We can’t always make electricity just when we need it, especially when we are using renewable energy sources such as solar panels. The answer to this problem is to store the electricity that we make so that we can have it to use it when we need it.

The best way to store electrical energy for household use is in rechargeable storage batteries.  However, rechargeable storage batteries provide direct current (DC) electricity and our mains-powered appliances need mains-style alternating current (AC) electricity.  To convert DC electricity from the storage batteries to AC electricity to run our mains-powered appliances we use an inverter.

How an Inverter Works

An inverter converts DC electricity to mains-style AC electricity to run mains-powered appliances. An inverter uses electronic switches to periodically reverse (invert) the direction of the DC electricity, turning it into AC electricity.  Each time
the electronic switches reverse the flow of the DC electricity, it snaps instantly from one direction to the other. A graph of these changes is called a waveform. The waveform that is produced this way is called a square wave and it looks like this:

The AC electricity made by a cheap and simple inverter has this square-wave waveform.

Inverter Waveform

Mains electricity is made with an alternator.  The AC electricity flowing from an alternator swings back and forth in a smooth waveform which is called a sine wave. A sine-wave waveform looks like this:

Many of the mains-electricity powered appliances that we use are designed to work only with a sine-wave waveform and don’t work well with a square-wave waveform — especially electronic appliances such as computers and televisions.

Making a Sine-wave Form

Electrical Resonant Filters:

Better quality inverters use electrical resonant filters to improve the waveform. The filters reduce the suddenness of the direction changes of the flow of electricity that occurs in a square-wave waveform, and smooth it out to be more like a sine-wave waveform. The arrangement looks like this:

Flow of electricity back and forth:

The most sophisticated inverters don’t simply switch the flow of electricity back and forth; within each same-direction flow of the electricity they produce a series of pulses of different length. 

The electrical resonant filters merge and smooth these pulses together into a near-perfect sine-waveform.

Transforming the Voltage

DC Electricity Into AC Electricity

As well as changing the DC electricity into AC electricity with the right waveform, the inverter must change the low voltage from the storage battery bank to mains voltage.  This can be done simply by using a device called a transformer to change the low voltage AC electricity to mains-voltage electricity.  The arrangement looks like:

The transformer that is required to change low voltage AC electricity that has a mains frequency of 50 Hertz, to mains-voltage electricity is bulky and heavy.

Two Inverters in One:

A transformer that operates at a higher frequency is compact and lighter. To avoid the need for a bulky and heavy transformer some inverters convert the low-voltage DC electricity to high-frequency AC electricity, and then use a compact transformer to increase the voltage. The high-frequency, high-voltage AC electricity is then converted to high-voltage DC electricity using a device called a rectifier. The high voltage DC is then inverted into mains-voltage AC electricity.  
The arrangement looks like this:

These inverters are actually two inverters in one; however, the complexity is justified because it avoids the need to use the bulky, heavy, and expensive 50 Hertz transformer.