Im using APC Back-UPS RS 550 ups. So will it charge if I only plug ups to wall socket with ups power off, or should I turn ups power on?
It doesn't matter whether the UPS is on or off. You can use your computer while its charging.
No not like that. If I want to charge my ups should I turn on the ups or it ll charge even its off?
You just have to put its cable in the main socket. It doesn't matter whether the switch in front of your UPS is on or off. It will always charge (or not charge if battery is full) when the main supply is on.