All the available energy on earth; whether it be oil, bio-fuel, or wind power; comes from the sun.
Well mostly... geothermal energy comes from fissures in the crust of the earth which release energy derived from the intense gravity experienced in the core of the earth which causes rocks to melt into magma deep within the earth allowing us to capture steam. Which is similar, in the sense that gravity causes all the hydrogen to compress, to the sustained fusion reaction that is our sun.
Anyways, outside of geothermal, all energy on earth comes from the sun. Oil and coal are just ancient organic material (things that used to be alive like plants and animals) that has been allowed to sort of ferment into a primordial soup that we can extract and crack in a refinery and use for fuel. It is literally stored sunlight. Wind power is made possible by the sun heating up the earth as it rotates causing warm and cool spots which changes the density of air and causes it to seek an equilibrium density - that is, move. Hydroelectric is just solar energy causing water to evaporate, rain, and then give up its potential energy as it runs downhill. Solar is pretty self explanatory. Nuclear is the exception. Then again it's also imitating how the sun produces energy... or the exact opposite. Whatever.
The total amount of energy hitting the earth is 174 Peta watts (10^15 watts) per day. Of that about 89 PW actually make it through the atmosphere and hit the earth (that much energy could run a 14 watt compact fluorescent light bulb for 730 billion years... or about 50 times longer than the age of the universe). Without going into space, without using stored energy, assuming 100% efficiency, by stealing light from every other organism - that is the amount of energy available to us. Our Current usage rates seem to be on the order of 15-16 Terra watts (10^12)(as of 2005).
So... (8.9 x 10^16 watts)/(1.6 x 10^13 watts) ~ 5600
Meaning that if we could capture 1/5600 or .018% of the energy hitting the earths surface we could meet all our current power demands.
Currently about 80%-90% of our energy comes from non-renewable resources like coal and gas. That is, stored sunlight. So that means that 10-20% comes directly from the sun. Let's take the more optimistic of both figures; 20% renewable and 16 TW total usage and you get 3.2 x 10^12 watts.
Total watts from sunlight available for collection (89 PW) divided by current watts actually collected (3.2 TW) equals roughly 28,000. So currently we're collecting 1/28,0000 or .0036% (at best) of the energy available to us. Way to go team.
The reason why understanding this is fundamental is because it proves that given the appropriate amount of time solar and its related technologies will become viable. We haven't even come close to tapping direct solar energy as a resource. Conversely, things like clean coal may be necessary in the short run, but they are clearly not ultra-long term solutions. The solar industry says that they are already competitive without any of the huge leaps that the media and government have said were needed to make it competitive with fossil fuels.
Side note: If fossil fuels are properly taxed to include their negative externalities this will instantly make renewables more competitive by revealing the true cost of burning fossil fuels. I know it seems crazy, but paying more for your gas (in this case due to a rise in taxes) will help everyone in the long run.
So what's the latest and greatest on these technologies? Wind turbines are being rethought to make them cheaper. This includes turning them around, allowing the blades to flex so as to capture additional gusts of wind, and using just 2 props. Solar-thermal, one of my favorites due to its sheer simplicity, just got modular. GMO's in the form of bacteria that eat CO2 and sunlight (sounds like my mum) have been engineered to create an ethanol like fuel. And then there's the grand daddy, literally, of them all; fusion. Long shot? This approach might be, but one day in the not so distant future (less than 80 years is my prediction) this will be the single greatest source of energy for humans. Not including the sun...
It's worth noting that the majority of your electricity bill is determined by costs associated with transmission and generation; not the cost of whatever is being burned to produce the energy, so realistically nanopower - that is, self powered buildings should become the norm.