A maximum of 8 Powered Mills can be built.
* assuming that Rally isn't used to speed up building or production
"Payback Time" refers to the length of time for the production of the new upgraded Powered Mill to match the production of the unupgraded Powered Mill. The first Payback Time column above takes only wood into account. The second (All Resources) Payback Time column adds wood and metal units together. This may assist players in making the best possible decision regarding building and upgrading.
Here is a simple example to illustrate payback time:
Upgrading from Rank 3 to Rank 4 costs 28050 metal and involves a production time of 0.83 day (if not using Rally). From the time the upgraded is started, it will be 24.06 days before the upgraded mine has caught up to the production of the mine if it had not been upgraded:
(Production for Rank 3) = (4.5 wood/min) * (60 min/hr) * (24 hr/day) * 24.06 days = 155,909 wood
(Production for Rank 4) = (5.5 wood/min) * (60 min/hr) * (24 hr/day) * (24.06 - 0.83) days - 28050 wood = 155,931 wood