IEA TCP WIND Task 25 ”Design and Operation of Energy Systems with Large Amounts of Variable Generation” compiled Recommended Practices for power system impact studies, traditionally called wind and solar integration studies. This provides research institutes, consultants, and system operators with the best available information on how to model and study power and energy systems with large amounts of wind and solar energy. The Recommendations are presented as checklists that are also useful in benchmarking any integration studies: what has and has not been taken into account. The Recommended Practices contain the following sections: Input data; Scenario set up; Adequacy; Operational impacts; Dynamics; Analysing and presenting the results. Each section highlights the issues with the main recommendations, for small/medium wind and solar shares in the power system and also for wind and solar dominated systems. In this paper, Recommended Practices checklists are compared with modelling practises applied in a study of electrification of the transport and industry sectors in Sweden. Key challenges and trade-offs when performing studies of energy systems dominated by wind and solar power are presented together with suggestions on how the modelling community could facilitate the implementation of high quality studies of systems with high shares of wind and solar power.