The connection of renewable energy sources to local low-voltage networks is becoming more accepted as electrical power networks progress to higher renewable penetration. Renewable energy resources, for example, wind and solar are highly dynamic and intermittent compared with more traditional generation sources, which imposes increasing challenges to the electrical network operator in terms of effectively managing the resource to maximise energy transfer and maintaining system stability. Therefore, transient energy storage systems (TESSs), for example, electrochemical batteries with fast charging/discharging capabilities are suitable candidates to improve the availability and reliability of connected renewable systems. In this study, two potential TESS technologies are presented, the lithium-ion (Li-ion) and sodium–nickel chloride (NaNiCl2) battery, and their feasibility to improve power systems in terms of power delivery and frequency fluctuations are compared. Experimentally validated battery models are presented and used to investigate the TESS performance in terms of state-of-charge, terminal voltage variation, peak current, power, energy and efficiency. The models and general design procedure may be applied to systems of different ratings and duty variations.