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.

Done and done

A momentous day has past - two major milestones.

Firstly, the September total has been met already - a series of good weather days at the start of the month, followed by good mixed weather afterwards has made the daily target of 8kWh per day very doable. Job done and still 12 days to make a profit.

Secondly (and probably the most important) that's also the point at which the total for 2012 has been reached. After double-checking the meter under the stairs, I can confirm I'm comfortably past the 2952 kWh annual target with over 3 months of the year to go. Admittedly these will not be the most productive months of the year, but it's all pure profit now, which is a nice feeling to have. It helps that so far this year I've beaten the target every single month, which given the poor weather is no mean feat, so I wonder what the output would be in an unusually good year. Maybe 2013 will show.

On an aside, I've now had to change my home laptop, which has been an experience in itself. So I've reinstalled the OWL software, but need to transfer over the database at some point. Hopefully no big deal. However, the interactive charts on the blog have been interesting - works in Firefox but not Explorer. So now I've no idea what's going on. I shall have to ask the nice man at work for his thoughts.

Watts going down

Recently I received an updated set of power consumption figures from my electricity supplier and I was interested to see by how much the solar PV system had reduced my imported electricity levels. Based on the figures supplied (which are far from ideal, being spaced at 80 day intervals) I got the chart below.

It's not a perfect way to work, but it does go some way to show the impact of the system. At the moment we are relatively high power users - part of this is the garden pond which can use 500 watts of continuous power in the summer (less in winter when some of the devices can be switched off) plus the air-conditioner in the summer can get the meter whizzing round too. (Although the latter has seen much less usage this year).

Based on a few rough calculations this would seem that I'm using about 7 kWh less of power a day since the panels went in - which seems a little high since this would suggest only 20% of power was being exported. I suspect we're being a little more economical than before and running the dishwasher and laundry during peak sunny times probably helps make the best of the system, hence not all of the drop is from the panels, some of it may just be our changing behavior.

7 kWh a day are equivalent to about £25 a month off the electricity bill, which is not to be sniffed out - the equivalent of £300 a year of additional return on the system (in addition to FIT and export payments). Not bad at all.

August

So that's August done and dusted. 

Weather-wise this was a strangely good month - without ever really getting going, it kept fine and bright and produced many good days that meant it felt like exceeding the prediction was a real possibility. By finishing the month 20% ahead of prediction on 411 kWh means it was the best month yet - here's to an Indian summer and a chance on completing the annual target by the end of September. Who knows, it might happen.

A few good days

A few good days of weather and we're back on track. In fact the last two days have been about as good as it gets with 20kWh a day flowing from the panels under clear blue skies. This has improved the generation stats and closed the deficit against the predicted generation levels - hurray. 

Although the weather has been near-perfect, the peak output is not as good as I'd hoped - more in the upper 2 kWhs rather than in the lower 3's. I suspect this might be the slightly hazy nature of the sun, but I cant really complain after all this rain we've had. It's great to see some output.

Overall, the meter under the stairs says 2237kWh to date, which means I've passed the £1000 point since the system was first commissioned. It's taken a little time, but I'm still ahead and it's nice to see the investment starting to pay back. I look forward to submitting my next meter reading in mid-August.

Weather, Owls and Spaniards

So they tell me that the weather is about to get better. Apparently, the run of poor weather we’ve been seeing of late is courtesy of the jet stream being too far south and hence the mixed weather to the north is fairly and squarely ensconced over the UK. The smart people in the met offices of this world now tell us that this is about to move, by the weekend the jet stream will move northwards and the lower part, at least, of the UK will start to see the benefit of improved weather. (This could be a major boon in time for the Olympics which might otherwise have to be held underwater).

You’ll have to pardon my scepticism at this point. It’s not that I don’t believe the science behind weather forecasting, it’s that based on recent experience, and the chaotic nature of weather (that’s chaotic in the mathematical sense) I think I’ll believe it when I see it. It would be nice to think we’ll get a summer at last – which would be handily in time for the school holidays, my solar panels and the contents of my greenhouse. But I hope to be amazed shortly.

