The paper reveals a comprehensive approach to economic efficiency assessment of large-scale investment projects in the energy sector. Authors consider scientific approaches to integrated assessment and substantiate its advantages when applying it to the assessment of effectiveness for the energy sector. At present, the accuracy of the estimates of the efficiency of investments in the energy sector is of particular importance in terms of the limited financial resources and the increasing capital intensity of deposit development. However, existing methods of determining investment efficiency for complex projects often tend to lead to incorrect results. The integrated assessment approach proposed in the article aims to overcome the shortcomings of existing approaches. Its peculiarity is to consider investment in a complex project as a single system consisting of subsystemsindividual projects. The whole set of projects is classified according to the nature of the mutual influence, which makes the assessment process much easier. The implementation of an integrated approach requires special care in the performance evaluation and is possible in cases of simultaneous implementation of the projects included in the complex, aimed at achieving the same effects and having synchronized time frames of entry. The positive effect of increasing the accuracy of the assessment and, as a consequence, of significantly reducing the risk of economically unjustified investments, far exceeds the cost of making the evaluation procedure more complex under the developed method.