In future energy systems, storage technologies for electrical energy are considered to be a key component for increasing the share of renewable energy use. Pumped thermal energy storage technologies represent a promising approach to complement established storage technologies such as pumped-hydro power storages without their geological restrictions. Assuming an ideal, reversible and adiabatic energy conversion process, the stored electrical energy can be entirely recovered. However, the efficiency of real processes is limited by irreversibilities. These exergy losses can be compensated by the integration of low temperature heat. The exergetic efficiency can be further increased by using thermal energy provided during discharging. In this paper, a fully heat-integrated, subcritical PIES using butene as the working fluid is presented. The results of a detailed numerical simulation of the cycle regarding exergy losses during heat transfer, efficiencies of machinery and parasitic energy consumption are shown. A maximum net electrical power ratio between charging and discharging of 125% is obtained, while the maximum exergetic efficiency is 59%. The conducted numerical simulation includes pressure losses and pinch points, allowing for a more in-depth understanding and for a pre-optimization of the hydraulic design. (C) 2018 Elsevier Ltd. All rights reserved.