The introduction of modular design increases complexity within product development and especially within Requirements Engineering. To handle this complexity, the approach of Model Based Systems Engineering is applied in this contribution. First, the origins of complexity in modular product development are introduced. Key challenges within Requirements Engineering are drawn up and then verified. A modeling method for requirements of modular kits is introduced and validated in the series development of hybrid drivetrain systems at a German automotive manufacturer. Increases in quality and a requirements reusability of over 80% were achieved. Thus, the capability of the modeling method is verified.