The Apple 15" MacBook Pro (2018) came equipped with a Apple 87W charger supporting USB Power Delivery 2.0 fast charge technology over USB Type-C for the Mac's own power supply. Let's find out how the Mac draws power from a 27 watts based third party charger like the Softbank SELECTION USB PD USB Type-C Rapid Charging AC Adapter (2018) which is also USB certified and uses USB Power Delivery.
To observe charging, we attached the Softbank charger to a Thunderbolt 3 based USB-C connector on the Apple 15" MacBook Pro (2018) using the Mac's own supplied Apple USB-C Charge Cable (2m) accessory. At initial 5 volts no current was pulled by the Mac but after the voltage stepped up to around 11.7 volts only the Mac began pulling current up to about 3.1 amps. This gave the highest point of charging for the Mac at around 36.3 watts (11.7 volts @ 3.1 amps) although it happened very quickly. The current then started bouncing up and down continuously from about .5 amps to slightly above 2 amps. The Mac was also seen to frequently pull around 2-2.3 amps to max out charging in between 23.6 watts (11.8 volts @ 2 amps) and 27.1 watts (11.8 volts @ 2.3 amps) with the Softbank charger. The Mac uses aggressive power management schemes with the charger such that even while the charger was connected to the Mac, the current repeatedly went up close to 2-2.3 amps at repeated intervals. Click here to download the trace captured by the GRL-A1 power analyzer to see more details.
The Softbank SELECTION USB PD USB Type-C Rapid Charging AC Adapter (2018) was able to deliver decent charging to the Mac close to the max power level of the charger even though the charger exceeded its own power limit with the initial 36.3 watts. For a much faster charging at 20 volts, the Mac should still use its own shipped 87W charger as observed here.