traveling long distances. That’s why home appliances are operated on alternating current and not on direct current. The constant value of direct current could burn the appliances. Alternating current, on the other hand is made up of a sine-wave which represents the oscillation of electricity on a particular frequency (usually at 60 Hz), which reduces the risk of high peaks.

By calculating the product of current and voltage present at the system connected to the UPS you can determine one-way power. This product can also help you determinate the VA requirement for the system connected to the UPS. The product of the current measured and the voltage gives the "apparent power" of the load that is made of two main parts - a part that does the useful work (called “real power”) and a part that generates a heat byproduct but does no useful work (called “reactive power”).

If you want to learn some more about UPS units don’t miss "Choosing the Correct UPS Backup (Part Three)" where we would outline some additional tips on the topic.