This paper studies the practicable of hydrogen as a renewable energy source. The study proves a theorem on indirect energy conversion, suggests a monetary unit measured by unit energy, in which, comparing values of different industrial products and comparing different techniques used in production of the same product are better than that of money measured by gold, and proves an assertion, in which, the energy by decomposing a unit water is greater than that by composing a unit water, based on the law of conservation energy. According to the above, we get a conclusion: investment in hydrogen engine must be ended with failure.