This checks out! Kinetic energy = 0.5 mv^2, where v = 0.1c => E = 0.005mc^2 (relativistic effects not too important at this speed). Fusing a kilo of hydrogen co

This checks out! Kinetic energy = 0.5 mv^2, where v = 0.1c => E = 0.005mc^2 (relativistic effects not too important at this speed). Fusing a kilo of hydrogen converts ~6g to energy; at E = mc^2, that's 0.006mc^2. So the energy needed to accelerate a given mass of hydrogen to 0.1c is almost exactly equal to the energy produced by fusing that hydrogen.