We propose a new model of a small open economy with efficient energy use to investigate the inflationary dynamics along the green transition. The model incorporates the production of green energy that substitutes exogenous brown energy sources in energy production. Production is characterized by low substitutability between resource and traditional inputs that firms can alter through directed input-saving technical change. We study transitional dynamics induced by exogenous increases of brown energy prices and/or changes in the brown energy taxation; green subsidies and green public investment. Increases in brown energy prices and taxes decrease the usage of brown energy but do not expand significantly the green sector, they simply improve energy efficiency use, surging firm’s marginal costs leading to greenflation and output losses. Public investment and subsidies effectively increase the usage of green energy. Green investment expands output and reduces green energy prices as it increases the productivity of the green sector. Subsidies imply a slower transition with small output costs and elevated green energy prices. We discuss the fiscal costs and welfare implications of the transition using different welfare metrics.