Overall our project portfolio performed 5.4% below target in the year April 2016 to March 2017, with the Portworthy project performing 10% below target.
Whilst there have been some minor operational performance issues, that are now largely resolved, the most significant reason for the lower performance has been lower than average sunshine levels.
Analysis suggests that sunshine levels during the last two years have been significantly below average at all our sites.
Evidence suggests that longer term performance should return to predicted levels.
Performance against targets
Our total project portfolio performed 5.4% below target during the financial year to end March 2017 compared to a just below target the previous year (Fig. 1). Fig. 2 breaks down performance for last year by location.
Because the publicly available data is modelled rather than actual observed data we also wanted to check how accurate the modelled data might be compared to the real world. So the graph also provides data recorded at the Meterological Office Station at Yeovilton, which is in the same area as the Crewkerne site.
The actual observed data at Yeovilton shows a strong correlation with the modelled data we are using at Crewkerne. The strong correlation between the observed and modelled data here increases our confidence in the modelled data for our other sites as well.
The long-term data from Yeovilton, illustrated in Fig. 4 below, emphasises the unusual nature of the last couple of years. The data suggests that the last time sunshine levels were as low as this was in the early ‘90s. The long term average from actual observations at Yeovilton, is within a couple of percent of the 8 year average shown in Fig. 2.
The overall trends in the modelled data summarised in Fig. 3 shows a good correlation with our project generation performance over a longer period outlined in Fig. 1.
Overall, this means that the last two years have delivered significantly below average levels of sunshine at all three locations, suggesting that in the long term we should see better performance.
 Sunshine hours are defined by the met office as the amount of time that direct solar radiation exceeds 120 w/m2. This is not as accurate as measuring actual solar irradiance, which is what we do at our larger sites. Sunshine hours do not take into account indirect solar radiation, of which we get a lot. However, it has not been possible to source historical irradiance data for comparison.