The number that sets the time is the input, not the output
Every power bank carries two power figures, and they do different jobs. The output wattage, the one printed large on the box, is how fast the bank can charge your phone or laptop. The input wattage is the one that matters when you plug the bank in to refill it: it is the most power the bank will accept, and nothing you connect can push past it. Anker's own guide to charging a portable charger sets out the range, noting you "might see something like 5V 2A (10W) on older models, while newer USB-C PD power banks can support higher levels like 15V 3A (45W) or even 20V 5A (100W)." A bank that accepts 100W refills in a fraction of the time one capped at 10W needs, even when the two hold the same charge.
The wall charger then has to meet that input to make any use of it. Anker's advice is that "the easiest way to charge faster is to use a more powerful wall adapter," and that if the bank supports fast input you should "pair it with a charger that matches or exceeds its rated wattage." Feed a high-input bank from an old 5W phone plug and it crawls; feed it from a matching USB-C Power Delivery charger and it wakes up. The cable counts too: Anker warns that "even if your charger supports fast input, a weak or low-rating cable can hold things back."
What that looks like in real recharge times
Makers publish their own figures, and they show how wide the gap is. In Anker's table, a 10,000mAh bank takes 4 to 6 hours on a standard charger but 2 to 4 hours on a fast one, and a 20,000mAh bank takes 8 to 10 hours on a standard charger against 4 to 6 hours on a fast one. Same capacity each time, a very different wait, decided by what you plug it into.
At the top end the contrast is sharper still. Anker rates its Prime 27K power bank to reach "a full recharge in under 43 minutes" when it draws "a maximum of 140W input via the USB-C ports." That is a large bank refilled faster than a small one on a slow plug, purely because its input rating is high and the charger behind it can deliver.
How to recharge yours faster
Start by finding the input figure on the bank itself or its product listing, usually written as a voltage and current such as 5V 2A, or as a plain wattage. Match it with a USB-C Power Delivery charger rated at least that high, and run it through a cable rated to carry that power rather than a thin freebie. Those three things, the bank's input, the charger and the cable, are the whole story.
If a fast refill is high on your list, our 20000mAh power bank guide compares packs by their USB-C wattage. And if your real question runs the other way, how many phone top-ups you get from a full bank rather than how long it takes to fill, that is answered in how many charges you get from a 20000mAh power bank.
FAQ
Q: Does a more powerful charger always recharge a power bank faster? A: Only up to the bank's input rating. That rating is the ceiling on how much power the bank will draw, so a 100W charger cannot refill a bank that tops out at 20W input any faster than a 20W charger can; the spare wattage just goes unused. Matching the charger to the input is the goal, not overshooting it.
Q: Does a bigger power bank always take longer to charge? A: Not necessarily, because capacity is only half the picture. A large bank built to accept a high-watt input can refill faster than a small one that only takes a slow trickle, so size alone does not tell you the recharge time.
Q: Can I use a power bank while it is recharging? A: Sometimes. Charging the bank and a device at the same time is called pass-through, and not every power bank supports it, so a bank that does will say so in its specifications. If it is a feature you rely on, check for the word pass-through before you buy.
Q: Why is my power bank charging so slowly? A: Almost always the plug or the cable rather than the bank. An old 5W phone charger, a charger without USB-C Power Delivery, or a thin low-rated cable will each hold the input well below what the bank could take. Swap in a Power Delivery charger that meets the bank's input rating, with a cable rated for that power, and the time usually drops sharply.