"I need to power the system in case of power outage,I was thinking of a capacitor which could power the Uc till the data was logged . "

Instead of a capacitor you can have a look at the rechargeable batteries that they use in emergency lamps. The "National" make batteries are available from a company called Base. With a proper charging circuit these batteries provide you reliable backup for hours. No need to worry about discharge time of Capacitors or the power consumption of the data logging circuit etc.

Most members from Europe and US may not be aware of the power outages that happen here and may be thinking it is rare and only for a few seconds or minutes !

If you are scared about IIC chips for RTC, then look at the non- IIC variety. But there is nothing great about handling the IIC chips - once you know how ;-)

Raghu