I will answer this as a question on the different possible ways you could answer what is the universe's usable energy.
Well, the energy of the universe is all positive energy, if you consider the energy in the stress energy tensor and the dark energy. The dark energy has negative pressure, but positive energy. I say below something about what it is if you include gravitational energy also.
Note: you can calculate numbers and get to answers below. Whether that makes sense as the source of energy for your theorem I won't speculate.
So if you want the total energy now (in the cosmological comoving time now) just calculate the critical energy density. We're pretty close to that, if you count the dark density and both normal and dark matter. And then multiply by the volume of the observable universe. Since you're probably going to be comoving that is your available total energy.
The numbers are easy enough. Critical density is about $10^{-26}$ kg/$m^3$. This includes the mass equiv. The radius is 46 Gly, and volume you can compute to be about $10^{80}$ $m^3$. The total mass is then about $10^{54}$ kg. Multiple by $c^2$ and you get about $10^{70}$ joules. See some of the numbers from which you can calculate and verify at https://en.wikipedia.org/wiki/Observable_universe
However, the energy is not conserved. Over time the observable universe grows, and the density of matter energy decreases, except for the dark energy density which remains the same. That's 68% of the total density, or in order of magnitude the same as the total energy density. That density is about the same, and the universe becomes mostly a deSitter universe, expanding exponentially, which keeps growing to infinity. So total energy (which is all then dark energy) is infinite. In a comoving frame (remember the energy depends on the coordinate frame, if you change it it changes since the universe has no timelike symmetry). Also, although the observable universe at any one time is finite, the total universe is infinite, and so it is a not a bounded compact spacetime nor an asymptotically (I space or conformally) flat spacetime, and total energy is not conserved like it would be in an asymptotically flat spacetime. So in this view it just grows without limit. You can calculate it at $10^{100}$ years, it's about $10^{340}$ joules, order of magnitude assuming linear growth [I multiplied the $10^{70}$ joules now (roughly) by a factor of $10^{90}$ cubed, for a quick ROM]. This is a lower limit since the growth is exponential, so it may be closer to $10^{10^{340}}$ joules.
That would be one story. But dark energy is something we still don't understand. Another story would say most thinking leads to it probably being some kind of vacuum energy. Over time it is probable that that vacuum energy, could decay (say from false ground state), create matter energy, and then maybe form some other universe or bubbles of them. Whatever happens after our unknown dark energy starts decaying it really is impossible to predict now. This is highly speculative.
If you meant energy w/o the dark energy your number is above, as of now. Over time it goes to 0.
Either way, good luck with your Margolus-Levitin theorem. I know nothing of it. But the energy of the universe, depending on which energies you think you can use, are easily calculated as above. The less well defined in general relativity, the gravitational energy, you ignore unless you want to get to a total of 0, if you take the Einstein and the stress energy tensors. Or depending on how things fall out, you can get anywhere from 0 to about $10^{10^{340}}$ joules.
