On the Overo Waterstorm (GUM3703W), "VBACKUP" is connected to pin M14 on the TPS65950 (http://www.ti.com/lit/ds/symlink/tps65950.pdf). Table 5-22 provides the typical power consumption for the use case where no main power source is connected: 
Note that 3.2V is used as the typical backup battery supply voltage for this calculation (see page 19 of the datasheet):
Hope this helps!