I have only other and nuclear power plants, but I'm still using over 2bbl of petroleum a year. Why is this happening?

By accessing the 5th button of the second panel labeled Consumption/Usage Levels it is possible to see where any product is being used and what raw materials it consumes. Here we can see that indeed, 3.2M barrels are being consumed through power generation. This can be attributed do the fact that power plants placed on the map are not the only source of electricity. Larger cities produce not only some electricity but also Consumer and Industrial goods. The quantity of each is assigned during scenario design.

However, it is also clear that electricity is not the primary demand for petroleum in this case. Civilian usage is accounting for 560M while industrial goods production is consuming 1.6B barrels, by far the largest drain.

Possible solution: lock industrial goods and military goods production from minister control. Set production of industrial goods to 100% of demand and military goods to 0%. Since the player already has a surplus of military goods, they can live off of these temporarily. This will help in two ways. First, the 240M Barrels being used for military goods construction will not be needed anymore. Second, the demand for industrial goods will decrease (industrial goods are used in the production of military goods) and by only producing to demand the player should be able to cut petroleum use for industrial goods by as much as 40-50%.

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