We can calculate energy by using following equation:
E = hc/ λ
h=6.626x10⁻³⁴Js
c=3x10⁸ms⁻¹
λ=656nm = 656x10⁻⁹m
1 gram of hydrogen atom is = 1 mol of hydrogen and it emits photons = 6.023x1023 photon
Energy of one photon (E) = (6.626x10⁻³⁴ x 3x10⁸) / (656x10⁻⁹m)
Energy of one mole photon (E) = (6.023x10²³ x 6.626x10⁻³⁴ x 3x10⁸) / (656x10⁻⁹m)
1.82x10⁵J = 182 kj