There's always a lot of talk promoting wind power and solar power as 'alternative' energy sources to coal and other 'dirty' sources of electricity. But the truth of the matter is that wind and solar cannot ever be considered as 'alternatives' because they simply are not.
To say that they are alternatives implies that some day we could replace all our coal fired power plants with wind turbines or solar panels. In truth this is impossible, hence they are not alternatives. Why is it impossible? Because the inherent instability of the supply of wind power means that an alternative supply of electricity must be found that may be turned on and off at whim. Fortunately, we already have a source that can be used: coal. Or alternatively, natural gas, oil or biomass.
Denmark, a nation lauded by many environmentalists as being ahead of the gang when it comes to wind power will, ironically prove my point. This graph, taken from an article published in the Proceedings of the ICE back in 2005 by Hugh Sharman , demonstrated the supply of wind power and the electricity demands for a one week period in Denmark.
Figure 1: Electricity Demand (grey) and Wind Generated Electricity (red) over 1 week 
As anyone may see, the supply of wind generated electricity was for some days zero. Which means that the electricity would have to come from somewhere else. Generally, in Denmark that place is a coal fired power plant. In fact, as is convincingly argued in many places [1,2] that only 10-14% of the Danish electricity comes from wind turbines, not the 20% floated about by environmentalists. What happens to the excess electricity? It is exported to Scandinavia in the north or to Germany in the south. A useful benefit of being tied into those grids.
This is where we can see the limited usefulness of wind power. If Denmark did not have those markets to which to send electricity it would have to shut down the wind turbines and let them sit idle.
But, one might say, at least the Danish wind farms are reducing the need for electricity from coal right? And in that you'd be correct, partially.
Figure 2: Electricity Generation by Source 
It is true that the electricity generated from coal has decreased by an admirable 48 TJ from 1994 to 2007, and that electricity from wind has increased by 22 TJ over the same period. However, electricity from other sources (notably natural gas and the burning of biomass) make up for the rest and as was pointed out above, only 50-70% of that 22 TJ may be counted as contributing to the net decrease in electricity generation from coal. (Note: Solar and hydroelectric power is negligible in Denmark in these years)
So talking of wind turbines as 'alternatives' to coal, natural gas or even nuclear power is untruthful and not backed up by the available data. Until the wind will blow on demand or an alternative method of energy storage is viably combined with wind power it will not replace coal, only reduce the need for it.
 Hugh Sharman, "Why Wind Power Works for Denmark" Proceedings of the ICE, Civil Engineering 158, May 2005 [link]
 "Analysis of Wind Power in the Danish Electrical Supply in 2005 and 2006" Techconsult [link]
 "Danish Annual Energy Statistics 2007" Danish Energy Agency, 2007. [link]