He produced 1 liter of hydrogen gas, which is 0.044643 moles, because 1 mol H2 = 22.4L
I thought it produce a 1 liter mixture of hydrogen and oxygen? That would be 0.66L (0.03 mole) Hydrogen and 0.33L Oxygen. It could even be less because the mixture was bubbled through water at 91 degrees so the mix is probably not at STP.
I agree with your figure for the input power.
Revising the figures for the output gives..
The output energy (from combusting the hydrogen is potentially) =
(0.03 moles H2) * (237.5 kJ/mol) = 7125J
with the efficiency working out at about
(7125/15163) * 100 = 47%