Solar panels rely on a steady stream of solar radiation to reach the panels and provide energy to later be turned into electricity. However, the sun's light shines directly at the Earth from one point and covers the entire globe. Due to the shape of the Earth different parts of globe receive different amounts of solar radiation depending on the season and most importantly the latitude.
It is much like shining a flashlight onto a surface. If you point the flashlight directly at a surface you get a smaller more concentrated area of light. If you tilt the flashlight the light then spreads over a larger area. The heat being radiated by the flashlight also is more concentrated along with the light on a more direct shine, while the heat on the tilted light is lessened.
In the above example the flashlight obviously represents the Sun. The direct light shines perpendicularly on the equator. This means most of the solar radiation hits more directly and in a more concentrated area near the equator. Solar panels in this region would be the most effective (discounting the extremely common storms in this region).
The farther North in latitude you travel, the less direct the shine of the Sun on the Earth. Towards the poles, where the light is almost hitting at a transverse angle, very little light is concentrated and is scattered over a much larger area. This makes the solar radiation weak and thus a very poor location for solar panels.
Longitude represents directions East or West of the Prime Meridian and have very little impact on solar panels, beyond the tilt of the Earth and the seasons. The Earth will continue to rotate, and all longitudes over the course of a year will receive equal amounts of sunlight.
In summary: the less direct the sunlight (the farther North and South of the Equator) the less powerful the solar radiation and thus the area will show less productivity with solar panels. Longitude does not really come into effect beyond local weather patterns.