Here is the wikipedia article I found with chart comparisions of the costs of different power options.
http://en.wikipedia.org/wiki/Cost_of_electricity_by_source

From thinking on this overnight I think the initial purchasing cost will need to be factored in to give good balance in the game as well. I really like what you have done for the MW output levels so far and am basing most of my thoughts around balancing for those numbers and the numbers in the article.

- Oil appears to be a better cost per MW over coal in the charts (basing it off of natural gas). The purchase cost for an oil plant should be much higher than a coal plant but provide better maintenance costs per MW. This will make the oil plant less expensive in the long run but take longer to even out its value over coal.

- Wind appears to be quite cheap to maintain compared to most but provides low yields. Perhaps an increase in initial purchase price but a very low maintenance cost comparable to oil so that it is slightly better than coal.

- Coal as a startup power plant in game is at quite a good price for purchase right now. I approve the pollution increase as well. If the adjustment for maintenance costs brings oil to slightly cheaper then I think will bring some realism to where eventually the technology moves the builder to use the higher tier power options.

-Nuclear looks great with the increased output. The MW cost in the charts is more expensive than most sources in the charts. As long as the cost per MW isnt too much worse than coal then I think the in game mechanics of water demand and lack of need to transport fuel will balance this out and make it an ideal source for huge high density megalopolises.

-Hydro currently seems very well balanced with a huge startup cost but the potential to be the most efficient source by far. Currently in the vanilla game a dam needs to be able to produce a minimum of 427 MW to be the most cost effective power source; 426 MW puts it just a hair below Solar.

-Solar I am a little lost on. I like the output you have chosen for it. I am not sure how to balance the cost to make it a viable in game option. Perhaps remove the sound pollution and have the initial purchase cost less than other power options available at that tier; this would give builders an option to really go green and sink a large budget into to make worthwhile.

So I have been playing with numbers the last couple of days and come up with the following hypothetical values trying to balance gameplay to the realistic outputs making each option useful in specific scenarios. I hope this is helpful and easy to understand.

I am organizing things as follows:
Type - MW output - Purchase cost - cost per MW - Weekly operating cost - 50% Operation Cost

Chart 1
Wind - 4 - 5,000 - 8 - 32 - 16
Offshore 10 - 10,000 - 8 - 80 - 40
Solar - 55 - 50,000 - 12 - 660 - 330
Coal - 200 - 15,000 - 10 - 2000 - 1000
Oil - 250 - 80,000 - 9 - 2250 - 1125
Nuclear - 750 - 200,000 - 11 - 8250 - 4125

Chart 2
Wind - 4 - 4,000 - 6 - 24 -12
Offshore - 10 - 16,000 - 6 - 60 - 30
Solar - 75 - 50,000 - 12 - 900 - 450
Coal - 200 - 10,000 - 10 - 2,000 - 1,000
Oil - 350 - 100,000 - 8 - 2,800 - 1,400
Nuclear - 1,000 - 500,000 - 11 - 11,000 - 5,500

Chart One is the first set of values I came up with that seemed in some ways correct and closest to what your current setup is. Chart 2 is the second set I came up with and the one I feel is most balanced around being realistic and playable in game.

