Forecasts play a central role in the development of energy infrastructure. Since building energy infrastructure is long and costly, energy system planners try to anticipate future demand to avoid both shortages and overcapacity. But energy demand forecasts aren’t neutral: they represent a certain vision of the future that forecasters hope to bring into being. This article uses a historical case study to open the black box of forecasting and the world it contains. It studies electricity demand forecasts made by Hydro-Québec, one of the biggest industrial firms in North America, from the 1960s to the 1980s. Based on linear extrapolation models forecasting exponential demand and endless growth, the state-owned firm embarked on huge hydroelectric megaprojects with deep consequences on the environment and Indigenous lands. The energy crisis of the 1970s, by disturbing energy systems, led to criticism from the provincial government and civil society towards Hydro-Québec’s bullish forecasts that justified its expansionist agenda. This uncertain context favored other methods of predicting the future, like scenario analysis, and brought scrutiny towards the hydroelectric powerhouse’s business. At the crossroads of business history, energy history, and science and technology studies, the article argues that energy forecasts are used by actors like energy suppliers and governments to produce and project power relations onto the future. They become performative when powerful interests coalesce around their vision of the future to implement it.