A power inverter, or inverter, is an electronic device/circuitry that converts direct current (DC) into alternating current (AC). The input voltage, output voltage and frequency, and overall power handling depend on the design of the specific device or circuitry.

An inverter mainly consists of 1) batteries, 2) Oscillator & 3) transformer

Batteries – Battery is used as a DC power source. Ac supply is used to charge the batteries through charger.

Oscillator – The function of oscillator is to cut continuous DC signal (say 12V) into multiple positive & negative signals (magnitude 12V) which are in square form at a fixed frequency (say 50 Hz). Output waveform depends on the design of the oscillator. The low magnitude oscillated signal can be amplified (say 50V) by using an Amplifiers.

Transformer – The function of transformer is to step up input signal (which is the output of oscillator) from 50V to 220V.Output voltage of transformer depends on winding ratio of transformer.


When normal AC supply is available, the load is fed directly from normal AC supply & this AC supply charges the batteries through battery charger.

When normal AC supply is not available, this is sensed by sensors &change over relays and oscillator starts producing 50Hz signal. This signal is further amplified by amplifier circuits. This amplified signal is fed to inverter transformer which produces output signal of 220V.

Changeover from AC mains to Inverter takes a few seconds. That is why inverters are suitable for domestic equipment like lamps, fans, fridge, radio etc but not suitable for electronic equipment like Computers etc where this delay in changeover may corrupt/lose the data. So UPS is the best option for Computer applications where changeover takes place immediately (without any interruption of power).

Output waveform from Inverter

An inverter can produce a square wave, modified sine wave, pure sine wave, pulse width modulated wave (PWM) or sine wave depending on circuit design.

a)     Square waveform

This is one of the simplest waveforms an inverter design can produce and is best suited to low-sensitivity applications such as lighting and heating. Square wave output can produce “humming” when connected to audio equipment and is generally unsuitable for sensitive electronics.

b)    Modified sine wave

A modified sine wave inverter has a non-square waveform that is a useful approximation of a sine wave for power translation purposes.

Most inexpensive power inverters produce a modified sine wave rather than a pure sine wave.The operation of the modified sine wave inverters will be same as a square wave inverter. These modified sine waves can be used by most household electrical devices. It is extremely common to see this type of inversion in industrial quality inverters. It acts like a pure sine-wave, modified sine waves are mainly used for very high efficiency equipments such as microwaves, drills and clocks or motors.

c)     Pure Sine wave

A power inverter device which produces a sinusoidal AC waveform is referred to as a sine wave inverter. Such inverters are very expensive & use rotating electrical machinery & mostly used in industries. A pure sign wave inverter improves the functions of equipment.



Harmonics are the frequencies that are produced along with the main or fundamental frequency which is produced by inverter. These additional/extra frequencies/harmonics reduces the efficiency of the invertor by heating the equipment. When harmonics are present in the system then motors make noise while in operation. Harmonics are produced in multiple wave forms, normally 3 step, 5 step,..frequency so these harmonics are called 3rd or 5th or…harmonics.




1 Change-over time There is no time gap when power goes off & power is supplied by UPS to the load There is some delay of few seconds in changeover by inverter.
2 Design technology Batteries, charger & inverter are used for UPS Batteries, charger, Oscillator, Amplifier & transformer are used for inverter
3 Output voltage Pure sign wave output voltage is produced by UPS Various types of out voltages are produced by Inverter depending on the design like square wave, modified sign wave, pure sign wave
4 Harmonics/distortion Harmonics produced which are subsided by suitable filters Some harmonics are present in the inverter depending of the type of design. Motors makes noise during operation if harmonics are present in the output voltage
5 Maintenance No maintenance is required as sealed batteries (maintenance free batteries) are used Battery maintenance is required as Led Acid batteries are used
6 Application It is suitable for computer back up, sophisticated electronic equipment etc It is suitable for domestic lighting, fans, TV & other appliances
7 Prize UPS is expensive as compared to inverter as there is no gap in power change-over Its less expensive than UPS as some time delay is there in power change-over