This time last year (May 6th) I was delighted to report that my system had passed the the official MCS output prediction figure a full month before it was due. Link to original piece
This year, despite the awful weather in 2012 and resultant lower monthly generation, I've achieved the magic figure of 2185 kWh on May 12th, just four days later.
I ought to be pleased, but the changes to the MCS regulations which are used to give customers an accurate prediction of future output have changed. Significantly...
This year, despite the awful weather in 2012 and resultant lower monthly generation, I've achieved the magic figure of 2185 kWh on May 12th, just four days later.
I ought to be pleased, but the changes to the MCS regulations which are used to give customers an accurate prediction of future output have changed. Significantly...
Old formula
All of us who have had systems fitted since April 2010 have had the following formula used to predict output.
The formula was: 0.8x KWp x Solar radiation x shading factor
0.8 multiplied by the panel capacity in kilowatts peak (kWp) multiplied by the amount of solar radiation in kilowatt hours per sq m per year (KWh/m²/year) for a given roof orientation and inclination multiplied by the extent of shading.
My details:

14 Schuco PV panels rated at 190w: 14 x 190 = 2660 divided by 1000 = 2.66kWp

*My roof has a 30° slope and faces SE. = 1027

**I have no shade on the roof at any time of the day. = 100%

0.8 x 2.66 x 1027 x 100% = 2185 kWh/year 
The tables below show solar radiation for different roof inclinations and orientations and the multiplying factor for different degrees of shading.
*Solar output in kWh/m²/year
Roof slope

South

SE/SW

E/W

NE/NW

North

30°

1073

1027

913

785

730

45°

1054

997

854

686

640

60°

989

927

776

597

500

Horizontal

961

961

961

961

961

Vertical

746

705

582

440

371

**Shading multiplication factor
Shading description

% of sky blocked by obstacles

Multiplication Factor

Heavy

>80%

50%

Significant.

>60% – 80%

65%

Modest

20% – 60%

80%

None or very little

< 20 %

100%

0.8 x 2.66 x 1027 x 100% = 2185 kWh/year
Using my details you can see the formula arrives at a predicted yearly output of 2185 kWh.

New Formula
Now comes the complicated part...
From May 2013, installers must use the revised MCS guide formula. As you will see from the map below, they have addressed the obvious flaw in the previous calculation (which were based on Sheffield's figures) each region now has it's own level of sunshine/daylight. The figures on each area show how they've altered from the previous expectation. I live in the East Midlands and my figures are predicted to be 8% higher.
The new formula also takes into account the "shading factor" in a much more scientific way. To calculate the output projected for a system installed from May 2013 is much, much, more complicated, but essentially the same!
Standard Estimation Method
The approach is as follows:
1. Establish the electrical rating of the PV array in kilowatts peak (kWp)
2. Determine the postcode region
3. Determine the array pitch
4. Determine the array orientation
5. Look up kWh/kWp (Kk) from the appropriate location specific table
6. Determine the shading factor of the array (SF) according to any objects blocking the horizon  using shade factor procedure set out in MCS guide.
The estimated annual electricity generated (AC) in kWh/year of installed system shall then be determined using the following formula:
If you're interested in calculating your figures, you'll need to use this link to see the numerous charts and shade guides.
kWp: 14 Schuco PV panels rated at 190w: 14 x 190 = 2660
Kk: Region 6 chart, 30 degree roof slope, SE facing = 928
SF: I have no shade at any time on roof.
Annual AC output (kWh) = kWp x Kk x SF
2660 x 928 x 0 =2468 kWh annually.
(Using my figures once again to give an indication of the changes involved.)


It's all change...
As you can see, my delight at achieving 2503 kWh in 20112012 seem a little premature now the revised and probably more accurate formula is used!
Instead of a whopping 14% improvement on the targeted 2185 kWh, I' m now left with a measly 1.4% gain.
I suspect I'll do very well to match the new predicted output figures for my 20122013 output, but I'm still glad I made the decision to install PV panels on MY roof in June 2011.
How about YOU?
Icarus
Twitter: (@solaricarus)
Very interesting! I have applied the new formula to my figures after reading your blog. Here are the results...
ReplyDeleteOld Formula
*Solar output in kWh/m²/year, South facing
30° angle
=1073
*Shading multiplication factor, none or very little
1.0
This Old Formula: 0.8 x 3.92 x 1073 x 1.0 = 3365 kwh
New Formula
From May 2013, installers must use a revised MCS guide formula. Each region now has its own level of sunshine/daylight. The figures on each area show how they've altered from the previous expectation. I live in the West Pennines and my figures are predicted to be the same. The new formula is therefore:
Standard Estimation Method
The approach is as follows:
1. Establish the electrical rating of the PV array in kilowatts peak (kWp) = 3.92
2. Determine the postcode region M29 = West Pennines [zone 7E]
3. Determine the array pitch 22.75° = 844
4. Determine the array orientation = South
5. Look up kWh/kWp (Kk) from the appropriate location specific table
6. Determine the shading factor of the array (SF) according to any objects blocking the horizon  using shade factor procedure set out in MCS guide.
The estimated annual electricity generated (AC) in kWh/year of installed system shall then be determined using the following formula:
[use this link to see the numerous charts and shade guides]
http://www.microgenerationcertification.org/images/PV%20Book%20ELECTRONIC.pdf
kWp: 3.92
Kk: Region 7E chart, 23 degree roof slope, SOUTH facing = 844
SF: Beyond 10m away there *may* be some shade factor from tall trees. A shade assessment has NOT been undertaken using the standard MCS procedure but it is estimated that this method would yield results within 10% of the actual annual energy yield for most systems. For the moment I have attributed 1.0 or 'no shade' as the figure used.
Annual AC output (kWh) = kWp x Kk x SF
3.920 x 844 x 1.0 = 3308 kWh (a 1.7% reduction on previous MCS estimate so hardly ‘no change’!)
Actual output figures [1 Jan – 31 Dec]
2012 3179 (3.9% less than estimated output 3308)
2013 3391 (2.5% more than estimated output 3308)
The original formula estimated 3365. My 2012 and 2013 results according to that formula were:
2012 3179 (5.5% less than estimated output 3365)
2013 3391 (0.8% more than estimated output 3365)
Conclusion: the new MCS formula seems to position me more accurately. The estimate it gives is almost central to the figures I achieved in two complete calendar years – so far. The (potentially minimal) shading calculation may thus ultimately make little difference.
Hi Crisso.
DeleteI agree. Despite the physiological boost given by the inflated old method from MCS, it must be more sensible to have an accurate regional projection.
In reality it's all water under the bridge! We've made the decision, paid the money and live with the decisions we made.
I'm pleased I made the move to solar and if nothing else it's given me a new interest !
It's interesting that your "new" projection differed so little from the original figure.
DeleteI suspect it was because the original figures were gathered in Sheffield. http://www.solarpowerportal.co.uk/editors_blog/at_a_glance_what_the_updated_mcs_pv_guide_means_for_installers
Mine are VERY different yet I'm only 74 miles down the M1. Just think what a difference they must make if you live even further South.
Very interesting post.
ReplyDeleteDo you have any legal obligation to present to the grid operator or other the production forecast?
Both as a particular user and as an industrial power plant?
There isn't any legal obligation to provide forecasts, but when you make your initial application for FIT payments the provider will apply the relevant formulae to check that your subsequent quarterly readings seem correct. They will verify these readings when they make the bi annual meter reading at your property.
ReplyDeletethanx, but what about roof pitch chart
ReplyDelete