Given such figures it is quite easy to make a rough estimate of the annual
output from a PV module or array using the concept of peak sun hours. If
the energy received throughout the year is compressed into an equivalent
duration of standard ‘ bright sunshine ’ (1 kW/m 2 ), then the number of peak
sun hours is the same as the global annual figure. For example London has
about 1050 peak sun hours in a year, so a PV module rated at 200 W p can
be expected to produce around 1050 × 200 = 210 000 W h = 210 kWh per
year. However this is for a horizontally mounted module – unlikely in
London. It also assumes ideal ‘ bright sunshine ’ conditions, whereas much
of London ’ s sunlight is diffuse. Direct and diffuse light have different
spectral distributions and solar cells do not generally respond equally to
them; nor are most cells equally effi cient in bright and low - level light. So
estimates of array output based on peak sun hours are only very approximate,
especially for locations with a large proportion of diffuse light. In