Increasing energy efficiency in industry is crucial to reach the climate objectives in the European Union. Many policy instruments have been implemented to increase the speed of implementation of new efficient technology. But actual implementation is slow and even profitable technology is often not implemented. The Netherlands have a long history of energy efficiency policy in industry. Voluntary agreements have been implemented since 1990, among other instruments. The data gathered during these agreements offer a unique opportunity to analyse the way in which companies invest in energy efficiency techniques. This thesis describes how companies in the Netherlands annually implement multiple projects to improve efficiency. These projects have a significant role in the change in energy consumption, but their effect is smaller than the effect of other factors. Especially the impact of changes in production volume is important. Energy savings are often not the main reason for implementing these projects. Economics remain an important driving force, but this economic advantage does not origin from energy costs savings alone, but rather from the total of all advantages. A focus on payback period as a decisive criterion for investments without accounting for all benefits leads to suboptimal outcomes both for companies as for government agencies stimulate those investments. Our analysis has clearly shown that different methods to calculate efficiency do not show the same development of energy efficiency improvement and therefore should be presented as separate methods.