The price volatility exhibited by wholesale electricity markets has stymied the movement to restructure the industry, and may derail it altogether. Market designers argue that prices are superior to regulation for directing long-term investments to the proper location and function, and that price volatility is a natural manifestation of a robustly competitive market. However, episodes of prices that soar to previously unimaginable heights try customers' patience and cause policy makers to reconsider if the prize is worth the consequences.

Utilizing load, price, and survey data for 119 large customers that paid competitively determined hourly electricity prices announced the previous day between 2000 and 2004, this study provides insight into the factors that determine the intensity of price response. Peak and off-peak electricity can be: perfect complements, substitutes, or substitutes where high peak prices cause temporary disconnection from the grid, as for some firms with on-site generation. The average elasticity of substitution is 0.11. Thirty percent of the customers use peak and off-peak electricity in fixed proportions. The 18 percent with elasticities greater than 0.10 provide 75 percent of the aggregate price response. In contrast to Industrial customers, Commercial/Retail and Government/Education customers are more price responsive on hot days and when the ratio of peak to off-peak prices is high. Price responsiveness is not substantially reduced when customers operate near peak usage. Diversity of customer circumstances and price response suggest dynamic pricing is suited for some, but not all customers.