UPS vs Inverter vs EPS


What is a UPS?

A UPS allows a computer to keep running for a short period of time so you can safely save your data and shutdown the computer when there is a power outage also acts as a surge protector as well.

Types of UPS’s:

  • Offline UPS/ Standby
  • Online UPS/ Double conversion
  • Line interactive

Meanings of the different types of UPS’s

Offline UPS/ Standby:

Offers only the most basic features, providing surge protection and battery backup. The protected equipment is connected directly to the incoming utility power.

In a short summary:

  • Lower cost than other units
  • Under mains power unit charges batteries and power passes directly to the load
  • During mains failure batteries provide power to DC/AC inverter to provide 230VAC power to the load
  • Square-wave or Sine-wave output
Online UPS/ Double conversion:

The online UPS is ideal for environments where electrical isolation is necessary or for equipment that is very sensitive to power fluctuations. When power loss occurs the rectifier simply drops out of the circuit and the batteries keep the power steady and unchanged. When power is restored the rectifier resumes carrying most of the load and begins charging the batteries, though the charging current may be limited to prevent the high-power rectifier from overheating the batteries and boiling off the electrolyte. The main advantage of an on-line UPS is its ability to provide an "electrical firewall" between the incoming utility power and sensitive electronic equipment.

In a short summary:

  • Constant duty inverter
  • Design inherently improves power quality and reliability
  • No break
  • Sine-wave output
  • Static bypass improves reliability
Line Interactive UPS:

The line-interactive UPS is similar in operation to a standby UPS, but with the addition of a multi-tap variable-voltage autotransformer. This is a special type of transformer that can add or subtract powered coils of wire, thereby increasing or decreasing the magnetic field and the output voltage of the transformer.

This type of UPS is able to tolerate continuous under voltage brownouts (Voltage drop) and overvoltage surges without consuming the limited reserve battery power. It instead compensates by automatically selecting different power taps on the autotransformer. Depending on the design, changing the autotransformer tap can cause a very brief output power disruption. Which may cause UPSs equipped with a power-loss alarm to "chirp" for a moment.

In a short summary:

These UPS offer enhanced power protection over the basic off-line designs because they provide additional line conditioning. They can also cope with a wider range of input voltages without resorting to battery.


What is an Inverter?

An inverter is an electronic apparatus that changes DC (Direct Current) to AC (Alternating Current). Direct current is created by devices such as batteries and solar panels. When connected, an inverter allows these devices to provide electric power for small household devices. The inverter does this through a complex process of electrical adjustment. From this process, AC electric power is produced. This form of electricity can be used to power an electric light, a microwave oven, or some other electric machine.

  • Sine wave inverters produce good-quality AC power. They are expensive.
  • Modified sine wave inverter produces a lower quality of AC power, with strong power system harmonics but is cheaper. Various modulation strategies are used in cascaded multilevel inverters to reduce the harmonic contents.

Difference between an Inverter and a UPS?

While both provide backup power during mains outage, with the UPS the switch is instantaneous whereas with the Inverter there is a gap of a second or two. This gap is OK for household gadgets such as lights, fans, fridge, etc. but not OK for computers

The AC supply has a pure sine wave whereas in an offline UPS the output is a near sine wav, not a pure sine wav whereas for a inverter its step wave. This is the reason that we hear a hum when we attach an inductive load to a UPS or an inverter but we do not hear anything when it’s on AC supply.


UPS: The mains power comes to the UPS. The AC is converted to DC and this DC is constantly charging the battery. The output of the battery is fed to the Sine wave inverter and it converts DC to AC and this feeds the equipment. Since power out is always drawn from the battery, there is no time lag when mains switches off; it just stops the battery from being charged and the UPS continues to supply power till the battery runs out.

  • CHANGE OVER TIME: An off-line ups (the standard) switches to the batteries in 3 to 8 milliseconds, after the main power has been lost.
  • SIZE: Off-line UPS's go up to about 2kVA
  • VOLTAGE REGULATION: UPS's have automatic voltage regulation (AVR) set on average at 220v


Inverter: The mains power comes to the Inverter. This is directly sent to the output but the AC is also converted to DC and this DC is constantly charging the battery. A sensor and relay mechanism checks whether the mains is ON or OFF. When the main switches OFF, the relay actuator triggers to switch from mains to inverter. Rest is same like the UPS. Because of this sensor and relay, there is a gap between triggering.

  • The typical Inverter changes over in about 500 milliseconds
  • Inverters go to 16kVA, 3 phase
  • Inverters output voltage is the same as the mains input voltage (South Africa = 230V)
  • An inverter can transform a voltage from say 12volts dc and up to 230v AC.

EPS (Emergency Power System)

An emergency power system is an independent source of electrical power that supports important electrical systems on loss of normal power supply. A standby power system may include a standby generator, batteries and other apparatus. Emergency power systems are installed to protect life and property from the consequences of loss of primary electric power supply.

Mains power can be lost due to downed lines, malfunctions at a sub-station, inclement weather, planned blackouts or in extreme cases a grid-wide failure. In modern buildings, most emergency power systems have been and are still based on generators. Usually, these generators are Diesel engine driven, although smaller buildings may use a gasoline engine driven generator and larger ones a gas turbine. However, lately, more use is being made of deep cycle batteries and other technologies such as flywheel energy storage or fuel cells. These latter systems do not produce polluting gases, thereby allowing the placement to be done within the building. Also, as a second advantage, they do not require a separate shed to be built for fuel storage.

Computers, communication networks, and other modern electronic devices need not only power, but also a steady flow of it to continue to operate. If the source voltage drops significantly or drops out completely, these devices will fail, even if the power loss is only for a fraction of a second. Because of this, even a generator back-up does not provide protection because of the start-up time involved.

To achieve more comprehensive loss protection, extra equipment such as surge protectors, inverters, or sometimes a complete uninterruptible power supply (UPS) is used. UPS systems can be local (to one device or one power outlet) or may extend building-wide.


By ANTONIA ILIEVA    2018-11-01 08:37:24