Most transformers change one level of voltage to another, but the size of the transformer depends on the amount of overall power it must transfer. The formula P=IE, means power (P) equals current in amps (I) times volts (E). Since power equals volts times amps, some manufacturers, engineers or technicians use the term "volt amps" in place of "watts." For most practical purposes, the terms are interchangeable. To find the volt amp rating for a transformer for home appliances, you need to include the ratings of all the appliances the transformer will power.
Inspect each appliance to find out its power consumption. Each appliance should have a tag that tells at what voltage it operates on and how much current it draws.
Multiply volts times amps on each appliance. For example for a fax machine that draws 1 amp at 120 volts the volt amps is 120 (120 x 1 = 120). For a TV set that draws 3 amps, the volt amps is 360 (120 x 3 = 360). For a washing machine that draws 7 amps, the volt amps is 840 (120 x 7 = 840).
Add up all the volt amp ratings of all the appliances that you will power with the transformer. In the above case, the total volt amps is 1,320 (120 + 360 + 840 = 1,320).
Select a transformer with a volt amp rating equal to or greater than the total.
To calculate the current on the input of your transformer, simply divide the total volt amps by the input voltage of the transformer. In the above example, for an input of 120 volts - such as with an isolation transformer - the input current would be 11 amps (1,320 / 120 = 11). But if for a transformer that steps 240 volts down to 120, the input current would be 5.5 amps (1,310 / 240 = 5.5).
Using a transformer with a volt amp rating lower than the calculated sum will cause the transformer to overheat and eventually burn out. Make sure your transformer's input and output voltages match your application.