The solar PV system is, however, slowly doing its thing and generating power, albeit somewhat slower than you might expect for this time of year. We’ve made it through the equinox, and are on the slow run down to winter (makes me sound like a right pessimist) but with the balance of July and the whole of August to come, one hopes that we might get to see something special at this peak weather time of year.

July so far has been pretty mixed, the odd nicer day mixed in with lots of cloud and many showers. I’ve had a couple of good days (mid teen production) but no wall-to-wall days when the kWh’s really started to flood in. It’s a little disappointing given the day lengths available, but on the other hand I should be pleased that even with these limited days we’re fairly close to the generation prediction. I know I’m running about 10% behind where I should to be, and a lot more behind where I’d like to be, but given the conditions, I still find it amazing that I’m able to generate so much, so easily.

As an update the Owl seems to be behaving itself again. Two problems connecting to the transmitter in three days was a little worrying, but all seems to be good. Talking of the Owl, I see on their website that there is a new gismo (or should that be toy) available that allows automatic transmission of readings to the web for access via iphone apps and the like. I haven’t seen any reviews of this, but I might be tempted to play with this at some point, since it appears that it may be possible to have this as a retro-fit using the existing transmitter. I think I will hold fire until I see some independent reviews.  

In order to share data more effectively, I’ve added some additional ‘pages’ to the blog, these are on an extracted page per month based upon generation stats courtesy of google docs. This was fairly simple once I’d worked out what to do, but it does lack a professional look to it (no colour scheme and no links). However, it is a start, since it appears most browsers are unable to see the java charts on the top and right hand side of the blog. (I’ve still no idea why this happens).

On a completely different tack, I’ve just returned from a couple of days in Barcelona, where the sun shone, the humidity rose and it was all-round too damn hot for my liking.  However, I was surprised not to see a single solar panel all the time I was there. Given the excellent natural climate and strong light I would have expected to see something. From my hotel room window I could look out over a fairly large swathe of residential housing (a good few hundred properties) and there was nothing to behold. Maybe the subsidy is not there, or the planning is too draconian – who knows? I would spend some time investigating if I was really that bothered, but as a point of interest, I would expect some panels to be blasting away given the suitability of the conditions.

Owl on the blink

So for the second time in three days the OWL CM160 unit has stopped recording any power. Thankfully this is not the fault of the system, more an issue with the OWL. 

I've changed the battery in the sender unit, and reestablished the connection, only for it to cut out again a few days later. I shall have to have a go under the stairs again to try and work our what is happening - but without waking everybody up.

I've also noted another potential issue with the unit - it tends to stop recording internally for short periods. It still displays a reading, but sometimes when you come back to it, it does not seem to have advanced as far as one would expect. So on a bright day when it's showing in excess of 2kW you can come back a couple of hours later to kind the cumulative count has not moved as far along as you might think. I thought it was just me, but a recent check of the meter suggests that I'm 40kWh behind what the meter under the stairs says, and that's accumulated over 6-8 weeks since the last time I aligned the readings by hand. I already have an error correction built in to the readings spreadsheet, so I don't think that that's the cause, but I suppose I shouldn't complain too much if the difference is in my favour.

I can feel a replacement unit on the way.

What do you see?

Whilst you're reading this there should be a table at the top of the page and a set of charts/graphs down the right hand side. if you cant see them then I'm sorry. I'd love to know the reason - I suspect it's a security setting of some type, but I'm not sure how you fix it. 

Answers on a postcard...

June, well sort of

Well, June has started like May - very poor weather, some dry spells (thankfully) but it's hard to see where the big 20 units days are going to come from. 

Latest forecast is that it'll be like this for the rest of the month, which is hardly great news. Still, I am producing about 8 units a day on average, so not bad given that we've not had any proper sun to speak of. Here's to a little sun before the longest day. Maybe.

Caught up

After a very slow start to May, with very little decent weather, it's all change. 7 straight days of clear blue skies and sunny weather has delivered a total of 143 kWh, which has made up for the poor start, In fact as of yesterday (27th May) the cumulative generation for the month was actually ahead of the cumulative target for the first time.

It looks now as though the weather will back off for the last 3 days of the month, but with 95% of the target complete, it's difficult to see how the panels wont complete the quota for the month.

