Performance and environmental aspects are important when designing ORC systems and choosing an appropriate working fluid. Thermal and thermodynamic aspects must be considered simultaneously, as shown by investigation of the fluids R245fa, R1233zd(E) and R1336mzz(Z) in small scale low temperature ORC systems. Performance in terms of net electricity production is very similar for the fluids, and the major difference is the high GWP of R245fa. Performance of components such as heat exchanger and turbine can be explained and predicted by thermal and thermodynamic properties. Even a small heat source temperature decrease of 30 °C results in a changed oversurfacing by 75 % and a 100 % increase in evaporator pressure drop. Further studies are required to analyze how ORC system design can be optimized from an overall perspective.