Power output (wattage): The higher the wattage, the faster—especially if your device supports USB-Power Delivery (PD) or Quick Charge (QC).
Ports: Dual ports (USB-A + USB-C) are helpful so you can charge two devices.
Compatibility with fast-charge standards: PD, QC 3.0, maybe PPS if your phone supports it.
Input range: 12-24V is good (many cars use ~12V, trucks etc ~24V).
Safety features: Over-current, over-temperature, surge protection.
Build quality / materials: Metal shell helps heat dissipation, compact design helps fit in tight sockets.