Thursday, 1 December 2011

Dismal November for Solar.

Even without the terrible news that the FiT tariff was to be reduced dramatically, and with panic ridden short notice, November was a gloomy month. 

Unfortunately, it's going to get worse as the days carry on shortening until December 22nd. After that we can begin looking forward to lighter sky and bright sunshine falling on our PV panels. (Not instantly you understand?)

I'd tried to predict the monthly output of my 2.66kWh system based on daylight hours back in June 2011. I have to say this may be based on entirely flawed logic, but at the time it was the only way I could produce meaningful data without actual experience of how solar power and PV panels work.

Whilst I've been delighted with the above projected output figures in the June-October 2011 period, Novembers figures have brought me down to earth with a bump. I'd hoped for an average daily output of 4.9kWh and have achieved a miserly 2.75kWh.

On the plus side the "average" output figures between 11:00 and 15:00 each day in November would seem to confirm my decision to purchase a low output .400 kWh electric fire. In reality we've only used it once, as fortunately, if the suns shining and the panels are producing electricity the front rooms are warmed by the suns output rather than the PV panels.

Those of you with a statistical nature will always be wary of averages. As the well known saying "Lies, dam lies and statistics" emphasises you can prove almost any fact using exactly the same figures....

For example: I've had 3 days of solar power. Day 1 = 10kWh   Day 2 = 4 kWh Day 3 = 1kWh.   The "mean average" of these  three figures is 5kWh

10+4+1 = 15 divided by 3 = 5

We always need to be wary of anyone telling us an "average figure". There was only one of the days when output was over 5. I may be kidding myself about enjoying free electricity at lunchtime whilst using my electric fire.

What do you think?



  1. Hi Ian

    A friendly SPV installer (Peter Davis) told me this in response to this kind of query, he's brainier than me about these things and he says:

    "Logically we have our panels at 45 degrees (or whatever) and therefore when the sun is at the same angle they will get the maximum number of rays.

    "Let me give you an example: the 15th October was a perfect day here and we generated 20.9kw. The day was 641 minutes long. (A perfect day in June is at least 50% longer and can be up to 56% longer). On the 15th Oct the max angle of the sun never got close to the 45 degree ideal, peaking at less than 30 degrees. For the whole of June the sun peaks at over twice this. Logically therefore the sun has to climb past the ideal angle and then fall back. When one thinks about it, we know that the extra length of the day should make my peak output about 32kw for a perfect day ignoring the angle of the sun. Once you factor the angle in, one could reasonably speculate that I could have a peak output of say 40kw or even more on a perfect day. After all the sun has to climb past the 45 degrees before falling back again. Sadly the maximum I can find that anyone in the UK has had for my (Sanyo) panels is about 2kw per panel or 34kw in my case. You would have to be an advanced scientist to explain why it is likely to be only an extra 2kw.

    "Even more difficult to understand is why the perfect angle for solar panels is 30 degrees. In order to make the panels as near the perfect 90 degrees to the sun, surely we should have to angle them at about 60 degrees given the sun conditions here.

    "I cannot tell you the number of times we were told the old phrase "there are lies, damned lies and Government statistics". To get an accurate model you have to build the model up from data. As we are dealing with the weather as the principal influencing factor, you are basically swimming in treacle. Put it another way - chasing a fool's errand."

    Peter also advised me that the percentage daylight hours table that I was using, based on your 'averages' above, was wrong. He referred me to German data which is as near as dammit for us in the UK and gives:
    3.5% Jan
    4.9% Feb
    9.5% March
    12.3% April
    12.1% May
    12.8% June
    11.3% July
    10.5% Aug
    8.9% Sept
    7.3% Oct
    3.9% Nov
    3.0% Dec

    Hope this helps!
    Chris ('Sunrig')

  2. Forgot to mention: based on above Oct and Nov percentages (7.3% and 3.9%) I expected to get about 7.6 and 4.0 kWH per day. I generated averages of 5.5 and 2.3 per day. Clearly we had less than half as much daylight in Nov as in Oct.

  3. More from Peter: "Hours of daylight is not proportional to the output. PV output is directly linked to the amount of light. Every day in June is brighter than December on a like for like basis. Sunny days have much stronger sun and even cloudy days will be producing more electricity. Research from Germany (where PV is very big and is geographically similar to us) shows that 70% of PV output falls between 21st March and 21st September. This figure has only 1.5% maximum variation. 4. The (70%) came from a guy who set up a solar company 8 years ago. He said the thorough Germans had proved that they get 70% (plus or minus 1.5%) from solar panels between the spring and Autumn equinoxes. That is true of here (England) too. The more info you look at this, the truer this seems. So you have to look at the level of sunlight intensity and the angle of the sun to the array not just the number of daylight hours. Obviously in the Winter the sun is lower and the angle to the panel more obtuse for a standard pitch, it is also further away and therefore a lower intensity. The values expected are also weather dependent and a cloudy month will result in less output than a clear month, plus the efficiency of the panel +/- the STC point of reference for the panel output for a given irradiation level and temperature. STC is standard test conditions. This is the conditions to ensure all panels are tested at the same temp, irradiance etc and the performance established and comparisons with other manufacturers can take place. It is what determines the specified output of your panel i.e. “200W at STC”. Panels rarely operate at STC and therefore performance is affected when they don't operate at STC, either negatively or positively."

    Again - hope this helps!