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UK Solar PV flyer - snake oil or just sloppy?

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Posted by Jim Backus on April 18, 2010, 9:09 pm
A leaflet delivered recently included the following:

"The Department of Energy and Climate Change will apply the following
tarriff for feed in:

    41.3p for every unit generated by your solar PV equipment
    plus 0.03p for every unit exported to the National Grid

What does this mean to you in real terms?

    Typically a PV solar installation of 3kW will generate approx 2.57kW
of power.
    41.3p x 2570W will mean a total annual generation cash back payment
of GBP1,061.

    If 1.28kW of the 2.57kW generated is not consumed by you in the home
and is
    exported to the grid you will receive and additional payment of

It continues in similar vein.

The first thing I notice is that the author uses 'kW' and 'unit'
interchangeably. A unit in the UK
is 1kWh. It may be that this is deliberate to avoid confusing the lay
reader but it sure
confuses me!

So working back from the cash back payment of GBP1061 and the per unit
rate of 41.3p this
implies that a 3kW installation is capable of generating 2,569kWh over
a year. Alternatively,
assuming an average of 12 hours 'sun-up' daylight per day over a year
this suggests an
average generation of 586W.

Question 1: is this a realistic output for the UK bearing in mind that
the latitude of the UK is between 50 & 60 degrees North?

The second point concerns the power exported to the National Grid. If
we assume that the 1.28kW quoted means 1,280kWh then the additional
payment should be GBP0.38.

Question 2: has my maths failed me or has the leaflet calculated the
additional payment based on GBP0.03 instead of GBP0.0003?

All comments gratefully received - TIA

Jim Backus running OS/2 Warp 3 & 4, Mac OS X and Win98SE
bona fide replies to j <dot> backus <the circle thingy> jita <dot>
demon <dot> co <dot> uk

Posted by nikcross on April 19, 2010, 9:30 am

Answer 1.

We (RenSMART Ltd.) have developed an on line tool to model solar pv in
the UK and give estimated output. It's free for anyone to play  with

I used it to give estimates for solar panels covering 4m x 6m (3.1kW
peak) on a south facing pitched roof. Using our tool you can place the
panels anywhere in the UK and it will look up the average solar energy
arriving and how much could be converted to electricity.

In Exeter the estimated yearly generation is 3000kWh and in 2400kWh

Installers that are MCS certified, as they must be for their
installations to receive feed-in tariffs, must use a standard
calculation to estimate energy produced by solar pv. It is documented
in SAP2005 which you can download here http://projects.bre.co.uk/sap2005/

The calculation is:
0.8 x kWp x S x Zpv

Where kWp is the peak output of the system, S is the annual solar
radiation and Zpv is an overshading factor.
They use the same solar radiation value for the whole of the UK
(2.8kWh/meter squared/day) The RenSMART model is independent of this
calculation and uses a solar map called PVGIS to give a different (we
believe better :-) ) estimate. We also publish an interactive solar
map of the UK http://www.rensmart.com/Weather/PVGISSolar

Answer 2.

The UK Feed-In tariff 'The Clean Energy Cashback scheme' stipulates a
minimum export tariff of 3p per kWh exported. The producer of your
flier made the mistake of suffixing with p rather than prefixing with

Apologies for the blatant plugs but I hope the information helps.

Posted by Jim Backus on April 19, 2010, 5:37 pm
 On Mon, 19 Apr 2010 09:30:25 UTC, nikcross

Thanks for your reply and Jean Marc's

It looks as though the flyer was more sloppy than deliberately
misleading; as an engineer these mistakes are very apparent. However
it is probably misleading to quote the benefit from the export tariff
when the average UK household consumption is significantly more than
would be generated by a 3kW array.

I'll have a look at the links offered. My house has a generally south
facing pitched roof and might accomodate both solar PV and solar hot
Jim Backus running OS/2 Warp 3 & 4, Mac OS X and Win98SE
bona fide replies to j <dot> backus <the circle thingy> jita <dot>
demon <dot> co <dot> uk

Posted by Jean Marc on April 19, 2010, 9:46 am

PV Estimation Utility




Posted by stevey on April 21, 2010, 7:52 am

There is no confusion for me.  I believe this is the British version
of FIT--
Feed-in-Tariff.  In other words the UK government will pay for the
you generate from your PV at 41.3 pence per kWh, and in addition, if
you have excess to sell to the grid at .03 pence per kWh.


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