As to many of the other posts, general economic theory is fascinating certainly, but it's a bit beyond the scope of this critique. But I don't think using energy in this context makes sense, since energy is something which is produced using reactors, etc, rather then by people and economic activity. Equally, not every nation can manage to 'keep the factories going' without money, especially depending on local political support and the scale of the war.
In a total war for survival, that might be possible, but many nations have failed that exact same test, even in extreme wars. Running out of money, as odd as it sounds, has happened, especially outside of our modern context of money, which may well be a passing fad.
Allow me to play devil's advocate a bit. Just for fun! 
You assume that a society HAS money. Not all the games I have played had money. At least half of them didn't have the concept of wealth.
Some were hive races.
Other were war clans where anything in the nation was allocated due to fighting prowess
I also played an utopian communist nation once.
None of these scenarios have money. Money does not even EXIST here 
And yes, all of these were obviously with civilians "off", so to speak, destroying the civs so they don't pop out.
Stored energy always works, even when money does not exist. There are in fact some commercial sci-fi games which use energy instead of money, for these very reasons. Money does not always make sense, especially in ultra advanced settings.
I considered bringing up the fact that not all player race concepts may necessarily require the concept of money or wealth to function. However the wealth concept seems fairly deeply engrained in the game so I do not think it will be removed any time soon.
That said I have also considered the concept of 'energy wealth' an interesting notion and at least somewhat realistic. Public utility companies in the US at least typically charge a nominal rate based on total energy consumption over a fixed period of time. The rate varies considerably due to technological and legal constraints but is a fairly wide-spread economic model. These companies are generally subsidized by the government in some way however so there is a tight relationship between the government and the utilities.
