The idea of space-based solar power consist of harnessing the energy of solar radiation with a system positioned in outer space. Free-floating field of photovoltaic solar panels orbiting the Earth, coupled with microwave antennas transmitting the generated electricity for terrestrial use is a typical example of possible Space-based solar power system. Apart from (geostationary) Earth's orbit, chosen Lagrange points of the Earth-Sun and Earth-Moon system and Moon's surface are among the appealing locations for such facility.
Space-based solar power is scalable. There is virtually no upper limit for energy that can be obtained in this way (in the perspective of current civilization's energy needs).
Space-based solar power is steady and weather independent. Putting solar panels to space offers the possibility to solve one of the most aggravating problems of Solar energy - intermittency. When positioned e.g. on geostationary orbit, the panels would receive insolation for 95% of the time and the periods when they would be shaded would be known in advance.
Insolation is stronger in space than after passing through the Earth's atmosphere. Even when there are no clouds in the sky, a nonnegligible portion of the electromagnetic radiation coming from the Sun is lost due to reflection from and absorption in the atmosphere. Solar panels placed at the surface of the Earth receive in the best possible case (noon, clear sky, Sun shining perpendicularly through the atmosphere - i.e. area near the equator) only around 70% of the the full amount of solar power (..which they could receive when positioned in space). In the perspective of energy received annually, the ratio is around 1:10 in favor of space; that is, a square meter in space (near Earth), locked on to the sun, receives ten times the energy per year of a square meter positioned on the best possible place on Earth's surface.
The concentration of sunlight is easier to achieve in space than on Earth's surface. In space, all of the sunlight comes from a small solid angle (from the Sun's disc), and could be easily concentrated e.g. with thin foil mirror. Such mirror could also be rather easily, inertially locked on to the Sun. On the other hand, due to scattering in the atmosphere, not all the photons hitting the Earth's surface comes from the direction of the Sun, lowering the benefit of the concentration. Moreover, a tracking system is required for a proper operation of a concentrator on Earth.
Space-based solar power would require only a negligible amount of land. The only ground-based part of Space-based solar power plant would be the receiver of the microwave or laser beam.
Transmission of the generated power from space to Earth poses a significant technological challenge. While microwave wireless energy transfer and/or laser beaming of the generated power seems theoretically possible, practical demostrations have been carried out only on a scale/distance much smaller than needed for the construction of a decently sized space-based solar power plant.
Solar wind would tend to move the solar panels. Large area solar panels would act like solar sails, they would be pushed away from the Sun by solar wind. Keeping them in the right position could prove very difficult.
Space radiation degrade solar panels. Earth's atmosphere protects the surface of Earth from various kinds of high energetic particles from space. When in space, solar panels are exposed to full amount of this radiation; both solar wind and cosmic rays degrade the performance of solar cells. This limits the choice of materials suitable for long-term deployment in space.
Lightweight, inflatable structures based on thin film solar cells may be suitable for deployment in space even with current launch systems. While solar cells presently used in space have too low specific power (power to weight ratio) to be used as a basis of an industrially sized space-based solar power farm, the research into thin film photovoltaics is making steady progress in last years, providing optimistic outlook for a significant increase of specific power in near future. Lightweight, inflatable structures based on these new thin film solar cells could make the concept of space-based solar power economically viable in the short term even without major breakthrough in space transportation.
Large scale deployment of solar panels in space is unimaginable without a major breakthrough in space transport. Current chemically powered rockets cannot be considered as a viable way to deploy solar panels in space on a mass scale. While new, possibly utilizable types of space transport are being investigated (e.g. the space elevator), they are still far from practical use.
Microwave or laser power beam could be used as a weapon. Both of the proposed power transmission systems carry a potential danger. When pointed to other place than the receiving area, the microwave or laser power beam could be easily imagined as a powerful weapon, especially when more than one large solar array would focus its beam on one place.