This solar project is nearing fruition and I'm back to waiting; waiting for Installation Company to acquire all the equipment needed, and then come and install it on my roof. It occurred to me that maybe I should compare my initial hopes for this project with the reality I have since become aware of as I have studied its details.
The major difference between late February and now is the change in the government's system of grants and loans. Then I had expected that nearly the full cost would be covered by a £10,000 interest-free loan and a £4,000 grant. This has since been descoped to a mere £4,000 interest-free loan. I had just missed a window late last year where these schemes would have been available and so most of the finance must come from our own resources.
A more subtle difference comes in the amount of power I can expect to generate. When a lady from Energy Saving Scotland visited, she pulled out tables that said a south-facing array of the size I was planning ought to generate 3,100 kWh annually. I had seen a table on the web that suggested an east or west-facing array ought to generate 90% of that figure. That gives 2,790. However, I was thinking of a 4-kWp array and it transpired that a smaller array, 3.5 kWp, was a better plan to avoid the power company having to do a feasibility study. Looked at now, that ought to generate 2,440 kWh, but maybe I would get a little more as my roof is 20 degrees south of facing west. Yet somehow, I was going through this whole process with 2,900 annual units in my head.
Part of this was because when Installation Company quoted for my system, they suggested it ought to generate just over 2,900 kWh. That seemed about right - it appeared to be around 10% lower than the figure I had heard originally and I had assumed that whatever figures they were using, they had taken the orientation of my roof into account. However, on speaking to them last Friday, I discovered this orientation was new to them. They thought I had a south-facing roof. Their revised quote that I got later that day was appropriately adjusted. Their calculations, based on government tables, and assuming an east-west roof, is now 2,480 kWh annually or 6.8 units a day average. That matches my own new calculations.
So a new reality is setting in, but I can accept this gradually declining expectation of my roof's generating abilities as the figures still look good in the spreadsheet. What does it say? Using the new figure for annual generation, the difference between installing and not installing the panels is now £25,300 in my favour over 25 years, down from about £32,300 before. I am looking forward with glee to the 365 days that will come after the installation is complete when I will see how accurately this final prediction compares to reality.
When I visited a house with a panel array already in place, it was cold but sunny. Being February, the days were still short. Yet this array, the same size as mine but south-facing, had generated 15 units of power that day. Its current performance is 9 units a day average over the months of February, March and April which translates to 3,285 annually. Given that this period is biased towards shorter days, this system seems to be outperforming its expectations by a significant margin and it has made me think that the government tables are somewhat pessimistic. Time will tell.