Authors' preferred estimation is state+year fixed effects with price treated as endogenous and instrumented (their fullest specifications, cols 7-8, are the ones plotted for headline building-code savings). || 'we instrument for price using predetermined lagged values of average price' (p.42); 'We will control for time invariant heterogeneity across states by including fixed effects in our preferred estimation' (p.42).
Aroonruengsawat, A., M. Auffhammer, A.H. Sanstad (2012): "The impact of state level building codes on residential electricity consumption." The Energy Journal, 33(1), 31-52.