I had a similar idea except i used a small 12v battery to power it (always with a 12v battery charger) and then i use the desktop computer's USB power supply to check for power out when it is..
the light will come on and buzzer goes off, i can also go to
www.myarduino.info (and view status directly).
But that RTC with SRAM is a really good option, i might get one soon to play with.