March Generation Stats

Here's the graph for March's generation from our array of solar panels. This is for our 2kW 8 panel solar array (Panasonic HIT panels), mounted on our south-facing roof at approximately 35-40° angle from the horizontal.

The green line is March, and I've included February (dark red) by way of comparison.  March has performed significantly better than the forecast.  The forecast generation for the month was 117.94kWh, an average of 3.8kWh per day.

The actual generation was 179.5kWh, a 61.56kWh excess (52.19% over). This gave an actual daily average of 5.79kWh.  It shows a record generation of 11.2kWh on the 24th - the best for one day since the panels went in last year.

Here's March on the yearly forecast:

As you can see, March has virtually equalled the forecast amount for April.  We have now caught up on the deficit we had on the forecast over winter, and we've generated 15.11kWh more than the forecast for the year to date. 

Here's the yearly sun strength graph:

The average March sun strength for our array is 0.53kW.  What I find surprising is that already you can see that this graph is skewed.  I had assumed that sun strength would rise and fall symmetrically around summer, but the rise during Jan-Mar is far steeper than the drop off shown in Sep-Dec. Note that the sun strength chart takes cloud cover into account, so the lower frequency of cloud cover during spring-summer will effectively increase the sun strength, and this may be what is contributing to the skew.  It's a bit of guessology, but by following the curve, I think there's a good chance that sun strength will top 0.6kW over the summer months.

As an aside, by comparing year-on-year records of this graph, it will be interesting to see if global warming has any beneficial effects on the amount of electricity our panels generate.

Why the divergence from the forecast?
I've had a mull as to why the forecast has over-estimated the winter months, and (so far) has underestimated the Spring months.  Whenever solar panels are installed, the suppliers give an SAP forecast for generation for the year. This is a standardised forecast based on a set of government calculations.  The SAP yearly prediction for our panels was 1645kWh.  To produce my monthly forecast amounts, I divided that up based on the changing amount of bright sunlight hours we receive per month, (from Met office average statistics from 1971-2010).   If you rummage back through the blog, there's a page about it with links.

The forecast therefore assumes that each hour of sunlight in each month has the same generating strength.  This is not in fact the case. Assuming a cloudless sky, at our longitude here in the UK, one hour of low winter sun generates far less than one hour of high mid-summer sun.  This is shown very obviously in the sun strength graph above.  

I am very optimistic about the next few months. I did a rough revised forecast, based on the conservative assumption that the sun strength will remain at 0.53kW between now and the end of summer, and we are looking to top 1800kWh for the year.