UPS Basics

Utility-supplied voltage is seldom a steady 120V AC. Power can dip down to around 100V and peak into the 130V region without major problems. However, if power falls outside this range, you can lose data and damage your hardware.

To prevent undervoltage--brownouts--and overvoltage--power surges--some on-line UPS products can correct utility power ranging from 85V into the mid-140Vs. They also keep you safe from spikes--those brief moments when lightning sends mega-voltage flowing through the line--and line noise. The UPS's Radio Frequency Interference (RFI) filter prevents line-noise interference that might come from other electrical products in the office (fans, microwave ovens, fluorescent lights, etc.)

There are three types of UPS products:

  • Standby, or "off-line," UPS products provide utility power during normal operation. When voltage or frequency changes become severe, the standby UPS starts an inverter that takes DC power from the battery and turns it into AC power to run your system. The standby UPS does not regulate incoming power and is not recommended for networking systems.
  • On-line UPS products are ideally suited to networks running mission-critical applications. On-line UPSs take in all types of voltages and frequencies and continuously create clean, regulated AC power. Some on-line UPSs can compensate for undervoltage and overvoltage without using the battery, saving battery time for use only in complete blackouts.
  • Line-interactive UPSs provide a measure of voltage regulation by "boosting" utility voltage up or "bucking" it down before passing it to your system. The line-interactive model uses the battery for this type of voltage regulation, so its battery life is shorter than that of on-line models.

The UPS's power rating is measured in VA or kVA. Here's how to determine the power rating you'll need:

  1. List all the equipment you need to protect.
  2. Multiply the nameplate voltage and amperage for each piece of equipment to obtain your VA (the product of volts and amperes). If the equipment is specified only in watts, divide the watts by 0.7 to obtain VA in Root Mean Square (RMS) values, which is how the UPS's capacity is calculated.
  3. Add the individual VA requirements to get your total system's needs. If your VA requirements are, for example, 770VA and you purchase an 800VA UPS, remember that your full-load runtime--the amount of backup time you can expect running at full load--is likely to be only five to 20 minutes at best. For extended runtime, you can purchase add-on battery packs from the vendor.