One thing that was a concern was the relatively low peak reading of the last week - never ever breaking 3000 watts. I know the sun of past the optimum angle slightly, but I did expect a little more. Maybe the sunlight was a little diluted by high-level mist, or maybe this is the limit for continuous running. Maybe we'll see as the year progresses. But given the yields observed in the last week, one can hardly complain.

Power to the people

Three days - 60 kWh added. Enough said. 

Shame the good weather isn't here to stay, but at least the generation chart doesn't look as bad as it used to.

Sun at last

At long last the sun has finally come out in May resulting in 18 kWh today. If the weather forecast is to be believed, tomorrow should be even better. I reckon I could do over 30 kWh in a single day, but it would need perfect weather and that's asking a lot at the moment. 

However, I'm not sure this weather is here to stay, so I'm not sure we'll hit the target for the month - but I'm hoping to be proven wrong. 

Fingers crossed.

April - done and dusted

I was a bit nervous about the target for April, with the need to generate over 10 kWh per day on average. However, despite the rather inconsistent weather the system did it's thing (thanks to several days of over 20 kWh) the month's quota was delivered with 10% to spare. 

This goes some way to show just how much a difference the improvement in the sun's angle made, along with the longer days. Suddenly 1000 watts in the middle of the day is quite possible even on a moderately cloudy day. Add that to the better day's weather and some decent productivity is possible. 

April was very mixed weather wise, I don't believe we ever had a 'perfect day' (strong sun all day) and the rain towards the end of the month was record-breaking - the rainiest April on record, apparently. Thankfully the good days balanced this out. It is not without a sense of irony that for most of April we have had a hosepipe ban due to drought conditions - this certainly would seem to be one of the wettest droughts on record.

May, on the other hand has been a bit of a struggle. As of May 5th, we haven't actually had any proper sunlight - just cloud and rain - and the forecasters tell us the weather is here to stay. Hopefully, they'll be proven wrong, but the 5 and 10 day outlooks on various websites are not that marvelous.

Fingers crossed for some decent weather.

4000 watts and counting

So today we topped the 4000 watt mark on a couple of occasions. Not for too long, but long enough to be recorded by the OWL unit. The highest we've seen on the monitor is 4094W, but the average minute values recorded were 0.67 KWH in a minute which is a 4020W average.

I'm glad at this point that I have an over-rated inverter as a few extra watts are always welcome in making the meter tick over that little bit faster. 

So far the weather this month has been very up and down. Generally the weather has been better than forecast, but we've not had any classically sunny days - lots of cloud mixed in, some rain, hail (twice today), rainbows, frosts and biting winds. As such I'm impressed the system is doing as well as it is, given the target requires over 10kWh per day I'm pleased to be on track and a little ahead (currently 15% up on the monthly cume). Whilst April will probably not be quite as good as March (with its exceptional 2-week spell of clear weather), it should really start to rack the units up. 

I expect we'll complete our first megawatt-hour tomorrow - we closed on 999.77kWh today according to my records. This would be a nice milestone to pass.

Peak Power

I'm just going through the point at which the sun is perpendicular to the solar array so am expecting some interesting peak readings in the next few days. Unfortunately today, the weather didn't quite play ball - early sun was quickly diluted by 10am but occasional breaks in the cloud gave some interesting high readings (if only temporary). 

The highest I noted was just over 3900 watts, and there were several readings well in excess of 3800, which is interesting given the system is only rated to 3430 watts, although the inverter is 4400 watts. There is a theory that says these peaks only occur when the sun is broken by cloud, the panels are cooler and more efficient, and hence you cannot get these peak readings on a clear day as the system cannot run at that level of output for long periods of time. 

I shall wait and watch for this over the next few days - we are due to get periods of early sun for the rest of the week, surely one decent patch of blue sky will coincide with the mid-day sun's alignment. Here's hoping.

Peace in our time?

The ongoing war of our time on the cuts to the feed-in-tarriffs seems to be coming to and end - the higher FIT rate will be extended to March, but there are severe cuts in the pipeline between now and the summer. (See telegraph article).

