The effect of methanol to oil molar ratio and catalyst concentration (BaO and KOH) on waste biodiesel yield using microwave method. Methanol-to-oil ratios of 6:1, 9:1, 12:1, and 15:1 were tested for both catalysts. In this study, for homogeneous catalysts (KOH), a molar ratio of 9:1 and 2% catalyst were found to be effective with maximum biodiesel yield of 92% (Figure 4). For BaO, a maximum biodiesel yield of 96% was obtained for 12:1 methanol to oil molar ratio and 2% catalyst concentration (Figure 5). It was observed for homogeneous catalyst that when the amount of methanol-to-oil molar ratio was increased over 9:1, excess methanol started to interfere in the separation of glycerin due to an increase in the solubility and resulted in lower biodiesel yield [27].In combine effect of excess alcohol and KOH catalyst leads to saponification resulting in lower biodiesel yield and lower biodiesel quality. An advantage associated with heterogeneous catalysts is that they can be recovered and reused several times. For homogeneous catalysts, the biodiesel separation and catalyst recovery from reaction mixture can be laborious. The reactivity of BaO catalyst was found to be quite different than KOH catalyst as it posses different catalytic activity, basicity, leaching tendency, and specific surface area, which influence the transesterification of oil [28].Energy requirements for the two heating methods (microwave and conventional) for waste cooking oil transesterification are presented in Table 1. It can be noted that that 6 min were sufficient for microwave heating while 105 min were required for conventional heating to achieve comparable biodiesel yields. This large difference in reaction time can be attributed to the limitations of conventional heating in which the energy is first utilized to increase the temperature of the reaction vessel and the higher temperature of the reaction vessel results in higher heat losses to the ambient. The energy required by the conventional method is found to be around 11 times greater than that by the microwave method to achieve same biodiesel yield for waste cooking oil. From our previous study for camelina oil transesterification, it was concluded that proper power dissipation control resulted in effective use of the microwave energy and further reduction in energy requirements [23].