One way would be to write rules to disable/enable Output 1 non-alarm voice messages during night time hours. That will silence not only the power fail message, but also any others, such as low battery.
If it is only the AC Fail message that you want to eliminate, you can set that particular message to all blanks in RP2 (under Automation|Voice
System Troubles). That will silence it permanently, until you change it back. Unfortunately, you can't change it via rules.
Note that the keypad will still beep. If you want to silence the beeps, that can be done with separate rules.