- In Both Charts I have adjusted the Wind Max output to 4 MW on the premise that there are not too many spots on maps, in theory, that will provide max output potential.
- Wind in Chart 2 is at 6 per MW which means the at 4 MW output it is 2nd best; at 3 MW it is tied with Oil; at 2 MW it is tied with Solar; at 1 MW it is the worst.
- Coal I don't think can have its output increased any more without breaking start up gameplay or long-term balance. A larger, slightly, more efficient coal plant would work better to simulate this; perhaps at 9 cost per MW based on Chart 2.
- Because Coal is now very expensive to maintain I have reduced the cost of the plant to give builders time to get population up early game to offset this cost; I think that the $10,000 in Chart 2 is the most reasonable for not killing players at the outset.
- I made the Solar plant cost cheaper to offset the cost per MW and buffed the output a little in Chart 2 to make it viable for large volumes of pollution free energy. This is also with the idea of 0 noise pollution in mind to make it rather attractive outside of money value.
- Nuclear is buffed to 1,000 MW in chart 2 to produce huge volume and the plant is now 150% more expensive to buy; I feel these numbers suit the idea of Nuclear being a great option for mega-high populations.
- Oil I buffed a bit in Chart 2 to give it a bigger difference for coal but also to make the weekly maintainance cost higher than coal while still being more cost effective over time.
- Wind and Solar plants have been reduced in purchase price to offset the fact that they are less viable sources of output. I feel that with the increased cost for Offshore, green energy is now something that really requires the builder to invest in it on a large scale to make it worthwhile.
- Oil and Nuclear Plants are much more expensive to require more forward planning to implement. A surprise power shortage would now likely be easily solved by dropping a coal plant but long term the maintainence and pollution would severly hamper the city; short term though a coal plant could keep the power flowing long enough to save up for an oil or nuclear alternative.

The EIA levilized cost of energy chart would be a good place to mine realistic values from.

http://www.eia.gov/forecasts/aeo/electricity_generation.cfm

The first thing you should note is that wind and solar are horribly expensivbe compared to most of everything else. You should also note that they all have low capacity factors, and I think with your variable wind power output you could simulate forced and planned outages, this means that the plants would shut down/reduce power randomly with the chance being higher for plants with lower capacity factors.

Now back on topic, the renewable problem is evident. They are largely more expensive than the other options (even wind if you include the need for batteries), and signifigantly less reliable, thus making them unattractive for players. One idea I had is to give them a minor (very, very small) park effect, thus making them as attractive for players as the other options without being unrealistic.

Now to the greater art of ballance.

As you can see coal is clearly among the cheapest, far cheaper than the wind, solar, nuclear, oil, or biomass. The cheapest is natural gas, but unfortunately that is not included in the game as an option. It should also be noted that the pollution from a coal plant is huge, so the game should reflect this with a pollution area of at least 50.

Next up is oil, you can approxamate the cost from the conventional combustion turbine data. As you can see it is expensive and has a low capacity factor (though that is because it is a populkar peaker plant, thus it is offline most of the time, driving down the capacity factor. It should really be comperable to coal in it's capacity factor) It is also less polluting than coal.

Then there is my favorite, nuclear. It should emit no pollution other than minor noise pollution and consume more water than it does now. In fact it should consume a quarter of the output of a water pumping station. You should also reduce the sewage output from what it is now, as most of the water is going to go out the cooling towers and into the air. It should also be cheaper to run than coal, oil, or the renewables, but cost much more than coal and oil to build, at least three times as much. It should also hire at least three hundered people, primarily well educated and above, to represent the hiring patterns of reactors in real life. If at all possible the reactor should have a slight negitive effect on land value, to reflect real life. It should also be noted that the cost of nuclear reactor construction is extremely variable, to about $1500 a kW installed capacity in france to nearly $4000-$5000 in the US. The construction cost represents the primary cost of any nuclear power station's electricty, and as such has a dramatic impact upon the cost of nuclear energy.

Now to the renewables, wind, hydro, and solar.

Wind should have an overall low capacity factor and (for renewables) low startup costs. The magic (decent capacity factors) with wind comes when you have a large number of wind turbines running at once, ballencing out each other's output, allowing for a reasonably assured power supply. They should be reduced in max output to about 3-2 MW to represent the output of unts of their size. They should also (if possible) reduce the efficency of nearby wind power plant units.

Hydro should have an absolutely enormous startup cost, but a very, very low operating cost. In other words, change little with it excepting making it even cheaper to run.

Now to solar. Solar should have the highest construction cost of all power plants. Due the one in the game being solar thermal it should copy the values listed in the table. It may be interesting to note that the max amount of energy avalible per sq. m for solar is 1 kW. Solar thermal has a efficencey of about 10-15%(high estemate) Thus yielding about .15 kW per sq. m or 9.6 kW per cities skylines square(CSS) of solar panel. Assuming a 12x12 CSS size for the solar panels you get 1.3824 MW out of the power station using realistic energy densities.