It depends what you mean by “efficient”. You need a lot less by weight for the same energy output via oil or natural gas or coal, but hydrogen is a highly voluminous gas, so you need a lot more by volume to get the same effect. However, the hydrogen we propose should be used is generated by sun, wind and wave power; of which we have a constant supply. There is enough wind alone to generate enough electricity, and hydrogen via this electricity, for the whole world. An annual extra of this clean gas hydrogen can be generated, so that needing more does not become an issue.