Transportation consumes approximately 29% of the world's energy and is responsible for approximately 25% of CO2 emissions worldwide. In Cuenca, Ecuador, fossil fuels for transportation represent 59.9% of total energy consumption. Ecuadorian legislation has specified gradually replacing traditional public bus transport units with electric units. Because the relevant costs include those associated with replacing equipment and improving route networks, alternatives that prioritize incorporating the first units on the best routes are being sought. This work establishes a methodological process with a set of tools to prioritize the gradual migration from conventional to electric buses according to the characteristics of each route. The Preference Ranking Organization Method for Enrichment of Evaluation (PROMETHEE) multicriteria technique is used to detect the best routes according to technical, social, environmental, economic and location criteria. A final rank of routes is obtained using Visual PROMETHEE software. After the values defining each criterion and its corresponding weight were processed, the final ranking makes it possible to prioritize one route before others. Given 23 lines and 475 bus units, the method enabled us to determine that in an initial phase, 68 electric buses are required to be used on five lines to reduce fuel consumption. The factors that most influence decision making are the number of passengers served, the length of the trip and the energy required by the route.