Authors' recommended model is Model 4 (mixed fixed and random coefficients), which they state performs best || 'The recommended model controls for regional differences assuming that the coefficients of regional-seasonal specific factors are fixed and different while the coefficients of economic and weather variables are random draws from a common population' (abstract, p.565); 'The mixed fixed and random coefficients model (Model 4) performs the best' (p.579).
Hsiao, C., D. C. Mountain, M.W. L. Chan, and K. Y. Tsui (1989): “Modelling Ontario Regional Electricity System Demand Using A Mixed Fixed and Random Coefficients Approach.” Regional Science and Urban Economics 19(4), 565-587