Hydrogen fuel cells and the economics of unmanned aerial vehicles (UAVs) are gaining global attention. With higher energy densities, fuel cells can overcome the range limitations of lithium battery-powered aircraft. This paper is to address two important issues often overlooked in research on fuel cell UAVs. It innovatively quantified the ranges within which fuel cells remain a better option and originally focused on the impact of altitudes on the power demand. A detailed literature review was conducted to determine the flight profile. Environment models, power demand models, and other models were established using MATLAB to analyse the impact of altitude on performance. Some parameters were obtained through fluid field simulations using Ansys. The study shows that using air-cooled fuel cells in a 3.5 kW UAV is a superior solution compared to lithium batteries when the energy demand exceeds 2.8 kWh. Furthermore, the power demand increases by 3.5 % per kilometre in altitude, and the maximum flight altitude is reduced by approximately 266 m per kilogram of take-off mass. It also emphasizes that solely increasing the air stoichiometric ratio cannot always enhance performance. Through sensitivity analysis, it was found that improving the stack's power density has the highest relative gain.