In this paper, a model is develped for determining the probability distribution of the output of a digital ader in case of 2-, 3- and 4-level quantization before summation. This probability distribution is then used to determine the efficiency of a system which determines the total power of the signal at the output of the digital adder. The sumsignal is also used in a VLBI correlation mode where the sumsignal is correlated with the signal from a VLBI partner. The effeciency, expressed in so-called Degradation Factors, is determined for 1- and 2-bit correlation and 2-, 3- and 4-level quantization before summation. Furthermore, the application of the calculated values (Total Power Detector Efficiencies and Degradation Factors) is illustrated.