Regarding charging from PC USB sockets:
- The current limit for bog standard USB 2.0 is 500mA, but some systems allow this to be increased via an optional software utility (eg, Asus Ai Charger for Asus motherboards for "50% faster charging"). Or use USB 3.0 if you have it, rated at 900mA as standard. Even better are USB Power Delivery ports, providing 2A at 5V. So reasonably fast battery charging is possible from modern computers.
Mains adapters providing so-called USB power:
- Depends on the model, can be more than 500 mA, but not necessarily.
- Beware of fake and crap chargers, which can electrocute/explode/ignite.