There is frequent discussion regarding the USB power needed to run the Raspberry PI.
I bought ‘a few dozen’ of these USB Charge Doctors (see vid link below) from eBay for under $2 each shipped free from China.
The Charge Doctor gives you Voltage and Current readings.
FWIW, my RasPI Model B with it’s $10 SDR, RasPI camera, and a pretty decent CPU load (it runs more that just PiAware) draws ~700mA.
These little Charge Doctors are pretty handy to have around and you will quickly discover that not all USB chargers are created equal.
14 sec vid at: http://www.youtube.com/watch?v=kGzVcvNhEHI