Excluding geothermal (which is only practical in very few places) all the energy we have comes from or came from the sun. As our reserves of coal, oil and natural gas are used up all we'll have left is the energy currently reaching us from the sun. But is it enough?

First we need to know the total amount of energy reaching the earth. We get this total by simply multiplying the solar constant, 1367 watts/meter², times the cross sectional area of the earth, 1.27*10¹⁴ meters²
1367(w/m²)*1.27*10¹⁴ m² = 1.74*10¹⁷ watts
or joules of energy every second. Multiply that out for a whole year"s worth of seconds
1.74*10¹⁷ W*60*60*24*365.24 = 5.5*10²⁴ joules a year.
Now the earth's albedo, or reflectiveness means 30%[1] of that total is immediately reflected back into space leaving us with around (5.5*10²⁴)*.7 = 3.84*10²⁴ joules a year.

Now that we know what we have to work with, what are we using right now? The total energy consumption for the earth in 1998 was 4*10²⁰ joules[2] or about a hundredth of a percent of our solar energy. On the surface this sounds like good news, but how much of the energy reaching us can we actually use?

About 60% of that sunlight hits the ocean powering algae, evaporating water and basically keeping us alive[3]. Of the remaining 40% let's assume we can directly convert 10% of this solar energy into human usable power [4].
(4*10²⁰ J)/(3.84*10²⁴ J*.4*.1) = 0.26%
i.e. about 0.26% of all land on the earth's surface, take into account inefficiencies of storage and transport and you're up to 1-2% of the earth's surface used for energy production. To give some perspective of size that's about 60-100 thousand square miles or the entire state of Utah.

Now that was at the 1998 level of energy consumption, the US Energy Information Administration predicts world energy consumption will rise to 7.6*10^20 joules by 2030[5], so now were up to Utah and Nevada. Getting back to the 1998 numbers for a sec, if we brought every human on earth up to the USA's level of per capita consumption we'd be at 2.2*10^21 or about 5.5 times higher. At that level of consumption there is no reasonable way we could collect enough sunlight to keep up with demand and keep in mind I'm using some very optomistic numbers here.

To be fair I'm also not counting on hydroelectric or wind power, which partially derive their power from solar energy falling on the ocean, but it does give you an idea of the problem, we've outgrown what the planet can easily supply. The sollution is either to find another source of energy, cut back or let nature take it's course and see how many die durring the crash when the oil runs out. Of course fusion or something else may step in to save the day, the consequences of that though will have to wait for another day and more bunnies.(written on June 25th, 2006)