A USB 3.0 Type A port on a PC is typically going to be about 4.5 Watts. If it's a dedicated charging port (rare). It could deliver 7.5 watts. Best I can find. Magsafe charging is about 75% efficient. So, you'll likely get ~3.4 watts from a typical USB 3.0 Type A port. A USB 2.0 port is going to be about 2.5 Watts (~1.9 Magsafe).
A USB Type-C port on a computer might deliver up to 15 Watts (~11.3 Magsafe). I doubt many laptops would offer 15 watts. Desktops are more likely to.