Faced with the challenges posed by fossil fuels and nuclear power, several European countries are implementing ambitious energy transition strategies. In Switzerland, the Energy Strategy 2050 is based on two complementary axes: reducing energy consumption and increasing the use of renewable energies. This requires efforts in all sectors, from electricity production to heating/cooling of buildings and mobility. Here, we illustrate the challenges of this transition with two concrete examples: the renovation of a building dating from the 1960s, and a plan for the whole of Geneva, which aims to cover more than 90% of its thermal needs with renewable energies by 2050.