Jbeale reviewed how much current small USB WiFi routers draw. His intent is to connect them to the Raspberry Pi which can only provide 140mA over the USB port. It turns out that from Internet connectivity the routers draw under 100mA, but if used in high speed LAN operations the current draw jumps over R-Pi’s threshold.
Just compared two small USB-WiFi N adapters from ebay. Both devices connected easily from my WinXP laptop to my Linksys WRT160Nv2 router, reporting excellent signal and 72 Mbps link speed. It is interesting to see that both devices keep under the 100 mA limit while working the internet at my DSL speed. But going to a machine on the LAN, the speed goes up and the current draw goes up with it, above 150 mA which is more than the unmodified RPi USB port can reliably provide. So I’d expect either of these devices would be perfectly reliable for internet browsing used direct in the RPi without a supply, but UNreliable for heavy LAN use (eg. high-bitrate video streaming). …Unless you have faster internet than I do, in which case, web use might hit overcurrent also.
Via the forum.