Hybrid-electric powertrains have shown the potential to reduce aviation climate impact. Since battery capacity is sized for a particular design mission, the emission reduction could be significant when operated at a payload-range combination below the design mission. However, this relation is sensitive to the design point, in particular the design power split ratio and design range. Furthermore, hybrid-electric powertrains would require airlines to adjust their operations. In this study, the interdependencies between hybrid-electric aircraft designs, their off-design performance, and the network's performance are evaluated. The effect of modifying the design range and the design power split ratio on the aircraft's off-design performance and network performance is evaluated. Several designs are constructed and several operational scenarios are generated. The Air Nostrum network is used as a case study. It is found that when the off-design performance of the hybrid-electric aircraft is considered in the fleet assignment and scheduling of an airline, CO2 savings equal to 15% can be attained while incurring a minimal loss in profit of 1.35%. This research highlights how modifying the design range of hybrid-electric aircraft has a larger impact on the applicability of the former in regional airline networks than the modification of the design power split ratio.