Apparently I was mistaken. They don't just make a weird hydrogen isotope. They make plain-old hydrogen. 4 protons become 2 protons and 2 neutrons. The difference in mass between a proton and a neutron is 2.3×10^-30 kg. So, naively, you'd generate 4.1×10^-13 J per helium atom. Or, roughly 11 GJ (3 Mwh) per litre of helium at STP.

Seems a little low, though I suppose 1L is only 1/5th of a gram of helium. I still might be missing something, too.