Six lv 1 Coal Mine are not enough. Many people will use Choking plant. I have seen high lv players using six lv 1 Coal Mine + Fourteen lv 3 choking plant. However, not many people use lv 2 Coal mine.
Before we run into other reasons behind this scenario, I want to figure out:
if there is any advantage to use lv 2 Coal Mine Only over lv 3 Choking plant Only.
Case A: Six lv 2 Coal Mine
Six lv 2 Coal Mine produce 12 Coal per 1.5 minutes = 24 Coal per 3 minutes.
That's equivalent to Eight lv 3 Choking Plant: 24 Coal per 3 minutes.
Case B: Eight lv 3 Choking Plant
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Now, let's see how many building we need to support these two cases.
Case A: Six lv 2 Coal Mine
Since we are considering if Coal Mine should be upgraded or not, we are going to ignore the cost of Coal Mine. In fact, we are just going to consider the Upgrade Cost.
How long does a Coal Mine last?
At lv 2, Coal Mine should be run out in 2900 Coal /2 Coal /1.5 production time (min) = 966.67 min.
For the total Upgrade Cost: You will need 300 Tool x 6 = 1800 Tool per 966.67 min.
How many supporting buildings we need?
A lv 3 Tool-maker generate 3 tool per 3 minutes. In 966.67 minutes, a lv 3 Tool-maker generate 966.67 tool.
That's a 1:1 ratio for Lv2 Coal Mine : Lv 3 Tool Maker; One Lv 3 Tool Maker also require One Lv 3 Copper Smith
Thus, Six lv 2 Coal Mines requires Twelve Buildings to support at minimum if you purchase Copper Ore.
Let's see if you are not purchasing Copper Ore, how many more building you need?
In order to support Six Lv 3 Copper Smith, you will need 9 Copper Ore per 3 minutes.
Six lv 1 Copper Mine produce 6 Copper ore per 2 minutes = 9 Copper ore per 3 minutes.
or
Three lv 2 Copper Mine produce 6 copper ore per 2 minutes = 9 Copper Ore per 3 minutes; with a cost of 150 tool, but 150 tool is so little that I think this is negligible.
IMPORTANT: However, you also need to reduce the total coal production rate by 9 Coal per 3 minutes.
Thus, we can conclude that, Case A: Six lv 2 Coal Mine produce 15 Coal per 3 minutes, if you don't buy any materials; total cost of building licence: 21 Building license.
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Now, let's look at:
Case B: Eight lv 3 Choking Plant
We are going to ignore the cost of this production chain because they stay permanent. So, their cost become negligible in long run.
In fact, we are just going to look at how much building licences are used.
How many supporting building we need?
Every Two lv 3 Choking Plant requires Two lv 3 Fir wood Cutter, Three lv 3 Fir wood Forester
For Eight lv 3 Choking Plant, we need Eight lv 3 Fir Wood Cutter, Twelve lv 3 Fir wood Forester
Total Cost is: 28 Building License.
Thus, we can conclude that, Case B: Eight lv 3 Choking Plant produce 24 Coal per 3 minutes, if you don't buy any materials; total cost of building licence: 28 Building license.
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Let's normalize the calculation results of Case [COLOR="rgb(255, 140, 0)"] A[/COLOR] and Case [COLOR="rgb(65, 105, 225)"]B[/COLOR]
24 x Six lv 2 Coal Mine = 360 Coal per 3 minutes, cost 504 building licence
15 x Eight lv 3 Choking Plant = 360 Coal per 3 minutes, cost 420 building licence
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Assuming I didn't make any mistake in the calculation, it's not worthwhile to upgrade Coal Mine to lv 2 at all.
lv 3 Choking Plant + Six lv 1 Coal Mine are the most-building-licence-efficient (is this even a word ?!)
Also, not to forget mentioning another main reason for not going lv 2 Coal Mine: Geologist takes a long time for Coal.
Unless you online 24-7......you are going to have a hard time to keep up Coal Mine production + Iron Mine production + Marble Production + Copper Production, etc
P.S. Is the color function not working? Or is that just google chrome?