Sunlight As An Energy SourceSpacecraft are usually designed with solar panels that can always be pointed at the Sun with the rest of the body of the spacecraft aimed independently. A tracking mechanism is often incorporated into the solar arrays to keep the array pointed towards the sun.
Surface AreaSolar panels need surface area, more exposure means more electricity can be converted from light energy. Satellites will purposefully orient panels out of direct alignment with the Sun. If the electricity needed is lower than the amount of electricity made, the extra power needs to be vented by a shunt into space to dispel heat.
DurabilitySolar panels are very hardy compared to alternative power sources. They wear out very slowly in space. The principal factor affecting a loss in power is solar radiation. Low Earth orbiting satelittes have their effectiveness decreased by one to two percent a year. A higher radiation environment (i.e. mid-altitude Earth orbits) can cause solar arrays to lose half their power within one year.
Photovoltaic concentrator solar arrays used for primary spacecraft power use lenses called Fresnel lenses that take a large area of sunlight and direct it towards a specific spot by bending the rays of light and focusing them. The same principle as using a magnifying lens to focus the Sun's rays on a pile of kindling or paper to start fires. Solar concentrators put one of these lenses over every solar cell. This technology allows costs to be cut significantly due to the utilization of less material.