The irradiation
varies with time of day but it can also vary considerably
from location to location, especially in mountainous areas.
Irradiation varies from an average of 1,000 kWh/m2 per
year for northern European countries such as Germany,
to 2,000 to 2,500 kWh/m2 per year for desert areas, as
shown in Fig 2.1 for horizontal surfaces. These variations
are caused by weather conditions and differences in the
relative position of the sun (solar elevation) in the
sky, which depends on the latitude of each location.
Although
some benefit may be obtained by tilting solar panels to
face the sun when it is not directly overhead, the improvement
is less than might be expected under these conditions
for two reasons:
Renewable Energy Sources 