There are several different types of structures that your nation can research and build that, instead of being defensive, are devoted to generating and storing manpower or energy. For example, the Fuel Tank and Earth Chakra store energy, while the Agro Colony and Hydroponic Garden store manpower. The amount of manpower and energy that can be generated and stored by each type of structure is different, with higher level structures storing much more than lower level structures.
As soon as you build one of these structures it begins filling up, and takes 48 hours to become full. However, if an enemy nation captures the structure it will steal the manpower and energy stored within it, and once your nation recaptures the structure it will start filling again from empty.
A nation can have up to 8 storage structures built at any one time. If you’ve already built 8, you won’t be able to build a new one until you’ve salvaged one of the old ones. You can check how many active storage structures your nation has any time, on the Nation panel’s Stats tab, under Storage Structures Built.
The energy and manpower that gets stockpiled in these storage structures aren’t for use by the nation that generates them, however; instead, they’re stored for use by the nation’s allies (see Alliances). When a nation runs out of manpower or energy, it is automatically donated to that nation by its allies (if any of its allies have enough manpower or energy stored and available to be donated). This provides a key advantage to forging alliances; allies are there to chip in some manpower or energy when you need it most. Under the Nation panel’s Stats tab you can check how much stored manpower and energy your nation has, that’s available to be donated to allies, and how much your allies have, that’s available to be donated to your nation should it run out.
However, there’s another reason to store up manpower and energy, rather than to donate it to allies. Each storage structure generates a certain amount of XP per hour. Different structures generate different amounts of XP, so it’s important to upgrade to the higher level structures to gain the greatest benefit. The amount of XP generated by a storage structure also depends on how full it is; a 100% full structure will generate its maximum amount of XP, but a 20% full structure will only generate XP at 20% of its normal rate. Therefore it’s important to defend your nation’s storage structures so that they aren’t captured and emptied by enemy nations. Using storage structures to generate XP (sometimes called “farming”) is an alternate way to level up your nation — one that doesn’t require engaging in warfare.