What area of solar panels would be required to satisfy the whole
world's present energy needs.  E.g. Panels the size of Turkey might be
enough.  Maybe of only Quatar.
I suppose the latitude at which the panels would be placed would make
a difference, but I'm not after anything too mathematical, just a
nugget of information.  Saying that, though, I'm not scared of maths.
Also, though, would there be any adverse affects of having such a
large reflective surface in one place - such as, literally, covering
the whole of a desert with solar panels?  Increased global warming,
perhaps?  Would it be better to have several smaller sources?
And, if you get into this question, how do the costs of obtaining the
energy through solar compare to obtaining the energy through oil? 
E.g. the costs of big cables going across the world.

Thanks

hello wilbur111,

Here are some good pointers, however the effects of the panels on
global warming or the costs relative to oil, were not readily
available within the scope of a $4.50 question, but the sites below
should give you some ideas.

"If the whole world consumed 500 quadrillion BTU's of energy in 2000,
and that's only a bit generous, than a square of photovoltaic cells
200 miles on a side would have produced 100% of the world's energy
requirements in that year. That's assuming 8 watts of output per
square foot of PVs, 6 hours of sun a day year-round, and 70%
efficiency after transmission and conversion."
http://www.ecoworld.org/Home/articles2.cfm?TID=259

Another possibility is to collect the sunlight in space and 'send it'
back to earth:
"But the technological challenges posed by the system are great. In
order to harvest as much energy as possible, the satellites would need
large surfaces of panels, measuring on the order of 7 miles long and 3
miles wide-over 21 square miles each."
http://hcs.harvard.edu/~hsr/pdfswinter2003/bechis72-75.pdf

"The solar energy released by the sun may seem like the best source of
energy for the world. It has many benefits: it is free, it does not
pollute, is very reliable when the sun is shining, and it is very
safe. With all of these benefits, it may seem odd that there are
extremely few solar power plants on earth. The reasons that solar
power plants do not power the world are summed up in two reasons: cost
and reliability. Solar plants made of solar cells are extremely
expensive to build. Thousands of square feet of solar cells are
required to generate enough power for even a very small town. A large
city simply doesn’t have enough room for a solar plant, or enough
money. The second main reason is the unreliability of the plants. If
the plant has a few cloudy days, there will be no electricity for the
town. This is not an option for many towns or cities. In the modern
age of today, we need to have power 24 hours a day, 7 days a week, not
only on sunny days, sometimes at night."
http://www.smartown.com/sp2000/energy_planet/en/trad/solar.html

Another alternative:
"With oceans covering 60 per cent of the planet, wave power offers a
source of energy that's still untapped - just 0.1% of the ocean's
energy could power the world for five years."
http://www.tve.org/earthreport/archive/03Sep2000.html

Best regards

THX1138

The location makes a big differece. Arizona, USA would likely produce
twice as much energy as Turkey even though Turkey is a bit larger.
This is because there are fewer cloudy days in Arizona. Close to the
Equator also helps but any very sunny location in the torrid zone is
good, and there are a few good semi tropical locations such as
Arizona. At present even 30% of the world's needs is impractical as
storing energy from afternoon to early evening (peak demand) is
costly, dangerous and/or not tested large scale. At present the cost
of big cables around the world would likely far exceed the cost of the
solar panels and the land they occupy. The known reserves of some of
the elements (copper) are not sufficient. The line losses are often
more than half at distances over about 200 miles, so many smaller
sources is better than several locations and one location for the
Earth is far more costly. Liquid nitrogen temperature super conductors
may be competitive with copper wire for high tension power lines soon,
but I have not heard any encouraging reports. I think one problem is
the line explodes if super conductivity is lost for more than one
second, unless the power input to the line can be cut in less than a
second.
 With solar panels about 20% of the energy become electricity, perhaps
40% becomes heat and about 40% is reflected back into the sky. The
direction of the reflected beam changes thoughout the day, even if the
solar panels are stearable, so I don't think there would be important
environmental problems except the mine tailings from producing the raw
materials and the land under the solar panels. I have participated in
perhaps 70 alternative energy threads at www.abuzz.com   You probably
can locate them with key words such as solar power tower. When you
find one where ccpoodle participated (that's me), click on ccpoodle
and you can skim though the 3600 threads I have been in. At least 30
are about alternative energy.   Neil

Here are my comments on the THX comment. The arithmetic may be
correct, but some of the assumptions are over optimistic. In the North
Temperate zone, December and early January, the 6 hours per day is not
realstic, as the pannels produce only about 1% as much power when the
sun is close to the horizon, even if the pannels are stearable. So for
about 7 weeks each year annother energy source is needed especially as
these are peak demand weeks in most of the North Temperate zone. The
70% efficiency may not be achievable even for customers within 10
miles of the solar array and may be as low as 20% conversion and
delivery efficiency for cities at 40 to 60 degrees North or South
latitude that are typically 1000 miles from a good location for
obtaining solar energy.
 The solar power sattelites would be incredibly costly to build and
keep running. What do you think about a one square KM rectenna
receiving ten billion watts next door to your house? That is average
10,000 watts per square meter = one watt per square cm. Hot spots
could be ten times more, like inside your microwave oven. A 1.2 KM
wide death ray, 24/7.  The rectenna can be larger, reducing the power
density to non-leathal, but land near lots of energy customers is
typically expensive. Whole body exposure to 1000 watts per square
meter makes you feel a bit warmer, and the health effects are still
being debated.
 As far as I know a one megawatt (enough for about 500 homes) tide or
wave generating facility is yet to be built. Apparently there are many
problems before either can compete with oil.   Neil

Thanks very much for all your responses.  All good stuff and none of
you official researchers.  Thanks.