An uninterruptible power supply (UPS), also known as a battery back-up, provides emergency power and, depending on the topology, line regulation as well to connected equipment by supplying power from a separate source when utility power is not available. It differs from an auxiliary or emergency power system or standby generator, which does not provide instant protection from a momentary power interruption. A UPS, however, can be used to provide uninterrupted power to equipment, typically for 5–15 minutes until an auxiliary power supply can be turned on, utility power restored, or equipment safely shut down.
While not limited to safeguarding any particular type of equipment, a UPS is typically used to protect computers, data centers, telecommunication equipment or other electrical equipment where an unexpected power disruption could cause injuries, fatalities, serious business disruption or data loss. UPS units come in sizes ranging from units which will back up a single computer without monitor (around 200 VA) to units which will power entire data centers or buildings (several megawatts).

Common power problems
There are various common power problems that UPS units are used to correct:
1) Power failure
2) Voltage sag
3) Voltage spike
4)Under-voltage (brownout)
5)Over-voltage
6) Line noise
7)Frequency variation
8) Switching transient
9) Harmonic distortion
UPS units are divided into categories based on which of the above problems they address, and some manufacturers categorize their products in accordance with the number of power related problems they address.

Technologies
The general categories of modern UPS systems are on-line, line-interactive, and standby. An on-line UPS uses a "double conversion" method of accepting AC input, rectifying to DC for passing through the battery (or battery strings), then inverting back to AC for powering the protected equipment. A line-interactive UPS maintains the inverter in line and redirects the battery's DC current path from the normal charging mode to supplying current when power is lost. In a standby ("off-line") system the load is powered directly by the input power and the backup power circuitry is only invoked when the utility power fails. Most UPS below 1 kVA are of the line-interactive or standby variety which are usually less expensive.
For large power units, Dynamic Uninterruptible Power Supply are sometimes used. A synchronous motor/alternator is connected on the mains via a choke. Energy is stored in a flywheel. When the mains power fails, an Eddy-current regulation maintains the power on the load. DUPS are sometimes combined or integrated with a diesel-generator[clarification needed], forming a diesel rotary uninterruptible power supply, or DRUPS.
A Fuel cell UPS have been developed in recent years using hydrogen and a fuel cell as a power source, potentially providing long run times in a small space.

Offline / standby

The Offline / Standby UPS (SPS) offers only the most basic features, providing surge protection and battery backup. Usually the Standby UPS offers no battery capacity monitoring or self-test capability, making it the least reliable type of UPS since it could fail at any moment without warning. These are also the least expensive, selling for as little as US$40. The SPS may be worse than using nothing at all, because it gives the user a false sense of security of being assured protection that may not work when needed the most.
With this type of UPS, a user's equipment is normally connected directly to incoming utility power with the same voltage transient clamping devices used in a common surge protected plug strip connected across the power line. When the incoming utility voltage falls below a predetermined level the SPS turns on its internal DC-AC inverter circuitry, which is powered from an internal storage battery. The SPS then mechanically switches the connected equipment on to its DC-AC inverter output. The switchover time is stated by most manufacturers as being less than 4 milliseconds, but typically can be as long as 25 milliseconds depending on the amount of time it takes the Standby UPS to detect the lost utility voltage.[citation needed]

Hybrid Topology / Double Conversion on Demand
Recently there have been hybrid topology UPSs hitting the marketplace. These hybrid designs do not have an official designation, although one name used by HP and Eaton is Double Conversion on Demand.This style of UPS is targeted towards high efficiency applications while still maintaining the features and protection level offered by double conversion.
A hybrid (double conversion on demand) UPS operates as an offline/standby UPS when power conditions are within a certain preset window. This allows the UPS to achieve very high efficiency ratings. When the power conditions fluctuate outside of the predefined windows, the UPS switches to online/double conversion operation.In double conversion mode the UPS can adjust for voltage variations without having to use battery power, can filter out line noise and control frequency. Examples of this hybrid/double conversion on demand UPS design are the HP R8000, HP R12000, HP RP12000/3 and the Eaton BladeUPS.

