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Is my roof too shaded for solar PV panels?

Posted by Chris Rudge on 14 January 2010 at 7:10 am

If you are thinking of having a solar electricity  system installed in 2010 to take advantage of the last chances of getting a lump sum grant before the Feed In Tariff starts in April, the first thing to do is look for a good location to install it.

Traditionally for domestic Solar PV (photovoltaic) installations, a pitched roof is the ideal location. Look at your roof, to check that it faces somewhere between south east through south to south west.

If it does, you need to see how much open roof space there is available. Solar PV panels take up much more space than solar hot water systems. Think about 8 square metres per kWp  for system size.  Once you have checked you've got space, look for possible problems from shading. Gable windows and chimney stacks are probable problems you will have to consider. The array can never be shaded at any point in the day by close structures as output could drop dramatically.

Looking further away from your house, are there any trees close by or right next to the house which you know will shade the array at certain times of the day? Trees further away from the PV array can also have an impact. For example, a wooded area on the other side of the road may not block the sun during the summer, but may create shade all winter due to low sun angle in these colder months.

At best, shading issues will simply mean less production of electricity from your PV array at certain times of the day/ year. At worst, it will cause overheating in some of the cells as they try to carry electrical current from all the other modules. If damage occurs, your warranty will be invalid.

If you are unsure how affected you will be, ask an established Solar PV installer for an opinion at the same time as a quote.

Photo by Randy son of Robert

About the author: Chris Rudge is a qualified electrician who specialises in renewable energy.

If you have a question about anything in the above blog, please ask it in the comments section below.

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4 comments - read them below or add one

Rudge Energy

Rudge EnergyComment left on: 29 September 2011 at 12:18 pm

Hi Heather

Your right, actually installing a system onto the tin roof will be quite straightforward and cost effective. However, the 7 degree angle is less than a standard PV modules recommended self cleaning angle, which is between 10 and 15 degrees (depending on manufacturer).

The way around this will be to ask your installer to quote for the rear edge of the modules to be raised by 10 degrees.

However this cost more, need more roof space (to avoid the front module shading the one behind), and as the array will be more than 200mm above the plane of the roof need planning permission.

But.. Its all achievable at a small aditional outlay.

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heatherComment left on: 29 September 2011 at 11:44 am

I have an outbuilding with a tin roof and an angle of 7 degrees which faces SE. I have been told that it is no problem to fit a 4KW system to this single storey roof. However, i have read that the  roof angle should be between 10 and 45 degrees. Can anyone clarify?

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Cathy Debenham

Cathy DebenhamComment left on: 23 August 2010 at 8:18 am

I just received the following question:

Hi I'm thinking of installing PV. My roof facing SSE is shaded by large trees ( 20m) situated 20m away. The PV installer calls this "low to moderate shading" ( 20 -60%) and entered this on a SAP calculation. He says this will not be a problem, but he then he has different interests to me. Any ideas how I might get independent advice?

We always advise people to get three installers to quote, even if it isn't a difficult site, as that will usually reveal any significant differences in predicted generation and in price. I'd also look for one that has a gadget to measure the impact of shading.

My PV panels are on SE facing roof, with large beech trees about a similar distance away in a neighbour's garden. In addition to the SAP calculation, my installer (Abacus Renewable Energy) also used a SolarPathfinder to measure the shading throughout the year, and provided me with a solar site analysis report. This predicted the system's generation, broken down by month, with and without the shading, and if it was in the ideal position. I haven't had the system a full year yet, but so far it has generated quite a bit more than was predicted.

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ePower-SolarComment left on: 15 February 2010 at 3:53 pm

A great tool to use is Google SketUp. Its free and it allows you to import data from Google Earth and to simulate sun/shadow for every day of the year. If used correctly you can perfectly evaluate which area of your house is shaded and which is not. Your installer should be able to provide with the required data and should lay out the solapanel accordingly 

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