We are hearing of a number of reports that the STAMINA power saving mode found in the Xperia Z does not appear to be functioning properly with the latest 10.1A.1.434 firmware. Switching STAMINA mode on/off does not make a difference with the estimated standby time i.e. there does not seem to be any battery saving when switching STAMINA mode on.
We have experienced the same with our resident Xperia Z, so we hope a fix is incoming from Sony. Clearing the data from the ‘Eco mode controller’ in Settings > Apps does seem to rectify the situation, albeit temporarily. Further details can be found on Danish site AppsAndroid.dk.