Flickering lights can be a "charming" feature of an old house. For any business that experiences such losses of power, however, this charming feature can be quite problematic.

Uninterruptible power supplies (UPS) are designed to protect electronic equipment like computers and phone systems against problems stemming from a temporary failure in the power supply. By providing a constant source of electricity, a UPS can help prevent damage or data loss that can occur with the unexpected shutdown of computers, phone systems, and other sensitive equipment.

UPS systems work by detecting decreases in the amount of electricity coming from the wall circuit, and boosting power to maintain a constant flow of electricity to connected equipment. This power boost is provided either by a transformer that enhances a weak electrical flow, or from an internal battery that substitutes for the normal power source in the event of failure.

Most UPS units also contain surge protectors, which help prevent equipment damage whenever there are power surges (sudden increases in the flow of voltage).

This guide is designed to give you the facts you need to purchase a UPS system for your business. The various sections are listed in the box to the right. You can choose to read this guide from beginning to end, or jump directly to a section of interest.

Types of UPS

All UPS systems are not alike. There are three basic types of UPS available.

Standby, or off-line, units switch from the regular power source to an internal battery when they sense a power disturbance. There is a brief lag time before the internal battery comes on-line. This downtime should be brief enough, however, to avoid causing a computer to shut down.

Line-interactive UPS systems add a transformer to minimize the need to use an internal battery with every power fluctuation. These units monitor the line voltage at all times, activating the power transformer when the voltage falls below certain parameters. The battery is activated when even lower voltages are recorded.

On-line UPS units constantly supply power to connected equipment from an internal battery, with battery recharging occurring on an ongoing basis. There is no lag time when power failures occur since these units serve as the primary source of power to equipment. In the case of a blackout, the UPS will continue to generate power, but will be unable to recharge.

Choosing the Right UPS

When deciding what type of UPS to purchase, you should determine how much an unexpected power failure would impact your business.

On-line units offer the most protection, since they run constantly. The additional cost of these systems is often deemed worthwhile for key equipment where a shutdown is unacceptable. They are usually recommended for mission critical applications such as phone systems or a computer server.

Standby and line-interactive UPS are more appropriate when an unexpected power outage, and the resulting loss of data or other information, would be more of an inconvenience than a major problem. These systems should be able to keep equipment going during most power fluctuations, but there may be the occasional situation where they do not quite work as expected. They can be a more cost-effective solution for equipment such as standalone computers or a fax machine.

Line-interactive UPS are best suited for situations where power fluctuations are common occurrences. Power fluctuations occur when "power-hungry" equipment such as an air conditioner is frequently turned on and off, causing brownouts and surges. With line-interactive UPS, these fluctuations can be handled by the transformer instead of the internal battery.

How Much Do You Need?

The power requirements of your equipment will determine the amount of backup power you will need. It is generally recommended that a UPS have a capacity that is at least 25% greater than the total power capacity requirements of the connected equipment. For example, a desktop computer that runs between 180 VA to 280 VA should be equipped with a 300 VA UPS.

You can check how much power your equipment uses by reading the plate on the back of the equipment. Keep your calculator handy, since these plates won't necessarily list the power requirements in the VAs that UPSes use. If the power figure is given in amps, just multiply by the line voltage (in the U.S., that's usually 110V). Then add up the various pieces of equipment together.

Firms will also need to decide whether they want to centralize or distribute their UPS. Offices that have multiple computers housed in one room will generally find that connecting all the equipment to a central UPS will be more cost effective than purchasing individual UPS units. However, this may not be a reasonable option when the equipment is spread over multiple rooms.

Power Life

Most UPS units are designed to provide about ten minutes of backup power. This should be enough time to appropriately shut down the connected equipment and avoid data loss. Since blackouts typically last no more than two minutes, this should also be enough time to work right through most power failures. (It should be noted, however, that this action is not recommended.)

Manufacturers report how long a battery will last under full-load or half-load conditions. Running under full load signifies that a UPS is working at its maximum capacity. A typical UPS should report a full-load time duration of about ten minutes. Under half-load conditions, when the UPS provides only half the power it is capable of generating, the time duration is often more than three times as long.

Another option is to purchase extra battery attachments. Expect these extra batteries to cost about one-fourth to one-half the price of the UPS itself.

Special Buying Tips

Battery Level Indicators
The UPS will be of no use to you if there is insufficient charge left in the machine. At minimum, there should be a visual signal indicating when the unit is drawing upon its internal battery for power. Audible signals are preferred when the battery power is almost depleted. It is also a good idea to look for a battery run-time display.

On a Budget?
Surge suppressers are an inexpensive way to protect against surges and spikes that can destroy equipment. Surge suppressers are quite cheap, starting at less than $10.

Multiple units vs. Larger Units
It can be more cost-effective to supply power for multiple machines with a larger UPS rather than multiple units.

Auto Shut Down
It can be useful to have power management software that will automatically shut down an unattended machine in the event of power failure.

Battery Life
You should plan to replace the battery in the UPS every three to five years. Check ahead of time where you can purchase additional batteries.