We are often asked to make market forecasts, so we decided to go back to some of our older forecasts and see how well we fared. We found that most forecasts had a fair 15-20% deviation from the actual figures (Exhibit 1). There were however outliers: on the one hand, we overestimated the global installed capacity of wave energy by an order of magnitude, while on the other hand we predicted the evolution of wind energy costs inside a 5% margin. In any case, the real value of forecasting is what you learn in the process, as it forces you to understand and quantify the forces that shape a particular market. In this article we share four lessons learned while making market predictions and forecasts.