This paper is about investigating the differences in fuel consumption of conventional and equivalent electrical subsystems of passenger aircraft. The goal is to develop a framework that can help evaluate fuel consumption of passenger aircraft, both conventional aircraft and More Electric Aircraft (MEA), for a given size of aircraft and a given stretch of flight. The work presented in this paper originates from pre-studies within MEA research performed by the Reliability Centered Asset Management (RCAM) research group at KTH in collaboration with SAAB that has been done on a passenger aircraft with comparable size to the Airbus A320. The main difference to the prior study is the addition of subsystem weight, passenger scaling effects, environmental dependencies and flight profile, all added to increase the accuracy and diversity of the model. The fuel consumption is based on studies of existing technology for several passenger aircraft from Airbus and Boeing. The main focus was the Environmental Control System (ECS). A numerical model of passenger aircraft including the ECS was constructed in MATLAB with different levels of electrification, a conventional ECS or a fully electric ECS. A special case with an Airbus A320 with 180 passengers doing a round trip between Copenhagen and Stockholm on a hot day was studied. The results show possible fuel savings in the magnitude of 4% to 8% when electrifying the ECS for the case studied and for aircraft with 156 to 700 passengers.