1. A decision method considering time and power consumption for offloading computations, which is used in a computing system, wherein the computing system comprises a plurality of computing units and is coupled to at least a server, the decision method further comprises: obtaining a computing mission; obtaining a plurality of static effect factors from an effect factor table of the computing system; collecting a plurality of dynamic effect factors; generating a power consumption and a computing time of each of the computing units and each of the servers according to the dynamic effect factors and the static effect factors, wherein the power consumption and the computing time are what each of the computing units and each of the servers consumes when executing the computing mission; calculating a cost value of each of the computing units and each of the servers executing the computing mission according to the power consumption and the computing time; determining a target unit to execute the computing mission according to the cost values, wherein the target unit is either one of the computing units or one of the servers; and transmitting the computing mission to the target unit in which the computing mission is executed.