The Apple 15" MacBook Pro (2018) can still get power supplied by other chargers like the AT&T 18W Type-C Wall Charger just in case the original Apple 87W charger is out of reach. The AT&T charger supports up to 18 watts of charging using USB Power Delivery over USB Type-C so let's see how it works with the Mac.
We first attached the AT&T 18W Type-C Wall 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. Note the Mac's battery was fully drained at plugin. At about 5 volts startup voltage, the Mac did not seem to pull any much current to charge properly with the AT&T charger. After the voltage stepped up close to 12 volts, the Mac only started to pull more current to initially charge at about 14.1 watts (11.8 volts @ 1.2 amps). In the first minute of charging the current was rapidly changing which even dropped to zero at certain times. During this time the Mac mostly drew power of around 9.4 watts (11.8 volts @ 0.8 amps) to 14.1 watts (11.8 volts @ 1.2 amps) from the charger. When the battery has charged for a bit the current dropped down to about 0.4 amps resulting in only around 4.7 watts (11.9 volts @ 0.4 amps) charging. After some time the current ramped up to allow the Mac to achieve around 16.4 watts (11.7 volts @ 1.4 amps) of steady state charging with the charger. This charging is only recommended when the Mac is off and you need to charge it overnight. View more details here by downloading the trace captured by the GRL-A1 power analyzer.