Volts x Ampere = VA (VoltAmpere)
Volts x Ampere x Power Factor = kW (Watts)
Power Factor = 0.8 (which is the max. power factor of home standard)
If 200W of appliances then,
200 / 0.8 = 250VA
So, prefer the inverter of 250VA or more. 300VA inverter will be the best choice
Similarly, battery is an integral part of the inverter. The performance of the inverter depends on the connected battery and vice-versa.
The battery backup time in hours can be calculated using following formula:
Backup Time = Battery AH x 12V x N x Efficiency of Battery / Load in Watts
Battery AH = Ampere Hour Capacity of Battery
N = Number of 12 V Batteries needed
Efficiency of Battery = Generally it is 0.8, which is the max. power factor of home standard
Note: The inverter capacity is not used in backup time calculation.
Suppose a 60A battery and load is 150W. With a single 12V battery.
Backup Time = ( 60 x 12 x 1 x 0.8 / 150 ) hours = 3.84 hours