If you are using your Z2 plugged into an electrical outlet with the Auto Off-On feature engaged (AOFF-ON) and you are also using the PowerShell and Battery, due to the possibility of an electrical outage, if the power does go out, the battery will engage.  When or if the electricity switches back on, your Z2 will automatically begin using the electricity again, because the auto feature is engaged.  

If you are using your Z2 plugged into an electrical outlet with the Auto Off-On feature turned off (AOFF-OFF) and you are also using the PowerShell and Battery, due to the possibility of an electrical outage, if the power does go out, the battery will engage.  When or if the electricity switches back on, your Z2 will not automatically begin using the electricity again.  Your battery will continue to power your Z2 as long as the battery charge allows.