This is inexpensive inverter circuit which able to convert 12V DC become 220V AC. IC CD4047 is use to generate the square wave of 50hz and amplify the current and then amplify the voltage by using the step transformer.
How to calculate transformer rating
The basic formula is P=VI and between input output of the transformer we have Power input = Power output
For example if we want a 220W output at 220V then we need 1A at the output. Then at the input we must have at least 18.3V at 12V because: 12V*18.3 = 220v*1
So you have to wind the step up transformer 12v to 220v but input winding must be capable to bear 20A.