Do Households Respond to the Marginal or Average Price of Piped Water Services?

Water utilities commonly use complex, nonlinear tariff structures to balance multiple tariff objectives. When these tariffs change, how will customers respond? Do customers respond to the marginal volumetric prices embedded in each block, or do they respond to an average price? Because empirical demand estimation relies heavily on the answer to this question, it has been discussed in the water, electricity, and tax literatures for over 50 years. To optimize water consumption in an economically rational way, consumers must have knowledge of the tariff structure and their consumption. The former is challenging because of nonlinear tariffs and inadequate tariff information provided on bills; the latter is challenging because consumption is observed only once and with a lag (at the end of the period of consumption). A large number of empirical studies show that, when asked, consumers have poor knowledge about tariff structures, marginal prices, and (often) their water consumption. Several studies since 2010 have used methods with cleaner causal identification, namely regression discontinuity approaches that exploit natural experiments across changes in kinks in the tariff structure, changes in utility service area borders, changes in billing periods, or a combination. Three studies found clear evidence that consumers respond to average volumetric price. Two studies found evidence that consumers react to marginal prices, although in both studies the change in price may have been especially salient. One study did not explicitly rule out an average price response. Only one study examined responsiveness to average total price, which includes the fixed, nonvolumetric component of the bill. There are five messages for water professionals. First, inattention to complex tariff schedules and marginal prices should not be confused with inattention to all prices: customers do react to changes in prices, and prices should remain an important tool for managing scarcity and increasing economic efficiency. Second, there is substantial evidence that most customers do not understand complex tariffs and likely do not respond to changes in marginal price. Third, most studies have failed to clearly distinguish between average total price and average volumetric price, highlighting the importance of fixed charges in consumer perception. Fourth, evidence as of late 2020 pointed toward consumers’ responding to average volumetric price, but it may be that this simply better approximates average total price than marginal or expected marginal prices; no studies have explicitly tested this. Finally, although information treatments can likely increase customers’ understanding of complex tariffs (and hence marginal price), it is likely a better use of resources to simplify tariffs and pair increased volumetric charges with enhanced customer assistance programs to help poor customers, rather than relying on increasing block tariffs.