This paper presents an analytical approach to study the techno-economic sustainability of hybrid ESS (HESS) for grid ancillary services. As an example, Li-ion battery and supercapacitor (SC) based HESS is considered for the recently developed enhance frequency response (EFR) service to improve the UK grid stability. The design objective is to optimize the shared capacity of storage units that enables extending battery lifetime and maximize the economic benefits. A heuristic method for power sharing algorithm is also proposed to efficiently define the shared power of SC with its state-of-charge condition. Extensive simulation and case studies are carried out for a 10MW HESS solution. Studies include the impacts of battery degradation model, power management system and the HESS design capacity/sizing on the optimal share of SC capacity and annual profit estimation. Results show that 10% error in battery degradation model (over/under) can also lead up to 18% increase or 25% decrease in profit estimation. Optimal SC share of 7% can be possible with a maximum annual profile of 137.26%. Share can be reduced to 5–6% by 10% over-sized of HESS capacity, but the overall profit then reduces to 133.36%. This approach can also be implemented for other HESS solutions.