Interesting, I ran the tests again.... and this time my power plants (gas, coal, oil) hit the 50% mark around +45 years instead of +70 years. Now, this time around, I fully utilized the power plants from the beginning, which may have contributed to their wear and tear values. That, or the pollution levels were higher, or something else. But I'm seeing a difference of 25 years in the lifespan of these plants, so there's definitely something else affecting the equations. (The particulars though for a given efficiency value were identical - a coal plant at 50% costs $375 and only outputs 3000MWh, regardless of the age of the plant.)