All minerals produced at base are immediately converted to Economy energy reserves ( each 2 minerals makes 1 energy)
The Stockpile Energy option is a wonderful thing. At its
most basic level, it's useful in that it gives your base something to do even
after you've built all the facilities and secret projects you want. This ability
is, of course, crucial if you're going for an economic victory, and can also give
your faction the cashflow edge it needs to hurry the Ascent
to Transcendence project and beat another faction to Transcendence.
Even before the endgame, however, it can be profitably used. Put a few ( or a lot ) of relatively high-producing bases on Stockpile Energy when your cash flow is slumpling, or when you're in a race with another faction for a key secret project ( or several ) and suspect you might have to buy your way to victory in the race. Remember, however, that the Stockpile Energy production orders are never "finished". Once you start a base stockpiling energy, it will continue until you turn it off by hand. On a related note, if you're not using the Base Governer, your production orders will default to Stockpile Energy whenever you complete a facility or secret project ( but not units - the game will default to a new unit of the same kind ). If you're not attentive, it's painfully easy to allow a base to slip into Stockpile Energy when you'd rather have it producing new facilities or units.
