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Our solar photovoltaic (PV) system was installed in February 2010, and features 28 hybrid solar panels, arranged in two arrays covering around 35 m2 of our main office roofs. The arrays face south-east and south-west respectively. The system has a rated capacity of approximately 5.9 kWp - this being the maximum instantaneous output under ideal conditions.
The system was made possible thanks to the former Low Carbon Buildings Programme (Phase 2) and generous grant funding from the ScottishPower Green Energy Trust.
The predicted energy yield of the system, based on known panel performance and orientation, was 5,036 kWh per year - a specific output of 856 kWh/kWp. The PV system reached that milestone more than 3 months ahead of schedule in its first year, and has now comfortably exceeded it in both full years of operation. Specific output of 950 kWh/kWp was achieved in year 1, and an even better 952 kWh/kWp in year 2.
Performance over this spring and summer has been disappointing, in line with the very unsettled weather. Total output is about 10% behind where it was in the previous 2 years, but there's still time to catch up as the weather improves. Typical year-to-year variations can be up to 15%, so the overall tight consistency in our first two yearly totals is perhaps coincidental. In terms of high-output weeks, it is notable that 2011 yielded only one week with output above 200 kWh; in 2010, there were 4 such weeks, and in 2012, there has been just one.
| Week Ending (FRI) | Cumulative Output | Weekly Output |
|---|---|---|
| 26 Feb | 78 kWh | 63 kWh |
| 05 March | 198 kWh | 120 kWh |
| 12 March | 310 kWh | 112 kWh |
| 19 March | 438 kWh | 128 kWh |
| 26 March | 526 kWh | 88 kWh |
The graph below shows the output curves on a near-cloudless sunny day, highlighting the time lag between the peaks of the two arrays, which are oriented 90° apart.
| Week Ending (FRI) | Cumulative Output | Weekly Output |
|---|---|---|
| 02 April | 614 kWh | 88 kWh |
| 09 April | 765 kWh | 151 kWh |
| 16 April | 927 kWh | 162 kWh |
| 23 April | 1,142 kWh | 215 kWh |
| 30 April | 1,301 kWh | 159 kWh |
| 07 May | 1,442 kWh | 141 kWh |
| 14 May | 1,596 kWh | 154 kWh |
| 21 May | 1,794 kWh | 198 kWh |
| 28 May | 2,023 kWh | 229 kWh |
| 04 June | 2,207 kWh | 184 kWh |
| 11 June | 2,336 kWh | 129 kWh |
| 18 June | 2,531 kWh | 195 kWh |
| 25 June | 2,769 kWh | 238 kWh |
The graph below shows the pattern of daily output during April and May, both of which have been months of above-average sunshine.
| Week Ending (FRI) | Cumulative Output | Weekly Output |
|---|---|---|
| 02 July | 2,992 kWh | 223 kWh |
| 09 July | 3,180 kWh | 188 kWh |
| 16 July | 3,327 kWh | 147 kWh |
| 23 July | 3,487 kWh | 160 kWh |
| 30 July | 3,626 kWh | 139 kWh |
| 06 Aug | 3,747 kWh | 121 kWh |
| 13 Aug | 3,870 kWh | 123 kWh |
| 20 Aug | 4,001 kWh | 131 kWh |
| 27 Aug | 4,110 kWh | 109 kWh |
| 03 Sept | 4,296 kWh | 186 kWh |
| 10 Sept | 4,399 kWh | 103 kWh |
| 17 Sept | 4,512 kWh | 113 kWh |
| 24 Sept | 4,598 kWh | 86 kWh |
The graph below shows the pattern of daily output from April to August.
By early August, the total projected output was around 3,170 kWh, but the system had actually achieved 3,800 kWh - nearly 20% ahead of target!
| Week Ending (FRI) | Cumulative Output | Weekly Output |
|---|---|---|
| 01 Oct | 4,669 kWh | 71 kWh |
| 08 Oct | 4,777 kWh | 108 kWh |
| 15 Oct | 4,840 kWh | 63 kWh |
| 22 Oct | 4,947 kWh | 107 kWh |
| 29 Oct | 5,025 kWh | 78 kWh |
| 05 Nov | 5,069 kWh | 44 kWh |
| 12 Nov | 5,119 kWh | 50 kWh |
| 19 Nov | 5,171 kWh | 52 kWh |
| 26 Nov | 5,213 kWh | 42 kWh |
| 03 Dec | 5,243 kWh | 30 kWh |
| 10 Dec | 5,277 kWh | 34 kWh |
| 17 Dec | 5,300 kWh | 23 kWh |
| 24 Dec | 5,306 kWh | 6 kWh |
| 31 Dec | 5,322 kWh | 16 kWh |
| Week Ending (FRI) | Cumulative Output | Weekly Output |
|---|---|---|
| 07 Jan | 5,334 kWh | 12 kWh |
| 14 Jan | 5,374 kWh | 40 kWh |
| 21 Jan | 5,421 kWh | 47 kWh |
| 28 Jan | 5,442 kWh | 21 kWh |
| 04 Feb | 5,488 kWh | 46 kWh |
| 11 Feb | 5,530 kWh | 42 kWh |
| 18 Feb (First Anniversary) | 5,591 kWh | 61 kWh |
| 25 Feb | 5,629 kWh | 38 kWh |
| 04 March | 5,692 kWh | 63 kWh |
| 11 March | 5,803 kWh | 111 kWh |
| 18 March | 5,912 kWh | 109 kWh |
| 25 March | 6,073 kWh | 161 kWh |
The above readings are taken at the end of the working week, at around 5pm each Friday. The graph below shows the annual pattern of weekly output during 2010.
