The ideal situation for solar panels is facing due south, although they are effective facing anywhere between south east and south west, and they should be free of shade. For best performance they should be angled at 30 to 40 degrees – although you will still catch a reasonable level of sunlight at angles of 20 – 50 degrees.
Shade falling on the roof will reduce the performance. How much it affects it by depends on type of shading: close structures such as gable windows or chimney stacks can reduce output significantly as they will tend to throw shade on the panels for most of the day. Further away objects such as trees or a neighbouring house may not block the sun during summer but create shade when the sun is lower in the sky in winter. Having your PV panels linked to a series of microinverters (rather than a single inverter) can help address any issues with shading or where you have an array of solar panels with differing orientations.
Solar panels weigh quite a bit, so your roof must be strong enough to hold them. If you need to re-roof, you can do so using solar tiles. These are more expensive than retrofitting panels, but if you are re-roofing anyway it can be more cost-effective to re-roof with solar tiles than to use conventional tiles and put panels on top. Alternatively, panels might be mounted on a frame on the ground or a flat roof.
The size of the system installed depends on how many panels you can fit on your roof. A 3.5kWp system requires about 20m2 of roof space. Panels come in different sizes and the power of a panel is measured in watts. More than one panel makes up an array. Typically, a domestic property will have an array of up to 4kWp installed.