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
  • Climate/Weather
  • Panel Efficiency
  • System Design




Roof Angle

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.

Panel Efficiency

UK Solar Power Panels Logo

Probably the best panel on the market is the Sanyo HIT 250W hybrid panel that has a 17.1% efficiency which produces about 10% more electricity a year more than our standard panel, the Upsolar 190W (14.9% efficiency).

For a 4KW system, the Sanyo's cost would be £3,000 more. From a 25 year investment point of view, Upsolar and Sanyo systems work out about the same as the feed-in-tariff is paid irrespective of actual power output.

Roof Direction

direction of roof

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.

Daylight Levels

daylight hours

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.

View ther UK irradiance map

System Design

solar panel 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).

Light Levels

solar panels design

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.



weather symbols - climate

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%.

UK Solar Power

solar design document

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