The Raspberry Pi message boards indicate that the RPi runs at around 2.5 or so watts. To this I have to add the overhead of the USB hub, a wireless card, a webcam, and the Arduino. Due to some power supply issues, namely a wimpy regulator that is supplying power to all the smart stuff, I have not been able to measure actual current draw from the RPi yet but I was able to see that all the rest of the stuff was drawing about 200-230 mA.
Assuming that the 2.5 watts (or 500 mA) is accurate, and adding 240 mA for the USB hub and the stuff it powers, I will burn 750 mA or 3.75 watts at 5v. My battery pack for the smart stuff is the same as for propulsion so with old technology rechargeable batteries I would have between 600 and 1000 mAh at 7.2v or 4.3 to 7.2 watt hours. Assuming that I lose 20% of my power to translation from 7.2v to 5v that leaves me with from 3.5 to 5.8 watt hours. Burning 3.75 watts means from 0.9 to 1.5 hours of run time.
|Power for the 'Brain'|
Here is a spreadsheet that calculates this stuff.
If you are doing ANYTHING with an RPi you will need to worry about power. I can testify that it is VERY sensitive to how you feed it.