Thursday, 10 January 2013

Reduced Solar Output from West facing installations in Winter.

Some of you may have looked at "Electricity and Income Generated" page, Link to page. and noticed I'm very interested in comparing output from different installations. The formulae I use enables systems which are different sizes or facing in different directions to be compared with others.

When I first asked people to send in their monthly output figures I assumed it would highlight the differences in panel/inverter efficiencies. The surprising thing is it seems to reveal a significant difference in MONTHLY efficiency between South and West facing systems.

I suspect that the majority of installations in the UK have a roof-top mounted system facing South? 

The government formulae used to predict yearly output generation places a premium on Southerly facing installations, they indicate a 15% yearly drop in output from a West/East facing system in comparison with a South facing installation. 
I assumed that the percentage drop in output would be a linear reduction spread over each month of the year. This doesn't appear to be so...

I'm interested in the results shown above which compare two systems, one of which is South facing and rated at 2,185 kWh and the other larger system rated at 2,922 kWh which faces West.
Unsurprisingly, the larger system generates a higher output during the bright Spring and Summer months. The surprise comes during the Autumn and Winter months when the smaller South facing system consistently outperforms the larger West facing installation.

Do you have a East or West facing system?

I would be really interested if your output figures confirm the graph above. 

If you record your monthly output or even if you have your quarterly figures which you send to your Fit provider. Could you email me some information and you could help solve this mystery.

  1. How big is your system. (Number of panels + panel Wattage.)
  2. What direction does it face?
  3. What is the slope of your roof. (Normal is 30 degrees.)
  4. Is your roof in shade for part of the day?
Please email me your output and system details to

Thank you in advance.

Twitter: (@solaricarus)

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