Using the initial array size and the modified insolation data from (4), the
ampere-hours generated throughout the year can be calculated. In these
calculations, allowance needs to be made for loss owing to dust coverage,
assumed to be in the vicinity of 10%, although this may be an overestimation
for the impact of dust. An Arizona study (Hammond, 1997) found that for
modules at normal incidence to the sun, soiling causes a maximum of 3% loss
between periods of rain but that the loss increased with incidence angle, to
4.7% at 24° and 8% at 58°. Bird droppings, however, can have a more serious
impact.