Solar Power – Reasons for Low Output

For each kw of installed capacity of solar panels on average you can expect to generate 3.5kwhrs per day. This can however vary considerably for a variety of reasons.

Monthly Variation

On a peak summer day you may be able to generate 6 or 7kw hrs per day.

In winter it can be quite different. typical reasons include:

  • Shorter Days – The panels may only get around 6 hours of sunlight.
  • Weather – For most of us winter means more clouds and rain which reduce the sunlight.
  • Less than Optimum Angle With the sun low in the sky you won’t have the panel fully facing the sun unless you can change the angle.

Partial Shading

One of the things that most people don’t realise is that solar panels are ‘team players’.

The power that each panel in a string(circuit) generates is limited by the performance of the worst performing panel in the string.

This means that if one 200w panel in a string of 6 is shaded by as little as 10% the loss can be 120w.

That’s 10% of the total string 6 x the 20w of the single panel)

To minimise this problem you need to ensure:

  • All obstructions are minimised. Even a TV aerial can make a difference.
  • If you have panels on different areas ( for example on each on a different roof) they should be on different strings with separate inverter inputs.


Any dirt on the panels will reduce the effectiveness so a regular clean will help retain efficiency. Its also worth thinking about keeping birds from perching on the top edge and crapping on the panels. (You can get some plastic spikes to fasten to the panels)


Typically panels are at their highest efficiency at a panel temperature of 25degees C. In the middle of summer the panel temperature can be 80 degrees. If you panel is mounted to allow air movement around the panel this will help control the temperature and help efficiency.


See Solar Electricity for more posts