You have 8 equations but you just want to solve for one single variable? This may work, but mostly doesn't as the equations are likely to be contradictory/conflicting,
But ... I see that you have in total 12 variables in your 8 equations which had not been assigned any value.
These are Sfmax, W, M.o, L, S.R, S.t, S.TO, S.H, thetaf.max, V, g.o, and h.o.
I guess that it was a typo that you wrote "e" (Euler number) instead of "E" in the fourth equation!? I also guess that V and W are meant to be variables and not the units volt and watt.
So actually you are about to solve a system with 12 unknowns but only have 8 equations!! This may work, but usually there will be an infinite number of possible solutions and Prime will chose just one of them which might not be the one you would prefer.
To solve that kind of system you have to assign a guess value for every 12 unknowns and you also have to solve for all 12, not just for the one you are interested in only!
In any case you should correct that and if it doesn't work you should attach your worksheet, not just the pdf.
P.S.: I am not sure if your equation/condition using the "max" function will work as desired in a solve block.
