No single elasticity is headlined; authors discuss all seven specifications and decline to single one out ('we shall discuss the results of all seven specifications, without singling out (7) for special emphasis', p.244). Specification (1), the simplest, is the one worked through as the numerical example (p.244-245), so its PK elasticity (-0.021) is the best proxy for the authors' illustrative estimate. || 'consider the estimate of our simplest model, specification (1)' (p.244)
Park, R. E., and J. P. Acton (1984): “Large business customer response to time-of-day electricity rates,” Journal of Econometrics 26(1–2), 229-252