Typically, the high-mass flywheel is used in conjunction with a motor-generator system. These units can be configured as:
A motor driving a mechanically connected generator,
A combined synchronous motor and generator wound in alternating slots of a single rotor and stator,
A Hybrid Rotary UPS, designed similar to an Online UPS, except that it uses the flywheel in place of batteries. The rectifier drives a motor to spin the flywheel, while a generator uses the flywheel to power the inverter.
In case #3 the motor generator can be synchronous/synchronous or induction/synchronous. The motor side of the unit in case #2 and #3 can be driven directly by an AC power source (typically when in inverter bypass), a 6-step double-conversion motor drive, or a 6 pulse inverter. Case #1 uses an integrated flywheel as a short-term energy source instead of batteries to allow time for external, electrically coupled gensets to start and be brought online. Case #2 and #3 can use batteries or a free-standing electrically coupled flywheel as the short-term energy source.

Capacitors
UPSs can be equipped with maintenance-free capacitors to extend service life .

Applications

N+1
In large business environments where reliability is of great importance, a single huge UPS can also be a single point of failure that can disrupt many other systems. To provide greater reliability, multiple smaller UPS modules and batteries can be integrated together to provide redundant power protection equivalent to one very large UPS. "N+1" means that If the load can be supplied by N modules, the installation will contain N+1 modules. In this way, failure of one module will not impact system operation

Multiple redundancy
Many computer servers offer the option of redundant power supplies, so that in the event of one power supply failing, one or more other power supplies are able to power the load. This is a critical point - each power supply must be able to power the entire server by itself.
Redundancy is further enhanced by plugging each power supply into a circuit (i.e. to a different circuit breaker).
While it is common practice by uninformed people to plug each of these individual power supplies into one single UPS, redundant protection can be extended further yet by connecting each power supply to its own UPS. This provides double protection from both a power supply failure and a UPS failure, so that continued operation is assured. This configuration is also referred to as 2N redundancy. If the budget does not allow for two identical UPS units then it is common practice to plug one power supply into mains power and the other into the UPS.

Outdoor use
When a UPS system is placed outdoors, it should have some specific features that guarantee that it can tolerate weather with a 'minimal to none' effect on performance. Factors such as temperature, humidity, rain, and snow among others should be considered by the manufacturer when designing an outdoor UPS system. Operating temperature ranges for outdoor UPS systems could be around −40 °C to +55 °C.
Outdoor UPS systems can be pole, ground (pedestal), or host mounted. Outdoor environment could mean extreme cold, in which case the outdoor UPS system should include a battery heater mat, or extreme heat, in which case the outdoor UPS system should include a fan system or an air conditioning system.

Internal systems
UPS systems can be designed to be placed inside a computer chassis. There are two types of Internal UPS. The first type is a miniaturized regular UPS that is made small enough to fit into a 5.25″ CD-ROM slot bay of a regular computer chassis. The other type are re-engineered switching power supplies that utilize dual power sources of AC and/or DC as power inputs and have an AC/DC built-in switching management control units.
All these connections are connected to a common point called 'Earth'.

Difficulties faced with generator use
The voltage and frequency of the power produced by a generator depends on the engine speed. The speed is controlled by a system called a governor. Some governors are mechanical, some are electronic. The job of the governor is to keep the voltage and frequency constant, while the load on the generator changes. This may pose a problem where, for example, the startup surge of an elevator can cause short "blips" in the frequency of the generator or the output voltage, thus effecting all other devices powered by the generator. Many transmission sites will have backup diesel generators - in the case of AM, the load presented by the transmitters changes in line with the signal level. This leads to the scenario where the generator is constantly trying to correct the output voltage and frequency as the load changes.