(copy of what I posted on the Maxis' BBS)

Well, I did some testing and it's interesting - sometimes a power plant (only tested coal-gas-oil) lasted for 70 years before hitting the 50% efficiency level, the second time I ran the series it only lasted ~45 years before hitting 50% efficiency. I'm going to say that it's related to the overall percent usage of the power system - because all of the gas-coal-oil power plants decayed at exactly the same rate (in fact all 6 blew up at the same instant when they wore out halfway through year 54). Higher utilization levels results in power plants wearing out faster.
Efficiency also impacts the monthly cost that you're paying for the power plant. A plant at 50% efficiency costs an extra 50% to maintain, which means the cost per MWh is 300% more then a brand new power plant.

Here are the decay values for a coal power plant where the percent utilization of all plants was always greater then 95% (selling off the excess power to the neighboring cities).

year $/mo $/MWh MWhCapacity PercentEfficiency
+00 $250 $0.0417 6000 100%
+10 $253 $0.0431 5868 97%
+20 $256 $0.0447 5729 95%
+30 $271 $0.0509 5326 88%
+40 $320 $0.0760 4213 70%
+45 $375 $0.1203 3118 51%
+50 $518 $0.3124 1658 27%
+51 $573 $0.4569 1254 20%
+52 $616 $0.6505 947 15%
+53 $664 $1.1160 595 9%
+54 $719 $3.7254 193 3%

In my test city, I went from making +$3k per month to losing $3k per month within a 10 year span as the (6) test power plants (2 each of coal-gas-oil) hit the end of their lifecycle. So there is a serious cost impact once you let your power plants get below ~75% efficiency (that gets exponentially worse).

On a side note - Wind Power plants seem to last a lot longer then coal-gas-oil plants, so their intial $/MWh of $0.20 may not be as high as it seems (Coal plants start at $0.04/MWh.) I'm not sure if Wind Power plants ever wear out (have not tested this yet).

If you replace your power plants at year 40 (70% utilization), using the above decay rate, here are approximate costs per MWh for coal-gas-oil (anyone have the proper calculations for this? these are back-of-envelope figures and are probably 5% high):

Coal - $10k to build
produces 211,360 MWh over 40 years
maint costs are $11k over 40 years
($10,000 + $11,000) / 211,360 = $0.0994 $/MWh

Gas - $9k to build
produces 106,000 MWh over 40 years
maint costs are $17,600 over 40 years
($17,600 + $9,000) / 106,000 = $0.2509 $/MWh

Oil - $17k to build
produces 246,590 MWh over 40 years
maint costs are $26,420 over 40 years
($26,420 + $17,000) / 246,590 = $0.1761 $/MWh

Summary:

Coal still seems like a very cheap power source... If you're going be selling your excess power, make sure you're getting at least $0.10/MWh if you want to make money doing the deal (most deals I've had are $0.10 to $0.14 for selling power to a neighbor).