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, wherein the step of calculating the cost value of each of the computing units and each of the servers according to the power consumptions and the computing times comprises: calculating the cost value according to equations (1), (2) and (3), 
 ε T P =( {circumflex over (T)} p −{circumflex over (T)} cpu )/ T cpu (1) 
 ε E P =( Ê p −Ê cpu )/ Ê cpu (2) 
 C P =α·ε T P +(1−α)·ε E P (3) wherein cpu is a main computing unit of the computing units, p is a comparative computing unit, the comparative computing unit is one of the computing units or one of the servers, C P is the cost value of the comparative computing unit, {circumflex over (T)} p is the computing time that the comparative computing unit consumes when executing the computing mission, Ê p is the power consumption that the comparative computing unit consumes when executing the computing mission, {circumflex over (T)} cpu is the computing time that the main computing unit consumes when executing the computing mission, Ê cpu is the power consumption that the main computing unit consumes when executing the computing mission, α is a real number which is equal to or greater than 0 and less than or equal to 1; and setting the comparative computing unit with the smallest cost value as the target unit.