Mega time

So the latest from the meter under the stairs. Five megawatt hours of production have now been logged since the system was installed.

That's an awful lot of (energy efficient) lightbulbs run for free. With a good summer (unlikely, the way things are going) we could well exit the year close to 8 mWh. Well, maybe 7 (fingers crossed).

May 2013

So that was May.

It started nicely enough with some really nice weather (just in time for the bank holiday) and finished in a similar manner for the final bank holiday, but there was some real grey days in between. The weather this year has been odd to say the least - too cold, too wet and too grey - you can see this in the garden (everything is a few weeks later than usual), and in nature (fishing is very slow this year) and it even seems to be effecting some people (a sort of SAD if you like).

Output wise, too many mid-single figure days diluted down the good days and led to a good performance (112% of predicted output) but one that should have been better. Still, I supposed I shouldn't complain - another month done a dusted and another target beaten, however output was lower than April 2013, which indicates just how mixed the weather has been.

Let us hope that the good start to early June carries on through the rest of the month.

Spring, finally.

It took it's sweet time coming, but spring has finally arrived in the UK.

It's still not quite right (the garden is well behind where it should be) but suddenly getting up in the morning doesn't seem so difficult - blue sky rather than dull grey cloud helps somewhat in this regard.

So the highlights of the month:

(1) passing the target with ease - 407 kWh generated vs a prediction of 311 kWh

(2) best ever single day's production on 20th April - 23.56 kWh, beating the previous record of 26th May 2012 of 23.24 kWh. On it's own this was not a memorable day, but clear skies and a good elevation angle might have squeezed out some additional watts. Who knows? Maybe we'll get a few good summer days this year (without the haze of 2012) and even better results might be observed

(3) finally seeing some separation in cumulative generation for the year

Here's hoping the nice weather continues for a little while.

Worst March Ever

According to today's papers March (and Easter for that matter) have been the worst ever. I will wait until I see the stats from the Met office to be sure, but I can well believe it.

Cold weather, clear nights, threats of snow, long dark grey days all featured too often in the month. Despite a few good days, the PV system just couldn't do it's thing often enough and the month finished on 213 kWh against a predicted value of 225 kWh, giving 95% of prediction. This is the second month this year that I've missed the target and the second time ever - I just hope this isn't the pattern for the year. (If I'm honest I suspect I've missed by a little more than that, I think the OWL CM160 is over-reporting, but I'm ignoring that for now.)

The killer issue is the lack of average days in the month - there were far too many days when the weather never got going and 3 kWh or less was the output. It's hard to compensate for that when a good day is 15 or so and you need to average 7.

It's a big contrast to last year when the weather suddenly turned on for two weeks and all the March targets were well and truly smashed, such a thing was extraordinary at the time, but it'd be nice to have a good week for a change.

Looking ahead the weather looks to be brighter for the next few weeks. Although this will keep things cool, the PC should at least get a good work out. Here's hoping.

It's been a while

So it's been a little while since my last post - I've kept on top of the tracking, but not the blogging bit. So hopefully a chance to put a few things right.

So the highlights:

December 2012 was amazing, the target completed in just 12 days, thanks to some unusually sunny weather. At one point it looked possible to complete about twice the quota, but the weather was not so consistent come Christmas, but you cannot really complain.

Lowlights:

Err... that would be January. Awful weather, deep grey skies, snow etc and too many days when virtually nothing happened. All-in-all, this is the first month that I've not beaten the forecast - that's how bad the weather was.

Eagle eyed readers will spot a few negative days in there - this is not quite accurate. There was some nominal production but once I take into consideration the background noise levels on the Owl CM160 in order to track the meter under the stairs, this is roughly what drops out. Either way the overall trend and values are close enough. So that works for me.

February on the other hand, looks a damn sight better, with ten days to go I'm nearly there. Hurrah.

As a change for this year, I've adjusted the generation targets to be slightly phased. This reflects the day length/suns angle better and back weights the months in the first half of the year, and front weights the second half. It's just a minor adjustment, but it pleases me. I may explain how I've done this another time - it's not perfect, but its close enough for me.

Sun vs PV

Since the system went in, I've been watching the output of the PV system (no surprise there then), but I've been wondering how closely this follows the weather stats. So, now, with a good few observations to look at, it was time to have a proper look.

Courtesy of statistics published on the met office website, I've downloaded the observational data from London Heathrow, which is not too far from me, for the analysis. This contains various data, by month for the last 50 years or so, most importantly this includes the number of hours of sun per month.

The first chart, shows the hours of sun reported, based upon the monthly records going back to 1957, with the mean, max and min shown as the lines. I've then superimposed the 2012 actual figures (diamond markers) on the chart.

The overall shape of this chart is no real surprise and looks familiar to the shape of the PV predictions, however the variations are quite interesting. Indeed in a good month it is possible to have 50% more sun than average, and in a bad month 50% less.

2012, as anyone who has been watching the weather will tell you, has been another strange year. March was unusually warm and sunny for the last few weeks, after which the weather took a distinct downturn, and it was only August and September that provided any semblance of Summer. June in particular is worthy of note - the least sunny June for over 50 years. No wonder my chilli plants didn't do well in 2012. 

Which brings me on to the real point of the analysis - does the PV system performance vary with the relative hours of sunshine? Intuitively this is obvious, but it's always good to run the analysis properly. So let's look at the results (chart below). 

 

Here we see each month (plotted as a diamond) comparing the monthly sun hours normalized against the seasonal norm, plotted against the PV system output (indexed against the PV output prediction), with a simple linear fit applied. Visually this appears to be a good correlation (r=0.957), although there is a little noise. This is not surprising - not all of the sun would hit the panels at a good angle, (if it's at the wrong time of the day) and not all of the sun would be at the same strength, but overall, this would appear to be a good fit. More sun = more PV ouput, is hardly surprising but it does seem to suggest a linear relationship. 

A quick t-test on the above data provides a value of p=0.00209 which suggests a fairly high degree of significance, it'll be interesting to revisit this in a year or so's time to see how the correlation and p-values develop, especially as the system starts to degrade.

One important observation on the above is there is a natural bias in the system performance - whenever the sun hours were on the seasonal norm (i.e. 100% on the X-axis), the PV system appears to be producing 120% of predicted output. This is good news from a generation and FIT payment point of view and back up the anecdotal views of the PV system installer. They stated that PV systems seem to out-perform the prediction, so hopefully as the system degrades, I will still see a good rate of output and system return. One lives in hope.

 

October

Well that's October out of the way. The month started really well, with plenty of clear days and strong sunlight. After looking like I would pass the forecasted output with ease, the weather suddenly changed and ten days of poor weather followed. Heavy cloud, rain, shorter days meant that output of 2-3 kWh per day were common and that really started to erode the output.

Two good days towards the end of the month finished things out nicely and that as they say was that.