Authors explicitly prefer the one-step GMM estimates in Table 1 (residential) and Table 2 (industrial) over OLS and Within; abstract and conclusion say GMM 'gives more intuitive results in terms of sign and magnitude' || 'We find that compared to conventional OLS and Within estimator, the one-step GMM estimator gives more intuitive results in terms of sign and magnitude.' (Abstract, p.1); 'Compared with the OLS and the Within estimator, the one-step GMM estimator gives more intuitive results in terms of sign and magnitude.' (Conclusion, p.16)
Liu, G. (2005): “Estimating Energy Demand Elasticities for OECD Countries: A Dynamic Panel Data Approach.” Discussion Paper 373/2004, Research Department, Statistics Norway