One needs to be a careful consumer of information when reading about the "rated capacity" of certain alternative energy plants.
Take a 1MW nuclear plant, run it for 24 hours, and you get 24 MW-hours, or something fairly close to that, of electricity.
Leave 1MW worth of solar panels out in the sun for 24 hours, you get much less total electricity, depending on where you put it. On an average day in New York City, you will get about 4 MW-hours. In one of the best solar sites in the word, my home of Phoenix, you get about 6.5 MW-hours per day. The key metric is peak sun-hours per day, and some example figures are here. So, even in the best solar sites in the world, solar panels run at only about 25-30% of capacity.
It turns out, not surprisingly, that the same relationship holds for wind.
It's not like it's a secret that wind turbines are an unreliable source of electrical power. Bryce points out that, "In
July 2006, for example, wind turbines in California produced power at
only about 10 percent of their capacity; in Texas, one of the most
promising states for wind energy, the windmills produced electricity at
about 17 percent of their rated capacity."
that there has to be nuclear, coal-fired or natural gas power plants
functioning fulltime as a backup to the pathetically unreliable and
inefficient wind farms. Moreover, what electricity they do generate
is lost to some degree in the process of transmitting it over long
distances to distribution facilities.
Now, this should not outright dissuade us from these technologies, but since no one has really licked the night-time / not-windy storage proble, it's certainly an issue. I have looked at solar for my house a number of times, and the numbers just are not there (even with up to 50% government subsidies!) without a 2-5x decrease in panel costs. Low yields can potentially be tolerated, but capital costs are going to have to be a lot lower before they make a ton of sense.