Red Hat ENTERPRISE LINUX 4 - INTRODUCTION TO SYSTEM ADMINISTRATION Administration Manual page 168

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8.1.3.2.3.1. Providing Power For the Next Few Seconds
Since the majority of outages last only a few seconds, your backup power solution must have two
primary characteristics:
Very short time to switch to backup power (known as transfer time)
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A runtime (the time that backup power will last) measured in seconds to minutes
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The backup power solutions that match these characteristics are motor-generator sets and UPSs. The
flywheel in the motor-generator set allows the generator to continue producing electricity for enough
time to ride out outages of a second or so. Motor-generator sets tend to be quite large and expensive,
making them a practical solution only for mid-sized and larger data centers.
However, another technology — called a UPS — can fill in for those situations where a motor-
generator set is too expensive. It can also handle longer outages.
8.1.3.2.3.2. Providing Power For the Next Few Minutes
UPSs can be purchased in a variety of sizes — small enough to run a single low-end PC for five
minutes or large enough to power an entire data center for an hour or more.
UPSs are made up of the following parts:
A transfer switch for switching from the primary power supply to the backup power supply
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A battery, for providing backup power
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An inverter, which converts the DC current from the battery into the AC current required by the
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data center hardware
Apart from the size and battery capacity of the unit, UPSs come in two basic types:
The offline UPS uses its inverter to generate power only when the primary power supply fails.
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The online UPS uses its inverter to generate power all the time, powering the inverter via its battery
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only when the primary power supply fails.
Each type has their advantages and disadvantages. The offline UPS is usually less expensive, because
the inverter does not have to be constructed for full-time operation. However, a problem in the inverter
of an offline UPS will go unnoticed (until the next power outage, that is).
Online UPSs tend to be better at providing clean power to your data center; after all, an online UPS is
essentially generating power for you full time.
But no matter what type of UPS you choose, you must properly size the UPS to your anticipated load
(thereby ensuring that the UPS has sufficient capacity to produce electricity at the required voltage
and current), and you must determine how long you would like to be able to run your data center on
battery power.
To determine this information, you must first identify those loads that are to be serviced by the UPS.
Go to each piece of equipment and determine how much power it draws (this is normally listed on a
label near the unit's power cord). Write down the voltage, watts, and/or amps. Once you have these
figures for all of the hardware, you must convert them to VA (Volt-Amps). If you have a wattage
number, you can use the listed wattage as the VA; if you have amps, multiply it by volts to get VA. By
adding the VA figures you can arrive at the approximate VA rating required for the UPS.
Chapter 8. Planning for Disaster