A maximum of 8 Powered Mines can be built.
* assuming that Rally isn't used to speed up building or production
"Payback Time" in the table above refers to the length of time for the production of the new upgraded Powered Mine to match the production of the unupgraded Powered Mine. Since Powered Mines only produce metal, payback time is calculated taking only metals into account (e.g., pretending that the upgrade cost for Rank 4 is only 14000 resources). However, since wood is not free a second column of payback times is provided that treats wood and metals as equal resources (e.g., pretending the upgrade cost for Rank 4 is 42000 resources). Since both perspectives have their pros and cons, both are presented to assist you, the player, in making the best possible decision. Here is a simple example to illustrate payback time:
From Rank 3 to Rank 4, there is a cost of 14000 metals and a lost production time of 1 day. From the time the upgraded is started, it will be 13.72 days before the upgraded mine has caught up to the production of the mine if it had not been upgraded:
(Production for Level 7) = (3.0 metals/min) * (60 min/hr) * (24 hr/day) * 13.72 days = 59,270 metals
(Production for Level 8) = (4.0 metals/min) * (60 min/hr) * (24 hr/day) * (13.72 - 1) days - 14000 metals = 59,267 metals