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Accuracy of predicted solar PV generation set to increase

Posted by Cathy Debenham on 2 April 2012 at 9:09 am

The amount of sunshine in Sheffield is currently used as the basis for all predictions of solar generation, where ever you live. This means that people in the south are likely to be delighted when their new system produces more electricity than promised. The outcome isn't so good for people in northern Scotland, who may find their actual output is lower than promised.

The good news is that new standards are due to be introduced with the publication of the third edition of the PV installation guide by the DTI. It is expected to be available from the end of April.

The new system, which will be aligned with SAP 2012, will use the Met Office's solar radiation data. This means that installers will be able to do a postcode search and make much more accurate predictions of how much electricity a system will generate. There will also be improved allowance for the pitch and orientation of the panels.

The new guide will also introduce a more rigorous system for estimating the impact of shade on a PV array. Installers will have to draw a horizon line and map the impact of any shaded areas.

The result: people investing in solar PV will be able to calculate the costs and returns of their investment more accurately, wherever they live.

Click on the picture to enlarge it.
Contains public sector information licensed under the Open Government Licence v1.0


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6 comments - read them below or add one


HiltingburyComment left on: 19 April 2012 at 7:39 pm

I have just tried the Marble Solar model. It is very smart and easy to follow but would be pretty optimistic in my case because the basic calculation does not take shading into account. If you then go on to edit your input there is a shading frig factor you can enter on a scale from 1 to 10 but who knows what to set it at? 

That is why I recommended this more precise model  which seems to take everything into account, even the drop in output caused by higher ambient air temperatures in the middle of the summer.

It does not compute payback periods but it does predict output kWh for each hour of the day, for each month and for the whole year. It is easy to calculate your income from there, at whatever tariff rate you currently enjoy.

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Marble Solar Ltd

Marble Solar LtdComment left on: 17 April 2012 at 3:55 pm

With the last changes in Feed in Tariffs, people want to know with even more accuracy how much electricity they are going to produce and also how long it's going to take for them to get their investment back.

For this reason we have developed an income estimation tool, that takes into account the geographic location, roof orientation, pitch and other factors to give a more accurate estimation than the basic SAP calculations.

The tool allows the user to draw an outline of their roof on Google Maps, and then find out how many panels are needed to fill the space selected, how much energy would be produced each year, how much income would be earned from the Feed in Tariff, and the total monetary benefit to the user of installing the system.

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banjaxComment left on: 11 April 2012 at 5:50 pm

Would this have implications for  the accuracy of solar thermal generation?

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HiltingburyComment left on: 3 April 2012 at 3:16 pm

I welcome the change. As a simple customer who has significant trees to the south of my array I am finding this model to be rather good:

As you can see it does everything you mention, Cathy.

Of course I also have the difficulty of drawing the horizon from the viewpoint of the array. Now this model allows you to plot daily, monthly and yearly outputs. So I guessed the horizon and compared it with my daily generated profile in Feb and March, adjusting the shape of the horizon file until it matched the shape of the daily output from the inverter. This won't help anyone to predict the output before the system is fitted but may enable many new entrants to better predict their output

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Solar Panels Cambs Ltd

Solar Panels Cambs LtdComment left on: 3 April 2012 at 11:16 am

For the irradiance element of the revised calculation you can use the Met offices data now, without waiting for this to be published.

The Met office have stations all over the UK that record sunshine hrs.  Anyone can access the historic station data from abour 15+ of these station at no charge by following this link:  The Cambridge office shows the average No of sunshine hours 2000 - 2009 to be 1522 hrs a year vs. SAP 1053.  This differance has a significant impact on the ROI / business case for both domestic and commercial proposals and has helped us win business. 

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Rudge Energy

Rudge EnergyComment left on: 2 April 2012 at 8:31 pm

This is something we have been sending to our customers in addition to the standard misleading (but safe in the South West), SAP figures.

Surveyors will need to take more information on site now. Sometimes horizon data is difficult to obtain without the use of a ladder and precarious balancing with a camera, etc. 

Now that the FiT has reduced, for some people who are looking at the investment element rather than the Planet will need to know their installation ticks all the rate of return boxes.

Remember that even with the new EPC requirement, falling PV panel costs are making systems even more affordable for people that found the concept of a PV system right out of their budget only a year ago!


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