There are 7 main factors that affect how much power your installation can generate:
Direction of the roof
Light Levels
Angle of the roof
Local daylight intensity at your location
Panel Efficiency
System Design

Roof Direction

In the British Isles the best orientation is South facing. In fact slightly to the West of that by 5 to 7 degrees.

East and West facing roofs can lose 10 - 20% of their effectiveness and North facing roofs a little more. Even northerly orientations are still worthwhile as the panels still work under daylight conditions.

Light Levels

When light levels fall below a certain threshhold then the power output of a solar cell falls rapidly. Better quality panels will continue producing power in lower light levels so they produce electrcity earlier in the morning, later in the evening and during particularly overcast days.

Roof Angle

Obviously the best angle is pointing directly at the sun but motorised systems that track the movement are not economically worthwhile.

Daylight solar panels operate on overall lighting levels, so anywhere between 25 and 70 degrees from the horizontal will work well. Beyond these limits you can lose up to 20% of the solar panels effectiveness.

Daylight Levels

The further South you are the less atmosphere there is to block the sun's rays. Also the thickness and nature of the atmosphere is important. In the British Isles the nearer the South West you are then the more power you can generate. Solar panels still work in the North but not as efficiently.

Climate / Weather

The length of the day, the number of hours of sunshine, cloud cover and the number of rainy days, all affect the amount of power output from the solar panels.

These seasonal and climatic changes are no longer as important because the design of modern daylight solar panels has improved so much. However, these factors can affect overall power output by as much as 30%.

Panel Efficiency

We monitor the solar market place daily and select the most efficient and cost effective solutions for our customers specific needs. At present we are favouring Jinko solar panels due to their high efficiency rating and cost. Jinko are one of a few solar panel companies that manufacture their own solar panels.

System Design

Design is very important. The solar panels are strung together by wires. Each string is only as powerful as its weakest link as they are connected in parallel. If any panel is not working optimally then the overall power drops, like a kink in a hosepipe. The inverter installed in your loft which collects the electricity, typically has two inputs. This allows you to mix panels of two different sizes in two separate wiring looms (strings).

UK Solar Power

View one of our design overview sheets; it explains this in more detail.