In this paper, a waste heat recovery (WHR) system is designed to recover heat from the exhaust of a diesel-gen-set having an engine of 26.57 kW. The Rankine Cycle (RC) and the Organic Rankine Cycle (ORC) are used to produce additional power using water, R113, R124 and R245fa as the working fluids. Water as the working fluid gives the best improvement of 13.8% power improvement with 12.2% bsfc reduction, but fails to produce any power at the lowest operating power of 5.8 kW due to lower exhaust temperature and higher boiling point of water. This is when the WHR system is designed at the rated power of 26.57 kW. Designing at lower power of 20.0 kW improves the enhancements at this and lower powers but reduces the improvement at the rated power of 26.57 kW. This design again fails to produce any power at the lowest power. On the other hand, R113, R124 and R245fa which have much lower boiling points manage to produce additional power at the lowest power, but the improvements at other powers are lower than those produced by water as the working fluid. At the rated power of 26.57 kW, the power improvements are 7.1%, 4.6% and 10.6% with corresponding bsfc reductions of 6.6%, 4.6% and 9.6% with R113, R124 and R245fa, respectively. These lower improvements are due to lower pressure ratios at the expander which are 76, 6, 4 and 9 for water, R113 R124, and R245fa, respectively at the rated power.