Chargers and power banks are sold on a jumble of numbers — mAh, watts, “PD”, “GaN” — that sound technical but rarely get explained. Understanding four of them is enough to stop overpaying, stop charging slowly, and stop being surprised when a “10,000 mAh” bank doesn’t fully charge your phone three times.

mAh measures a tank, not a delivery

Milliamp-hours (mAh) describe capacity — how much charge a battery holds. But the figure on a power bank is measured at the internal cell’s low voltage, while your phone charges at a higher one. Converting between them, plus normal energy loss as heat, means a 10,000 mAh bank delivers noticeably less than 10,000 mAh to your device — often around 60–70% of the headline number in practice. That’s not a defect; it’s physics. If you want a truer measure, look for the watt-hour (Wh) rating, which accounts for voltage.

Watts measure speed

Power, in watts, is what determines how fast a device charges. Watts are simply voltage multiplied by current, which is why fast chargers raise the voltage to push more power. A 20-watt charger fills a phone far quicker than a 5-watt one. When you see a big charging-speed claim, it’s a wattage claim — and your device has its own maximum it will accept, so a 100-watt charger won’t force-feed a phone that caps at 27.

USB Power Delivery is the handshake

USB Power Delivery (PD) is the standard that lets a charger and device negotiate a safe, higher power level instead of defaulting to a slow trickle. Without PD (or a compatible fast-charge standard the device recognises), you’re stuck at baseline speeds no matter how beefy the charger is. This is why a laptop-class charger and a cheap cable can still charge your phone slowly: the fast handshake never completes. For fast charging, the charger, the cable, and the device all need to speak the same standard.

GaN is about size and heat, not capacity

GaN — gallium nitride — is the semiconductor material inside newer chargers. It runs cooler and more efficiently than traditional silicon, which lets manufacturers make chargers dramatically smaller and lighter for the same wattage. A GaN charger doesn’t hold more energy or inherently charge faster than a same-wattage silicon one; it just does it in a brick a fraction of the size. For travel and multi-port desk chargers, that’s a real, tangible benefit.

Buying sensibly

Pick capacity for how long you’re away from an outlet (a 10,000 mAh bank is roughly one to two phone charges in reality). Pick wattage for the fastest device you’ll charge — and check your device’s own maximum so you don’t overpay for headroom it can’t use. Insist on USB Power Delivery for anything you want charged quickly, buy a cable rated to carry that power, and treat GaN as a nice-to-have when you value a smaller, cooler brick.