The most abundant source of energy in the solar system is the radiant flux from our nearest star, the Sun. The sun is a main sequence star that is undergoing a stable fusion reaction, consuming 700 million tonnes of hydrogen each second and converting it into 695.7 tonnes of helium.
But wait... Where does the remaining 4.3 tonnes of mass go each second? Well, it is converted into energy. I'd wager that you already know the formula:
$$ E = energy \; (joules)$$
$$ M = mass \; (kilograms)$$
$$ c = speed \; of \; light \; (299782458 \; m/s)$$
We can calculate the energy output of the sun by multiplying the missing mass times the speed of light squared. By doing so, you can see that the sun gives off 386 yottawatts of radiant power. (that's 386 million billion billion joules per second!) As the sun is only halfway through its lifecycle, we can expect it to produce about this much energy for the next 5 billion years, or so.
The sun is the source of all energy on earth, with just three major exceptions: 1) Geothermal energy, which is (in part) leftover heat from the formation of the solar system, 2) Tidal energy, which is caused by gravitation interaction with the moon, and 3) Nulcear energy, which is derived from mass itself (see the above formula).
Plants and phytoplankton use photosynthesis of sunlight to turn water and carbon-dioxide into sugar, thereby supporting the entire foodchain. The wind is caused by differential solar heating of the earth's surface. And fossil fuels are just collections of dead organisms that grew with the energy from the sun in the ancient past.
The first thing to know about harnessing energy from a star, is that distance matters a whole bunch. Solar energy leaves the sun in all directions, and because of this, the energy gets more diluted the further away a user gets from the source. The flux density of the solar energy is governed by the Inverse Square Law, which shows that if you double your distance away from the power source, you cut the available power to 1/4th of what it was before. This is why you will never get a sunburn from Alpha Centauri.
You can see the effect that distance has, by using the tools on this page to compare a similar photovoltaic array on Earth and Mars. You will see instantly that a solar array on Mars produces significantly less power than a similar system on Earth will produce. This is because Mars is about 50% further away from the Sun than the Earth is. Now compare that to Mercury and see the result. Cool, eh?
The second consideration to make when harnessing the power of the sun is interference. Planetary atmospheres reflect, refract, scatter, and absorb energy, especially if the atmosphere contains clouds or dust. If you are trying to collect energy from the sun, you will have better results in space than you will on the surface of a planet with an atmosphere. If you are on the surface, you can also collect more energy around noon than at sunrise, because the amount of atmosphere the light has to travel through is less.
However, not all atmospheres are created equal. As an example, you can look at Venus using the tools on this page. Since it is much closer to the sun than Earth, you may expect solar power to be a rousing success on Venus, but this is not the case. You will see that even a very large solar power array would not produce much energy on the surface of Venus. This is because only around 3% of the light that hits the planet can make it through Venus' thick atmosphere. Use the altitude selection tool to climb up through the atmosphere and you will see that power production improves significantly as you get dozens of kilometers above the surface.
The third major factor in collecting solar energy is geometry, or in other words: location, location, location. Each planet in the solar system is a (nearly) spherical object. Only one half of the planet can face the sun at a time. So any particular location on a planet can expect to spend 50% of its time in sunlight, right? Not quite. Because planets can be tilted, we have to consider the effect of seasons. Summer is when a hemisphere is pointing towards the sun, and winter is when a hemisphere points away from the sun. Earth and Mars both have a large axial tilt, and you can see the effect on the day and night cycle by looking through the months of the year. Press the "speed up" button a bunch of times to animate the map for enhanced effect.
It is important to think of your planet as a ball, rather than a flat map, because the sun doesn't strike the surface of entire planet evenly. Think of a soccer ball, with its black and white segments. If you hold the ball in front of you, each segment faces you to a different degree. Some will be facing directly at you, others you will only catch a sideways glance of. Now imagine that you are the sun, and the soccer ball is your planet. The panels that face you more directly will collect more sunlight than than others because they have a smaller angle of incidence. Because of this, locations on a planet near the north or south poles, generally do not get as much sunlight as locations nearer the equator. This can be seen by using the tools on this page to move around the station marker. Click around the map to see that extreme northern and southern latitudes produce less power.
Another aspect of geometry is orbital eccentricity, or the fact that sometimes a planet is closer to the sun at certain times of its orbit than at other times. Mars's southern hemisphere has its summer months near the planet's closest approach to the sun, the perihelion, so southern summers have more sunlight than northern summers on Mars.
Orbital eccentricty also brings an odd periodicity to the day/night cycle on Mercury. Because Mercury rotates so slowly, and orbits the sun so quickly, a year on Mercury is half the length of a solar day on Mercury! This is odd enough in and of itself, but combine this with the high eccentricity of Mercury's orbit, and we have a very interesting day/night cycle indeed. Use the tools on this page to look at the Mercurian day/night cycle by clicking the "speed up" button 22 times. You will then be looking at Mercury 4194304 times faster than normal, and you can see that Mercury's days and nights seems to come in pulses. If you were an observer on the surface, the sun would hang in the same place in the sky for a long period of time, even traveling backwards for a short time.
Fourth and finally, we can consider the design of the solar power system itself. The two factors that are directly proportional to the amount of power the system will produce is the collector area and the system efficiency. On Earth, for example, a user on the surface may expect about 1000 watts of solar energy per square meter of land when standing directly under the sun. If the user had a 25% efficient collection method, and had a collector of 1 square meter, the user would collect 250 watts of power. If an Earth user needed 10 kilowatts of power, the user would need 40 square meters of 25% efficient panels in this example.
Systems that are not directly under the sun may benefit from being set up at an angle from the ground. The rule of thumb that you will get the best results is: set up a solar array at an angle euqal to your latitude, facing due south if you are in the northern hemisphere, and directly north if you are in the southern hemisphere. Tilt is the angle measured between the ground and the panel, and azimuth is the angle you are facing on the horizon, measured in a right or clockwise turn from north.
More advanced systems will actively track the position of the sun to optimize exposure to sun (essentially keeping the angle of incidence as close to zero as possible. You can set this on this page by changing the angles of freedom for pitch and azimuth. A final advanced consideration is low sun-angle obstructions. Terrain, buildings, veggitation, and even other solar panels can cause shading on your system when the sun is low in the sky. Oftentimes it is more economical to pack in arrays close together if land is expensive, even though it cuts down on the overall efficiency.
