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Samsung's own Fast Charge Wireless Charging Stand supports up to 7.5 watts of charging through Samsung's branded Fast Charge Wireless Charging technology. It also conveniently supports the Qi Low Power (5 watts) wireless charging standard for compatible phones such as the LG G6 (see review here).

The Samsung Fast Charge Wireless Charging Stand comes with a 15W based Samsung Adaptive Fast Charging (AFC) Charger that connects an AC outlet to provide power to the wireless charging stand. When we connect a more powerful Samsung AFC enabled charger like the Samsung 25W USB-C Fast Charging Wall Charger to the Samsung Fast Charge Wireless Charging Stand, we find out how much power the Samsung Galaxy S8 phone can draw when being charged on the wireless charging stand, by observing the power trace on the GRL-USB-PD-A1 analyzer from Granite River Labs.

The Samsung 25W USB-C Fast Charging Wall Charger claims to output 25 watts (12 volts @ 2.1 amps), 15 watts (9 volts @ 1.67 amps), and 10 watts (5 volts @ 2 amps) over its USB Type-A port which we connect to the Samsung Fast Charge Wireless Charging Stand. Once we place the Samsung Galaxy S8 phone with its battery fully drained on top of the Samsung Fast Charge Wireless Charging Stand, the phone starts charging by pulling quick current at around 5.1 volts in the first 60 seconds while switched off. When the voltage goes up to about 8.9 volts, power is drawn at close to about 9.8 watts (8.9 volts @ 1.1 amps) over a current band of 1-1.2 amps. As the phone boots up, it consumes higher power to charge at an average 11.7 watts (9 volts @ 1.3 amps) in between 1.2-1.4 amps current. 

From this observation, the Samsung Galaxy S8 can achieve near to 12W charging (assuming the wireless charging stand also consumes power to charge itself) with the Samsung Fast Charge Wireless Charging Stand using the Samsung 25W USB-C Fast Charging Wall Charger.  This allows the phone to charge much faster than the 7.5W offered by the Fast Charge Wireless Charging Stand connected to the 15W Samsung AFC charger. In fact, >11 watts is also slightly higher than what the phone was observed to pull when directly connected to its own charger.