You have to refresh the game. In order to do that, you can do any of the following:
- claim a neutral territory
- send drone to harvest or deposit resources from an adjacent square
- harvest the "area" you are in (not the structure)
- deposit resources into a bank
- move the screen to the edge of the map until it reloads
Let's say your power is at 25%. Then you do something else for two hours without completely shutting down the game (its still running in the background). When you come back and check the game, your power is the same because the game never reloaded. So if this happens to you, just try claiming another territory or anything listed above. It should work.