What makes a site suitable for solar energy?

Solar PV arrays can be mounted in many places and a large percentage of buildings are suitable for Solar PV. So what are the criteria for determining is a site is suitable?

PV arrays can be mounted in a range of locations but the roof of a building usually provides the best and simplest site.
The ideal roof is one that faces south with a slope of around 30-40°. However, other orientations and pitches can be viable with relatively little drop in performance (see table). North or northerly facing solar arrays will still generate electricity but the returns will be very much lower and usually not cost effective.
PV systems are modular, allowing a great degree of flexibility in design and specification. In practice, the size of a system is often determined by the available roof space. A standard 1kWp PV array will occupy at least 8m² of roof. The diagram shows the range of sites and orientations that can produce an effective Solar PV system.
Any shade, such as from trees or neighbouring buildings, can make a large impact on the performance of a PV system. To work effectively, the whole PV array needs to be free from shade for the majority of the day. Do you have any chimneys, trees, or surrounding buildings that cast a shade over your roof?
Although Solar PV systems are not normally heavy enough to cause structural problems, if your roof is in poor condition you should undertake a structural survey before proceeding with your installation.

Put simply, suitable sites generally face anywhere from South East to South West and are free from shade for most of the day. Problem sites are those that are east, north or west facing, are shaded by trees or buildings, are on north facing slopes or in valleys or that have problematic roof strutures, such as roofs in need of strengthening or repair or those clad in asbestos.