An off-line UPS is one that lies in wait for a power interruption or brownout. When the need arises, it switches on-line and provides emergency power. At one time, the term standby power system was reserved for this kind of UPS, and it may still be called a standby UPS. Further, UPS makers sometimes refer to the off-line UPS disparagingly as a "bucket of batteries" because it does little more than provide emergency power.

 In normal use, the off-line UPS merely connects the power line directly through to your computer. Its battery charger draws only a slight current to keep its source of emergency energy topped off. The batteries are in no way linked to your computer. They are switched out of the loop. As its name implies, most of the time the standby power system simply stands by, waiting for the power to fail so that it can leap into action.

When the power fails, however, the off-line UPS switches into action--and switch is the key word. A relay inside the off-line UPS switches the wires that go to your computer from the wall outlet (passing through the UPS), to an inverter connected to the battery pack inside the UPS. The inverter then supplies power to your computer with the batteries as the source.

The switching process requires a small but measurable amount of time. First, the failure of the electrical supply must be sensed. Even the fastest electronic voltage sensors take a finite time to detect a power failure. Even after a power failure is detected, another slight pause occurs before the computer receives its fresh supply of electricity while the inverter starts up and the switching action itself takes place.

 All available off-line UPSs switch quickly enough that your computer never notices the lapse. Most off-line UPSs switch in 3 to 10 milliseconds, compared to the 8 milliseconds in one-half of one cycle of the AC current, and about 50 milliseconds the most finicky modern computers can stand to be without power. Even the slowest off-line UPS has a wide safety margin when it comes to switching time.

The chief drawback of the off-line UPS is that its power protection features stop there. Without additional circuitry, the off-line UPS affords no protection against spike, surges, or even long-term over-voltages, and its protection against under-voltage problems is limited to switching to backup power--which means that battery capacity limits the protection period. Most UPS makers add surge protection circuitry to their off-line units, but the off-line technology cannot cope with sustained over-voltages or prolonged sags.

The off-line UPS is a low-cost product that protects primarily against a single power problem, the outage. It is not a complete power protection solution.

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