VERY Simply and basically, 1 Watt = 1 Volt x 1 Ampere. when a resistive load is used.
360 Watts at 120 V means it will draw a current of 3 Amperes. ( 120 Volts x 3 Amperes = 360 Watts)
If in India you assume 240 Volts as the supply, to deliver 360 Watts, means a current of 1.5 Amperes will be drawn.
In simple theory the 240 VAC to 120 VAC Step Down transformer will also need to be 360 Watts ( 1.5 amperes at 240 VAC).
However in practice:
1. The AVR will not present a purely resistive load. Hence will actually draw more ( probably 30% more ) current than 3 amperes at 120 Volts.
2. When you order a 360 VA Transformer in India, the supplier almost never delivers to specs. They will cut corners and its VERY difficult for a lay person to know if he has been short changed....
Keeping in view points 1 & 2 above, a 750 VA Transformer, Made in india will suffice to drive the AVR.
The Transformer will be :
750 VA ( Volt x Ampere)
Primary Winding: 250 VAC and 3 Amperes
Secondary Winding: 120 VAC and 6.25 Amperes.
Cheers