From Boeing's website
The 787-8 airplane utilizes two forward ground power receptacles and one mid-aft ground power receptacle. Receptacle ground heights are minimum 81 inches (206 centimeters) and maximum 108 inches (274 centimeters). Each receptacle is rated at 90 kilovolt amperes (KVA), the same as other Boeing twin-aisle production airplanes.
Similar to existing airplanes, the 787 utilizes power from the auxiliary power unit (APU) for engine start. The 787 is different in that it uses electrical power for engine start rather than the pneumatic power used on existing airplanes. If the APU is inoperative, an engine start can be performed using a minimum of two 90 KVA external ground power units (GPUs). Boeing recommends the use of three 90 KVA ground power sources to decrease engine start times and minimize ramp impact during ground operations. Same as the 777 and other twin-aisle airplanes, the ground power requirements must conform to the electrical power quality requirements specified in figure 5.
Why unusual to put GP on LHS? GP often comes from the jetway after all.