Unlike most USB powered loudspeakers, the Logitech Z205 is a single unit, fed by a single cable. Rather than using USB-only for power, it acts as a USB audio device, so the sound remains in the digital domain all the way to the unit's amplifier, and perhaps beyond, as we'll get to in a moment.
It is very portable, a lightweight unit that clips onto the top of your notebook's screen. At 180mm wide, it doesn't use much space either.
Logitech says nothing about the internal workings of this unit. You can see that there are two little speakers in it with what look like metal inverted domes 25mm in diameter. But power? Not mentioned. Claimed frequency response? Nothing.
For what it's worth, we'd guess that Logitech is using a so-called digital amplifier. Our 1kHz test signals revealed lots of noise spikes on odd harmonics all the way up to 19kHz, which tends to be diagnostic of digital amplification. That's not a complaint. The noise spikes are at such low levels (the worst is -40dB, or less than 1 per cent) as to be inaudible, and digital amplifiers are highly efficient.