It is not complete to focus on efficiency, and certainly not reversible energy efficiency. In one example, emphasizing minimizing overall energy efficiency leads one to conclude that building a photovoltaic device and an electrolyzer in a single unit is the correct approach. However, this raises the total cost significantly over what could be accomplished when the photovoltaic and the electrolyzer can be kept separate and each optimized separately. Then the PV can be run at its maximum power, and the electrolyzer can be run at a different temperature and at a significantly higher current density, at the optimum governed by a trade-off between capital and operating costs.
To bring a new product to market requires careful planning to insure that the product is useful and at an attractive price. Even more so when you are contemplating changing the whole energy infrastructure.
Perhaps cheaper than clean renewable energy like wind, geothermal, or solar would be to continue to burn fossil fuels but to capture and sequester the carbon dioxide produced and thereby mitigate climate change. It surprised me how much more attractive this method could be.