Estimate how long a device takes to charge.
Charging time is stored energy divided by delivered power, but two corrections make the difference between a useful estimate and a wrong one. First, capacity must be converted from milliamp-hours to watt-hours using the cell's nominal voltage, because a charger is rated in watts. Second, lithium-ion charging is not linear: constant-current charging runs to roughly 80%, after which the charger switches to constant-voltage and current tapers sharply — modelled here at about 45% of the earlier rate. That taper is why the last 20% routinely takes as long as the first 50%.
Charging time
Capacity (Wh) = mAh / 1000 x volts; time = Wh needed / (charger watts x efficiency), split at 80% state of charge
Energy drawn
Wall energy (Wh) = capacity x charge added / efficiency
Because the charger leaves constant-current mode at roughly 80% and switches to constant voltage, letting current fall away to protect the cell. Charging from 80 to 100 commonly takes as long as 20 to 60 does, which is why fast-charge claims are almost always quoted to 50% or 80%.
Only up to the device's own negotiated limit. A phone that accepts 25 W charges no faster on a 100 W charger, because the two negotiate the rate. Enter the lower of the charger's rating and the device's accepted maximum.
85% is reasonable for a modern USB-C power delivery charger. Older or cheaper adapters run nearer 75%, and wireless charging is substantially worse at around 60% to 70% — the difference appears as heat.