How roof orientation and tilt affects the output and performance of solar panels
An unobstructed, south-facing roof with an inclination of 30° from the horizontal is optimal, although solar panels can be installed at any angle, whether they are mounted on walls, or on a flat roof.
The majority of roofs in the UK have a tilt of between 30°-45°, though a roof tilt of between 0° and 50° from the horizontal should provide a good output.
As noted above, roofs facing due south are optimal, though by no means essential for solar electricity generation. The output of south-east- or south-west-facing roofs may be up to 10% lower than those that are south-facing. The output of arrays mounted on east- and west-facing roofs may be up to 20% lower.
If you have an east-west facing house, it is advisable to place one solar collector on each side of the roof, thus maximising the amount of solar radiation received throughout the day.
The Energy Savings Trust has a useful roof orientation and tilt table which shows the approximate system efficiencies on their website: