Fast charging and power banks: watts, mAh and USB-C PD explained
Why your charger's wattage matters, what mAh really means, and how to pick a power bank that actually charges your devices.
Chargers and power banks are covered in numbers. A few simple ideas explain almost all of them.
Watts: how fast
Power is measured in watts (W). A 20 W charger delivers more power per second than a 5 W one, so a phone charges faster — if the phone supports that speed. The phone always decides how much it takes, so a bigger charger will not harm it.
USB Power Delivery (USB-C PD)
USB-C PD is the common standard that lets a charger and device agree on voltage and current. One good PD charger can charge a phone, a tablet and many laptops. Some phone brands also use their own fast-charging systems; they reach full speed only with the matching charger and cable.
Cables matter
A cheap cable can limit speed. For laptop charging above 60 W, you need a cable rated for 100 W or more (often marked "5A" or with the wattage printed).
mAh: how much
Power bank capacity is shown in milliamp-hours (mAh). It is measured at the internal battery's voltage, and some energy is lost converting it for your device — so a 10,000 mAh power bank typically delivers roughly 60–70% of that to a phone. In practice, that is about one and a half to two charges for a typical phone.
Choosing a power bank
- Capacity: 10,000 mAh is pocketable for daily use; 20,000 mAh suits travel and tablets.
- Output: make sure its USB-C PD output matches your devices — 20 W for phones, 45–65 W or more for laptops.
- Ports: USB-C input and output lets you recharge it with your phone charger.
- Air travel: airlines generally allow power banks up to 100 Wh in hand luggage; check the watt-hour rating printed on it and your airline's rules.
Safety
Buy from reputable brands with certification marks, and avoid swollen, damaged or very hot power banks. Never leave a cheap power bank charging unattended overnight.