How powerful of a power bank do you need to safely charge a laptop?

Laptop power banks should be judged by watt-hours (Wh) and output watts (W), not milliamp-hours (mAh). A 10,000 mAh bank holds ~37 Wh; a 20,000 mAh bank holds ~74 Wh. A 70 Wh laptop battery drawing 14 W gains ~3.7 h from a 20,000 mAh, 45 W USB-C PD bank after 30% loss. The bank’s per-port output and USB-C PD rating determine if it can charge the laptop without draining its internal battery.