Roof Orientation

The direction the roof faces affects how much sunlight it will receive through the year, and how much electricity any solar PV system will generate.

South facing roofs are best, but anywhere between South East and South West facing roofs will produce almost the same amount of electricity over the year.

East and West facing roofs will produce in the region of 15-20% less electricity per year than a South facing roof, but if a larger system can be fitted on your roof then economies of scale often compensate for this and the payback times can still be good. 

More Northerly facing roofs are generally not recommended for use, although if they're at a shallow enough slope then it may be possible to generate a reasonable return from them

Roof Slope

The perfect slope angle for the panels to generate the maximum amount of electricity is around 38^o for south facing roofs, but anywhere between 30-50^o will generate almost as much over the year.

Steeper angled roofs reduce the overall performance up to a vertical wall mounted system which would perform around 30% worse than a perfectly sloped south facing roof.

Shallower sloped roofs perform less well on South facing roofs, but on East and West facing roofs, can actually improve the system's performance.