Single electron technology (SET) is one of the future technologies distinguished by its small and low-power devices. SET also provides simple and elegant solutions for threshold-logic gates (TLGs). This paper presents the design of an optimal TLG adder implemented in SET. This 16-bit Kogge-Stone style adder was fully designed and simulated using a Monte Carlo simulator. The simulation results give a quantitative estimate of both the delay and the power dissipation of the adder. The characteristics of our adder are compared with recent results estimating the energy-delay characteristics of advanced CMOS adders.