How the charging time calculator works
The taper curve
Lithium charging runs constant-current (fast, to ~80%) then constant-voltage (slow, topping off) — the last 20% takes as long as the first 80%. Marketing quotes the ideal math (Wh ÷ W); users experience the taper. The calculator's 25–30% penalty models it, and the 80% line is the planning number that matters.
Wattage: input vs output, again
A bank's input wattage recharges the bank itself (30 W input = a 20,000 mAh bank in ~4 hrs; 65 W input = ~2); its output wattage charges your devices. They are separate specs, often asymmetric (60 W out, 18 W in — annoying). Check the INPUT line when recharge time matters: travel days are recharge days.
The cable bottleneck
Above 60 W, USB-C requires electronically-marked (e-marked) cables rated for the current; cheap cables either negotiate down or warm up worryingly. A "slow charger" diagnosis should always start with the cable — the $10 e-marked replacement fixes more "broken fast charging" than any setting.