12v.. but bear in mind that the voltage regulators may be very hot if you use 12v

I've got several accu packs

6.0 volt 0.8 ah (small wall-e) can move and drive for 20 - 30 minuts 7.2 volt 2.2 ah (wall-e allmost done) can move and drive for over an hour 7.4 volt 6.6 ah still not empty ( counting 1 hour and 40 minutes already )
the last accu i bought in a small shop for 40 Euro.

and at least the accu holder with the 5 batts can hold also for about 20 minutes

sets are :

2 modified servos 2 servos for the hands 2 servos for the head 1 servos for the scanner 2 ir sharp sensors 1 audio board 5 leds for eyes and solar charge indicator 1 konig wireless cam

when the 2 wall-e's are complete i will show them

I got this from the manual Specifications 3.0 Input Voltage (min) 5 Volts DC Input Voltage (max) 14 Volts DC