Multi-unit combinatorial auction is a combinatorial auction that has some items that can bee seen as indistinguishable. Some approximation mechanisms have been proposed to be applied for a large-scale auction problem regardless of its computational intractability. Although theoretically such mechanisms could be applied for dynamic electricity auctions, there is little empirical analysis about how such a fast approximation mechanism can be applied to the actual electric power allocation problem when we consider the situation that there are sustainable electric power sources and the consumers have preferences for them. In this paper, we present a preliminary analysis about applying a fast approximation mechanism for multi-unit combinatorial auctions to an electric power allocation problem when it includes sustainable power sources. Also in this paper we discuss how such a mechanism can be implemented in a practical way.