Electricity prices that describe marginal costs can vary substantially over time. Fixed rates ignore changing electricity system conditions. Setting prices that differ for certain periods is an approach to approximating the real-time price. If such time-of-use prices are set in advance, they will necessarily miss the full variability of real real-time prices. A simple index indicates that even very good time-of-use rates would miss the majority of the efficiency gain that would result with use of actual real-time prices.