Uninterruptible Power Supplies

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Power outages and brownouts can severely disrupt information processing and result in data loss. UPSs enable processing to continue during short outages and let you shut down systems in an orderly fashion during long outages. A UPS also shields your equipment from voltage surges and reductions by maintaining voltage within acceptable limits.

You can purchase UPSs in a variety of capacities. Vendors use a VA (volts * amperes) rating to indicate the amount of power the UPS can deliver. For this Buyer's Guide, vendors supplied information about products intended to support one or two desktop PCs (up to 700VA), small server farms (up to approximately 5KVA), large server farms (up to approximately 15KVA), and data centers (three-phase models with capacities up to 25KVA).

Make sure the capacity of the UPS you choose exceeds your requirements by at least 20 percent—more, if you plan to add servers or desktop systems. Because PCs and servers can present different power loads to the UPS, the wattage rating should be your primary concern when making a purchase. To estimate how long your equipment can operate on UPS power, ask about the product's battery life and consider the typical duration of power outages in your area.

All UPS systems employ batteries and feature power-inverter circuitry. Online UPSs first rectify the commercial AC power to DC, then use the inverter circuitry to provide AC power to your computers. This process provides better output voltage regulation and purer power than line-interactive models. Online UPSs can also switch from commercial to battery power with no transfer time, while line-interactive models typically take from 4ms to 8ms to make the changeover (only the most sensitive scientific equipment would be affected by such a short interval). Keep in mind that online units typically cost more than line-interactive models.

If uptime is a crucial concern, you might consider a UPS with redundant batteries and power circuitry. Because UPSs sit idle most of the time, a failed battery might go unnoticed until it's too late. Some redundant UPSs also let you scale up their power handling and battery runtime capacities as you add new servers.

You need to also consider the management software included with the UPS. When power fails, UPSs communicate the loss of power through the server's serial or USB port to the management software running on the server. The software notifies the administrator of the power failure through email or pager and also notifies users so that they can save their data. When battery power drops to a predetermined level, the software begins to close applications and shut down servers in an orderly fashion. Some UPS management programs will let you specify the amount of time needed for each application to close before the server shutdown process begins. Other management applications let you remotely shut down servers to conserve battery runtime and keep crucial applications running as long as possible.

Finally, inspect the vendor's warranty and onsite service options. In a few years you'll need to replace the UPS's batteries. This process can be labor-intensive because the batteries typically are large and heavy. Securing an onsite service agreement from the vendor will free your IT staff for more important tasks.

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