What your computer expects to get from the power grid (in the United States) is 120-volt AC power oscillating at 60 Hertz (see How Power Distribution Grids Work for more information). A computer can tolerate slight differences from this specification, but a significant deviation will cause the computer's power supply to fail. A UPS generally protects a computer against four different power problems:
  • Voltage surges and spikes - Times when the voltage on the line is greater than it should be
  • Voltage sags - Times when the voltage on the line is less than it should be
  • Total power failure - Times when a line goes down or a fuse blows somewhere on the grid or in the building
  • Frequency differences - Times when the power is oscillating at something other than 60 Hertz
There are two common systems in use today: standby UPS and continuous UPS. A standby UPS runs thecomputer off of the normal utility power until it detects a problem. At that point, it very quickly (in five milliseconds or less) turns on a power inverter and runs the computer off of the UPS's battery (see How Batteries Work for more information). A power inverter simply turns the DC power delivered by the battery into 120-volt, 60-Hertz AC power.
In a continuous UPS, the computer is always running off of battery power and the battery is continuously being recharged. You could fairly easily build a continuous UPS yourself with a largish battery charger, a battery and a power inverter. The battery charger continuously produces DC power, which the inverter continuously turns back into 120-volt AC power. If the power fails, the battery provides power to the inverter. There is no switch-over time in a continuous UPS. This setup provides a very stable source of power.
Standby UPS systems are far more common for home or small-business use because they tend to cost about half as much as a continuous system. Continuous systems provide extremely clean, stable power, so they tend to be used in server rooms and mission critical applications.