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Facing some strange problems with my inverter: can anyone help?

I have bought an inverter last Sunday only and the inverter is not working properly. Something strange is happening. Like when there is no electricity all the lights that are connected with inverter runs well but all the fans which are connected with inverter stops working. I have called my neighbor also to show this but even he is not able to understand this. Can anyone tell me what the problem is what should I do?