The block erupters consume about 500 ma or .5 Amps of power (btw thats about 2.5 watts). That is the maximum amount of power that a single usb port can supply. So you need a power adapter for anything more than one erupter on a single hub. And its simple addition at that point. If you have 2 eruptors you need a 1 watt power adaptor, 3 erupters you need a 1.5 amp power adapter, etc.