While it's kind of neat that you can charge them via USB, the transmitters on similar units can last a fairly long while on regular batteries or rechargeable battery packs, and receivers are typically plugged into an outlet along with the amp that they're feeding.
What would be even better is if the transmitter unit that's plugged into the guitar could be self-charging. It's actually not very far-fetched, as the passive pickups in a typical electric guitar use alnico or ceramic magnets to generate current, which is then sent through a cable and amplified by the guitar amp. The pickups create a magnetic field that generates electromagnetic flux when the steel strings moving through the magnetic strings are moved, or rather, played. I imagine some of the power that's generated could be repurposed to instead recharge the transmitting unit, and the receiver unit (which should be plugged into a wall outlet by rights anyway) could be used to add gain to the signal to make up for the power lost powering the transmitter.