UPS can provide continuous, optimal power supplying in the event of a power down instead of utility line. The power supply can resume within 4 to 8 milliseconds, or even 0 milliseconds, which depends on different types.

2. What are the problems usually happen to the power supply ?
Blackout, power slump, power surge, continuous under voltage, continuous over voltage, frequency fluctuation, interference of computers, switching transient, harmonic distortion, etc.

3. The functions of UPS?
An UPS can perform
1, Power down protection
1. Under-voltage and over-voltage protection
2. Waveform distortion correction
4. Frequency stabilization
5. Voltage regulation
6. Normal mode noise rejection
7. Common mode noise rejection
8. Surge protection
9. Transient response protection
10. Power supply monitoring

4. What are the main categories of UPS?
There are basically three different types of devices by topology, which are offline, line interactive and on line ones.
For offline UPS, power is usually derived directly from the power line, until power fails. After power failure, a battery powered inverter turns on to continue supplying power. Batteries are charged, as necessary, when line power is available. This type of supply is sometimes called an "backup" UPS.
The quality and effectiveness of this class of devices varies considerably; however, they are generally quite a bit cheaper than online UPS. The time required for the inverter to come on line, typically called the transfer time, varies by unit.
Since appliances connected to the supply are basically connected directly from the power line, offline UPS provide relatively poor protection from line noise, frequency fluctuation, line spikes, and brownouts.

Online UPS

What we call as "true" UPS systems, those power supplies that continuously operate from an inverter. Obviously, there is no transfer time, and these supplies generally provide the best isolation from power line problems. The disadvantages to these devices are increased cost, increased power consumption, and increased heat generation.

line-interactive UPS

The line-interactive UPS uses a totally different design than any type of offline UPS. In this type of unit, the separate battery charger, inverter and source selection switch have all been replaced by a combination inverter/converter, which both charges the battery and converts its energy to AC for the output as required. AC line power is still the primary power source, and the battery is the secondary. When the line power is operating, the inverter/converter charges the battery; when the power fails, it operates in reverse.
The main advantage of this design is that the inverter/converter unit is always connected to the output, powering the equipment. This design allows for faster response to a power failure than a offline UPS. The inverter/converter is also normally fitted with circuitry to filter out noise and spikes, and to regulate the power output, providing additional power during brownouts and curtailing output during surges.

5. What is the benefits of a UPS?
A UPS has internal batteries to guarantee that continuous power is provided to the equipment even if the power supply stops providing power.
Advantages:
1. Computer jobs don't stop in case of the power failure.
2. Users do not inconvenienced when computer is out of power.
3. Equipment does not incur the stress of another (hard) power cycle.
4. Data isn't lost because a machine shut down without doing a "sync" or equivalent to flush cached or real time data.

6. How long can equipment on a UPS keep running after the power goes off?
It depends on how big a UPS do you have and what kind of equipment it protects? For most typical computer workstations, one might have a UPS that was rated to keep the machine alive through a 15 minute power loss. If you need a machine to survive hours without power, you should probably look at a more robust power backup solution. Even if a UPS has a very small load, it must still operate it's DC (battery) to AC converter, which costs power.

7: How are the "sizes" of UPS's determined?
Typically, a UPS has a VA rating. The VA rating is the maximum number of Volts * Amps it can deliver. The VA rating is not the same as the power drain (in Watts) of the equipment. Computers are notoriously non-resistive. A typical PF (power factor: Watts/VA) for workstations may be as low as 0.6, which means that if you record a drain of 100 Watts, you need a UPS with a VA rating of 167. Some literature suggests that 0.7 may be a good conversion factor, but this will depend heavily on the machine.

8. The main specs of UPS
The main specs for hardware are:
Operation model: offline, line interactive, online
Input voltage range
Input frequency range
Input power factor
Load power factor
Over load capacity
Transfer time
The stability of output voltage
The distortion of output voltage
Crest ratio
Ability of taking 100% unbalanced load
Cold start function
Bypass function
Compatible with generators
Battery management level
UPS efficiency

9、The capabilities of different topologies to utility line problems: