Hi, we are all suffering the same issue which is caused by background cellular activity. Throughout the user iPhone community (and all over the wider internet) the remedies proposed by Apple of restart, reset, restore from backup, restore as new device seem not to work.
In my case, I updated to 8.0.2 Sunday, and have been burning through a full charge in under 4 hours, this despite the fact that I have turned off cellular data for virtually all apps other than mail, phone, whatsapp and messaging. I do not expects the smartphone battery to last for ever, it would be nice to get through the day.
Having done some further research I have however noticed that 30% of total battery power over the past 24 hours has been used by Calendar and 27% by Contacts (both of which are linked to either my iCloud account or 2 Exchange accounts). This is about double the average of the last 7 days (about 15% each) and I suspect explains some of the problem.
Apple? any ideas? Is 8.0.3 on its way, hopefully?