A couple of quick notes.
First, I've been thinking about the alcubierre drive and my statement that at STL it doesn't need exotic matter. My mental models of the drive tells me that's BS and I can't find the article I read about it. So until that claim is verified somewhere else, ignore it.
Second, according to Wikipedia the two metastable isomers with the lowest and highest energy outputs are Thorium-229 at its first excited state and Halfnium-178 at its second excited state, respectively. Below, for your calculating pleasure, is the mass difference between the the ground and excited states of these two isomers. They are the limits to the possability of an isomer battery. For my own calculations, I would pick an intermediate energy point and use that for my assumed battery power density.
dM(Hf178m2) = 15e-6
dM(Th229m) = 35e-12
dM (H+O) = 150e-12
dM (Pu239) = 851e-6
dM(DD) = 975e-6
The (DD) fusion reaction at the bottom is for reference.
If my memory serves, and it might not, the difference in mass released by burning hydrogen and oxygen to get water is on the order of 1e-9. I'll have to calculate that and get back to y'all.
EDIT: Found a previous calculation of hydrogen burning. I am not sure what I used as assumptions while calculating, so take it with a grain of salt. I also added the mass change ratio for Plutonium fission. Further fusion reactions can be found in the work I linked to on fusion powered warp and rocket TOS Rom BOPs.