As someone who was fortunate to have solar PV installed in November 2011, I have watched with interest, but with the smugness that comes from knowing that I'm alright. 

What I always find interesting in these cases are the comments from the public that accompany these stories - I'm now a passionate advocate of solar PV. Whilst I can see the points being made on the basis of cost, there is no disputing that the systems are inherently productive and clean, and I hope at some point in the future there is a breakthrough to make the systems standard on all properties. Small changes in efficiency, cost and/or storage of excess energy built up over a period of years could revolutionize the way we generate and consume electricity, Here's hoping eh?

Hmmm

It's been an interesting month so far, the weather has shown many hints as to what the solar panels are capable of, but never really delivered that well. Hence the run rate chart for March is ticking along nicely, but without ripping up any trees. At the moment the yield for March is looking to be similar to February. I'm just waiting for the magical moment when the weather breaks for a few days (it might be now) and we get glorious uninterrupted sun for a while - that's got to be worth 40 kWhs in the bank in one go.

I have to say that now I can see the system doing it's thing day in day out I am impressed. Now that the days are longer and the sun's angle is much higher, even cloudy days create a healthy reading on the Owl meter - 700+ watts when the sun isn't out is not to be sniffed out, and peak readings over 3000 watts are always welcome! I need to reach about 900 watts to be self sufficient, so this is always the unofficial target to my way of thinking.

We finally received our first payment from the power company a few weeks ago, as expected this was just over £180 and a BACS transfer receipt came by mail. It's good to know that things can only really get better from here on as we go into the better part of the year.

I've been having some thoughts about adjusting the run rate charts to allow for varying sun angle and day length. For example during March the days are two hours longer at the end of the month than the beginning and the sun is many degrees higher. Hence the productivity of the system and the target should not be linear, and should accelerate towards the end of the month. I may well experiment with this and see what the results will look like, at its simplest level this will change the 'run rate' target into a form of parabola with which to compare performance.

Charts not working

Arrrrgh

Is about the easiest way to say it. Every month it's the same, as a try and roll the data forward a month the charts on the right start playing up. Last night after some playing I gave up. I know that's not the right attitude, but having had a very long week, with virtually no free time, I decided enough was enough.

So a few glasses of wine later, the chart were furthest from my mind. And hey presto today, we're working again. Eh?

Next plan is to create more add-ins but in unique code windows and take it from there. Here's hoping for a simple run at it and no major problems. As if I'd be so lucky.

Postscript:

So after much fiddling and having to regenerate every chart from scratch it's back up and running. And then not again. So more cut and paste of the scripts. This is getting tiresome - looks like the chart controls have been updated too, which could have a bearing on things. Not very impressed at the moment.

February Final Stats - 135% achieved

A few points...

So lots to say and no time to type it in!

A few highlights of the month to far:

1. Firstly a fantastic start to the month – four days of near perfect weather put nearly 50 kWh in the bank. This shows the value of the system even in the winter months. I’ve had a few good days since, but nothing like this for a run. It certainly makes the monthly target seem very achievable even after only a few days in.

2. Snow. I was very worried about snow having read a couple of blogs where it had completely killed the output for several days. However, despite having 2-4 inches on two occasions, the snow just fell off within an hour or so of dawn. I think the angle of the roof helped a lot, but either way productivity remained as per a normal cloudy day and all was well.

3. High power. A couple of days ago the Owl read an almighty 3.511 kW output – technically higher than the system (give or take for noise) and this was continuous. We’ve also seen extended periods of 3kW too, so hitting some of the summer targets seem quite possible. I’ve been tracking the day length and sun’s angle in detail, and there does seem to be a distinct change in February – as the angle improves, so does the day length, but also the peak power output (typically about 11:20am for me). At this rate I expect to hit closer to 4kW come optimal conditions – thankfully the inverter and wiring is rated for this, so no worries.

4. Money. I submitted my first meter reading - 410 kWh for the first quarter, which should translate to a little over 180 quid. I look forward to seeing it appear in the bank in the next few weeks, but come summer we could see some nice return on investment.

