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Are sunnier days always the best for solar PV generation?

Posted by James Page on 28 April 2017 at 11:35 am

People often remark that solar panels do not need a sunny day to generate. Turns out a cloudy day can sometimes generate more than a sunny one.

Three years ago, JoJu Solar installed a 10kW flat roof system on Artsadmin in Central London. It was the first project with panels facing East and West, rather than South, thereby increasing the number of panels and also the duration of generation during the day. At 10 degrees pitch, the output with low winter sun is not great, but this is more than offset in the summer.

Since the installation the skyline has changed dramatically (something we had foreseen, at least in part) with two new tall buildings in Aldgate which cause shading in the winter months, alongside an existing block. December 29th 2016 was a beautiful sunny day, but as you can see on our monitoring below, only about 2kWh was generated (on a good summer day 50kWh is often reached.) When this is compared with December 30th (an overcast day), over 50 % more was generated!

Light is a mixture of direct and diffuse rays. On the sunny day there is little diffuse light, and what there is (the mainly blue sky) is the wrong frequency for most solar panels.

So what are the lessons here? One thing, Solar generation is tough in London, at least until such time as planning officers consider things a little more. Micro inverters or Solaredge could be considered, although the shading objects are some distance away, so the benefits may not be large. If the shading was caused by a chimney on the building itself, some panels would be shaded for long periods and micro inverters would help. For the Artsadmin building on an average day things don't look so bad, and over the year generation wasn't far off the prediction. Even every London cloud has a silver lining.

Image credit: James Page, 2014

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About the author: James Page is a chartered engineer and is head of engineering at Joju Solar. All views expressed are his own. He stores solar energy under the kitchen floor.

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