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What to expect from your PV panels throughout the year

Posted by Cathy Debenham on 4 February 2011 at 9:20 am

We're often asked how much electricity you should expect to generate from a solar PV system, and how much it will vary through the year. I'm going to use my 2.1kWp system as an example to give you an idea of what you might expect.

Our system is mounted on the garage roof. It's a steep (45 degree), south east facing roof, and it is shaded for most of the morning in winter when the sun is low.

My installer (Abacus Renewable Energy) did a prediction of the amount the system would generate based on the Standard Assessment Procedure (SAP) for energy rating. This is the system that all MCS accredited installers are expected to use. He also used a gadget called a SolarPathfinder which gave a range of data, for monthly generation with and without shading, and if the roof were ideal (ie a slope of 38 degrees).

These were the predictions:

SAP                              1672 kWh per year
Actual with shade         1,589
Actual without shade    1,852
Ideal                              1,948

We're currently six days short of a year's generation and the system has produced 1,765 kWhs, so we're very happy to have generated more than predicted.

Not surprisingly, given the differing length of day and strength of sun, we do much better in summer than in winter. The best month for us is May with a predicted generation between 215 and 240 kWhs per month (figures with and without shade). This contrasts sharply with the December figure of estimate of between 36 and 45 kWhs.

We're based in the warm, sunny (wet) south west, so, as you go north, you'll get less sun, and correspondingly less generation.


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

Cathy Debenham

Cathy DebenhamComment left on: 31 January 2012 at 11:37 am

That's a pleasure Ken - I'm glad it was helpful

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FagashkenComment left on: 30 January 2012 at 6:13 pm

Hi Cathy,

Very interested with reading your findings on your installation of P/V panels. I have had 4kW fitted to my direct south house roof in Cambs,

The cost was £17.000 and was fitted just under three months ago I have been checking the progress of output and was a little disappointed with the kW it has been producing, i/e 350 kW I then read your findings which has settled my mind that it will do much better in the spring & summer months god willing ?

You never know with these new fangled products & being an older person it tends to concern you a lot more.

So well done for the info

Best wishes

Ken H 

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Cathy Debenham

Cathy DebenhamComment left on: 13 April 2011 at 8:50 am

The capital cost was £9,163 (I was lucky with my timing & got a grant of £2,100 bringing it down to £7,063).
Generation tariff: 1,765kWp @ 41.3p = £730
Exported: 882 kWp @ 3p = £26 (deemed at half, but actually exported c.1,200 kWp)
Saved from bills: 500kWp @ 14p = £70
Total annual income/savings: £826

Payback: 11 years without grant; 8 and a half years with grant.

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BEATComment left on: 1 April 2011 at 4:35 pm

Can you put the costs and a payback in here? Would be interesting to see -

At a guess £200 from export and savings off bill, and £730 from FIT - totalling £930? Total capital cost £10k?

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