5. Day length. The last few days have been interesting. Although the weather forecast isn’t great the lighter cloud and better solar angle means that I can get 1kWh out when there is light cloud and over 3kW when there’s a bit of blue. This is quite encouraging for when the weather picks up. It’s nice to see the Owl ticking over and knowing that the house is self-sufficient for long periods.

6. Month complete. Ok, not quite but the February quota is nearly done (all bar the shouting) with a good week yet to go. It’d be nice to hit 120% for month end, but that’s probably asking a bit much.

Good end of month

Having passed the monthly total for January on the 21st with ten days to go, it was looking like being a fairly productive month. This was helped by a period of 4 clear days that really got the panels going, followed up by various mixed days that still produced a handful of units each day. Plus a few surprises - a few instances where cloud was forecasted but better weather materialized. It all helps.
 

Since then, it's been fits and starts. Firstly the highest ever spot reading on the Owl unit (see below) on Jan 26th. Not bad for a 3.43 kW system. I suspect part of this is due to the uneven nature of the weather - I don't get these higher values when the sun is constant. I wonder what the system will be capable of come high summer, given that the inverter is rated above the panels, at least I know whatever they kick out will get used, maybe I can get closer to 4 kW?

Secondly, Jan 27th was another perfect day - wall to wall sun and over 13 kWh generated. We still had the dip at 3:15 pm when the sun descends passed a tree on the horizon, but other than that it was a perfect trace with over 2500W being kicked out in the middle of the day. Not bad for January, it just goes to show how the slightly longer days & higher sun angle increase the output, and that's just comparing to a few weeks back. Roll on some proper spring days!

Jan 14th - Perfect Day

January 14th was a near perfect day weather wise and was my most productive day to date with 11.3 kWh generated. Clearly this will get surpassed later in the year, but it's quite impressive for January, when an average of less than 3.5 kWh is predicted.

The detailed plot for the day (below) is almost perfectly smooth, with just a couple of dips, so I suspect this is about as good as it can get for January. The early bias on the chart show the impact of being SSE facing, but it's hardly a major problem. I think the slow pick-up at the beginning of the day and the early and late dips are caused by trees on the horizon. Although I don't suffer from shading on the panels, I do have a broken horizon with lots of trees in the distance. Hence why the plot isn't exactly smooth. As the sun rises higher in the sky these will cease to be a problem, so it's not something I should worry about - not that I can do much about it anyhow.

Jan so far

January has been a little mixed so far - some poor days and a couple of better ones. Plus a couple of moderately good days (well beyond the forecasted conditions). It's always nice to get a handful of units produced that weren't expected.

However, it now looks like we're heading into a period of high pressure - clear skies, frosts and plenty of winter sun for the panels to lock into. Today (13/1/12) looks like being a 10 kWh day, but it's hard to tell exactly, but either way we wont be far out. If this weather holds for next few days than an average 7-8 units for 5 days will do wonders for the performance against target. Whilst I don't expect to beat the performance target by much, it's always nice to sit back and enjoy all that free power.

It's nice to see the evenings starting to get lighter, its only a few minutes a day, but given that this starts to overlap with my evening commute home, it's most welcome. The mornings wont start to change for my morning commute for quite some time (maybe March), but it's nice to see some signs that the seasons are starting to change.

On a separate note, I managed to get up the loft recently and get a rough measure of the pitch of the roof. According to my rough calculations, the roof's pitch is a little over 45 degrees, maybe closer to 47 degrees. For some reason I'd always assumed it was closer to 50, but now I know. I've subsequently reworked the power factor values for the table at the top of this blog to reflect this. In theory this will make a slight change to the performance of the system, but is likely to be slight.

December 2011 - all over

So the month finished at just shy of 160% of predicted output. Not quite as great as the first part of the month had promised, but you cant really complain when you've gone through the shortest day and come out again. I look forward to seeing the sunshine stats on the met office website - it'll be good to know how the month compared to average and see if over time a correlation exists.

January 2012 could be interesting, the prediction is a little higher than December, but it is the time of year where we often get snow, which will kill the system dead until it melts off the panels. We've had a couple of harder winters in the last few years, so who knows what this winter will bring?

Now I need to push on and get the spreadsheet and graphs updated. I'm convinced there's a better layout than the one I use now, all I need to do now is find it.