The medium-altitude long-endurance (MALE) unmanned aerial vehicle (UAV) plays a key role in both military and civilian applications around the world. However, in a world where electric UAVs are increasingly becoming the norm, the MALE UAV is still primarily powered by hydrocarbon fuels and its design is very much constrained by existing general aviation reciprocating engines. The aim of this study is to investigate the design effects resulting from incorporating electric propulsion systems in MALE UAVs, while also investigating the effects of modelling fidelity on the results. Reciprocating engine, hydrogen fuel cell and battery models are incorporated into a multidisciplinary design optimisation framework, with the aim of minimising the maximum take-off weight at various levels of modelling fidelity. The results show that hydrogen fuel cells can provide around 40% reduction in the aircraft's maximum take-off weight, while batteries are not capable of enabling long-endurance flight. It is further shown that the effects of modelling fidelity of the aerodynamics and structural wing weight models are more significant when the aircraft design being optimised is substantially different from the baseline configuration.