If any Inverter/UPS (both are same things) auto cuts the electric power supply when there is no load for a certain period of time is called a no-load shutdown. It’s only possible when the Inverter/UPS is on the battery mode or using power-saving mode.
This feature is useful in many cases but could be annoying sometimes.
When it is useful?
It is useful when I’m going out and not going to turn on any load, still, I want to charge my Inverter/UPS using either grid or solar panels.
When it is not useful?
No-load shutdown becomes annoying when you need a 24×7 electric power supply even when there is no need. But the problem is most Inverter/UPS are designed to save electric power for long backup when there is no load.
What is the ultimate fix?
So to fix the issue, either disable power-saving or eco mode or have a small load always running like a colored 3 to 9 LED bulb. We found mobile chargers and similar charging devices do not use more than a few milliamps; better avoid them as 24×7 load.