The energy efficiency is one of the key metrics of a vanadium redox flow battery (VRFB), reflecting its ability to perform specific tasks for grid support. Due to ambient temperature fluctuations, this parameter can dramatically change, narrowing the applicability of VRFB. This study presents a comprehensive analysis of VRFB efficiency during prolonged operation under different ambient temperatures and realistic operating conditions. For this, the non-isothermal zero-dimensional model is used, which is validated against experimental data from a kW-scale setup, demonstrating high accuracy with voltage and temperature calculation errors below 2 % and 3 %, respectively. Battery efficiency and energy are systematically investigated across a wide range of load current densities (40–120 mA/cm2), pump power levels (20–80 W) and ambient temperatures (5–25 °C). The results reveal that the energy efficiency generally decreases with increased pump power due to the high influence of energy losses, while the absolute discharge energy can increase as a result of enhanced electrolyte flow rate. In addition, the multi-objective optimization analysis of the VRFB system is performed, highlighting the trade-offs between capacity, power, and energy efficiency. Specifically, high system power can be achieved at low temperatures due to better ambient cooling; however, it requires more power from the pumps due to the higher viscosity of the electrolyte. Conversely, high capacity and efficiency are achieved at room temperature, but the power can be limited to avoid overheating and ohmic losses. Using the Pareto front analysis, the balanced optimal point is found at 1.49 kWh capacity, 2.93 kW power, and 69.9 % energy efficiency, showing the compromise between all three parameters. The presented method serves as a tool for informed decision-making, based on the specific context and objectives. The findings offer practical insights for optimal operating conditions of industrial-scale VRFB systems, ensuring their efficient and reliable operation across a wide range of ambient temperatures.