In the liberalised market, the wholesale hourly price of electricity will be defined in real time, giving a signal of the effective available resources in each moment of the year. Though most customers will not see this hourly variability on their bill, wholesale prices are likely to act as a benchmark and to influence the structure of retail agreements and retail tariffs. Moreover, the efficiency of the wholesale market will also depend on the aggregate demand elasticity, which is affected by retail pricing policies and by the effective willingness to shift consumption over time by the end user. This paper focuses on the measurement of final customer demand responsiveness, analysing monthly data on medium size Italian industrial consumers facing TOU pricing between 2000 and 2003. The econometric model employs a nested Constant Elasticity of Substitution (CES) input demand function, which allows estimating substitutability of electricity usage across different hourly intervals within a month and across different months. The results show that monthly substitutability is easier than hourly substitutability, and highlight a wide heterogeneity in customer response, suggesting that different pricing policies may be pursued across different industrial sectors.