| Week Ending (FRI) | Cumulative Output (kWh) | Weekly Output (kWh) |
|---|---|---|
| 01 April | 6,171 | 98 |
| 08 April | 6,319 | 148 |
| 15 April | 6,482 | 163 |
| 22 April | 6,675 | 193 |
| 29 April | 6,852 | 177 |
| 06 May | 7,074 | 222 |
| 13 May | 7,233 | 159 |
| 20 May | 7,391 | 158 |
| 27 May | 7,575 | 184 |
| 03 June | 7,701* | 126* |
| 10 June | 7,861 | 160 |
| 17 June | 8,016 | 155 |
| 24 June | 8,180 | 164 |
| * electrical fault resulted in lost output during this week | ||
| Week Ending (FRI) | Cumulative Output (kWh) | Weekly Output (kWh) |
|---|---|---|
| 01 July | 8,344 | 164 |
| 08 July | 8,514 | 170 |
| 15 July | 8,698 | 184 |
| 22 July | 8,817 | 119 |
| 29 July | 8,987 | 170 |
| 05 Aug | 9,134 | 147 |
| 12 Aug | 9,275 | 141 |
| 19 Aug | 9,398 | 123 |
| 26 Aug | 9,544 | 146 |
| 02 Sept | 9,678 | 134 |
| 09 Sept | 9,777 | 99 |
| 16 Sept | 9,912 | 135 |
| 23 Sept | 10,014 | 102 |
| 30 Sept | 10,151 | 137 |
| Week Ending (FRI) | Cumulative Output (kWh) | Weekly Output (kWh) |
|---|---|---|
| 07 Oct | 10,278 | 127 |
| 14 Oct | 10,347 | 69 |
| 21 Oct | 10,447 | 100 |
| 28 Oct | 10,529 | 82 |
| 04 Nov | 10,587 | 58 |
| 11 Nov | 10,615 | 28 |
| 18 Nov | 10,664 | 49 |
| 25 Nov | 10,690 | 26 |
| 02 Dec | 10,729 | 39 |
| 09 Dec | 10,773 | 44 |
| 16 Dec | 10,806 | 33 |
| 23 Dec | 10,842 | 36 |
| 30 Dec | 10,859 | 17 |
| Week Ending (FRI) | Cumulative Output (kWh) | Weekly Output (kWh) |
|---|---|---|
| 06 Jan | 10,890 | 31 |
| 13 Jan | 10,933 | 43 |
| 20 Jan | 10,986 | 53 |
| 27 Jan | 11,027 | 41 |
| 03 Feb | 11,098 | 71 |
| 10 Feb | 11,119 | 21 |
| 17 Feb (Second Anniversary) | 11,184 | 65 |
| Year 2 beat last year's already-impressive total by just 10 units, for a specific output of 952 kWh/kWp! | ||
| 24 Feb | 11,254 | 70 |
| 02 Mar | 11,349 | 95 |
| 09 Mar | 11,465 | 116 |
| 16 Mar | 11,543 | 78 |
| 23 Mar | 11,680 | 137 |
| 30 Mar | 11,866 | 186 |
| Week Ending (FRI) | Cumulative Output (kWh) | Weekly Output (kWh) |
|---|---|---|
| 06 April | 11,972 | 106 |
| 13 April | 12,093 | 121 |
| 20 April | 12,256 | 163 |
| 27 April | 12,390 | 134 |
| 04 May | 12,456 | 66 |
| 11 May | 12,591 | 135 |
| 18 May | 12,761 | 170 |
| 25 May | 12,947 | 186 |
| 01 June | 13,142 | 195 |
| 08 June | 13,230 | 88 |
| 15 June | 13,358 | 128 |
| 22 June | 13,518 | 160 |
| 29 June | 13,670 | 152 |
| Week Ending (FRI) | Cumulative Output (kWh) | Weekly Output (kWh) |
|---|---|---|
| 06 July | 13,795 | 125 |
| 13 July | 13,939 | 144 |
| 20 July | 14,066 | 127 |
| 27 July | 14,302 | 236 |
| 03 Aug | 14,449 | 147 |
| 10 Aug | 14,598 | 149 |
| 17 Aug | 14,710 | 112 |
| 24 Aug | 17,834 | 124 |
| 31 Aug |
The chart below shows the typical yield distribution for a solar PV system through the year. Typically 75% of the annual total yield occurs during the seven months between March-September inclusive. The other five months account for only 25% of the yield.
Notice that the pattern is not exactly symmetrical about June, and is skewed slightly towards the spring (rather than autumn) months. Besides the weather, one reason for this apparent anomaly is the effect of panel temperature - the electrical efficiency of most types of PV panel drops off as their temperature rises - this is something we hope to analyse further as we continue to collect real data.