Dynamic tariffs such as spot pricing are meaningful as indirect load management tools only if customers are sensitive to inter-temporal price variations. However, little attention has been paid so far to understanding the mechanisms of cross-time price elasticity of demand, inter-temporal definitions of customer utility, and the interaction of these two effects with the supply side factors of least cost system operation and dispatch. The importance of these interactions is enhanced in circumstances where competition between suppliers is envisaged or when it is desired to use a common spot price for several disparate customers. This paper develops conceptual and theoretical models for this purpose, describes computerized solution algorithms and provides simulation examples.