Solar: how much you paid (£) how much you have generated (kWh) and date of install.

cepheus wrote: »

The theoretical rated output is irrelevant, it's the cost and actual output over time which is important, although I suggested the area is included so we get an idea of the size.

Oh not it isn't!

Using PVGIS we are able to instantly get an extremely accurate estimate of generation. This can then be used to evaluate an install on any scale.

Your method of asking for actual generation, means that you are using older installs, which are also more expensive, this will distort your calculations. All you really need to know is if PVGIS figures can be trusted, and therefore if my claims that a south facing system can generate 1,000kWh/kWp are true.

In reality, as I've pointed out previously, you can simply draw cost conclusions from the relative subsidy rates.

I'm starting to suspect that this is simply another time wasting exercise to avoid answering the question which is now 3.5 months and 3 threads old:

cepheus wrote: »

If you remember, my view is that solar is unlikely to become cost effective, for this country as a whole and we should be concentrating on more cost effective renewables such as wind and conservation. This situation remains the case in high latitudes such as the UK.

Martyn1981 wrote: »

but, how do you justify that position (pro wind, anti PV on economic/cost grounds) if PV is now competing directly with on-shore wind, and has beaten (hands down) small scale wind and off-shore wind already?

Since you made the statement, you should be able to justify it instantly.

Mart.

cepheus wrote: »

JimJames: Thanks for this, as far as I can see you are the only person who has been able to submit clear figures which can be used in a straightforward calculation.

The theoretical rated output is irrelevant, it's the cost and actual output over time which is important, although I suggested the area is included so we get an idea of the size.

Hi Cepheus ..

I don't know what you're attempting to do here .... effectively, if someone is currently looking to install a system it's the performance in terms generation, the current system price and FiT returns which they'd be interested in, not the price that someone paid five-or-so years ago and the return on those systems ...

If you're looking for data, then why not look for masses of data ... PVOutput ( http://pvoutput.org/ ) has somewhere around 20,000 systems registered globally, with a couple of thousand being in the UK, of which around 300-400 submit data on a live basis in timeslots down to every 5 minutes. You may even find that some have registered their installation dates & system prices ...

If you want to compare, it's all there to cut&slice it as you want .... nothing hidden and I doubt that anyone would have the time or inclination to sit and fiddle the figures ... pick a system local to yourself with a roof which faces the same direction and has a similar inclination and check the monthly output against either the SAP figures upon which installers base the estimates on quotations or compare it against PVGIS ( http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php ) which is the EU's official calculator ....

As for area, no problem there either, I fail to see the relevance but here goes .... Panels are around 15% efficient at converting light to electricity at a standard unit temperature of 25C and a 250Wp (15%Efficient) unit will be around 1.6sqm (1.6mx1m), with inverter efficiencies being around 95% - UK insolation is somewhere around 1000kWh/sqm per annum ( http://re.jrc.ec.europa.eu/pvgis/cmaps/eu_cmsaf_opt/G_opt_UK.png ) .... armed with this you can play with the figures however you want ...

... for example, take a 4kWp system facing south somewhere in 'average' UK in an 'average' year .... 1sqm of roof would receive 1000kWh if insolation, the panels would convert this to 150kWh of DC (1000x15%), which after system losses would be ~142.5kWh of AC (150x95%) ... 16 panels rated at 250W would make 4kWp and would cover ~25.6sqm(16*1.6) ... 25.6sqm should therefore, on average, generate 3648kWh per year (25.6sqm*142.5kWh) which, considering the simplicity of the calculation, is in line with what people are actually seeing ....

There's the physics and the resources, play with them as you will ...

HTH
Z

PS My daily PVGIS target for June is 12.97 kWh.

Yesterday, I recorded 20.20 kWh.

So my I would measure 20.20 kWh / 12.97 kWh = 1.56. That's a fair method of measurement.

Sterlingtimes wrote: »

The Wp does not seem to me to be a particularly useful measure when the inverters can be a limiting factor irrespective of their efficiency rating.

So my system is 3,780 Wp (14 panels at 270 Wp each).

My micro inverters have a maximum continuous power delivery of 215 W each. So my maximum AC output across the 14 panels is 3010 Wp (or perhaps Wi (i for inverter).

My best ever recorded 15 minute output is 789 Wh. This is a fluke but if it were sustained it would be 3,156 Wh.

All in all, I would conclude that I have system capable of 3,010 W (not 3,780. So I lose 20.38%.

On this basis my PVGIS reduces to 3,100,000 Wh a year.

The "O" based on kWp gives some correspondent here a significant advantage over others.

The PVGIS seems to be the best baseline comparator.

Hi ST

In reality the system output is limited more by irradiation than inverter capping ....

Our 4kWp system is currently generating 741W .... If the inverter capped the output to 3kW then we would still be generating 741W, as we would if we had a 5kW inverter .... however, if we had a 3kWp system with a 3kW,4kW or 5kW inverter cap we would only be generating ~556W ....

... Yes there would be a difference with undersized inverters, but this should only make a difference when the combination of bright/cool/windy/semi-cloudy conditions apply ... in normally excellent generating conditions very little generation is achieved at >90% of rated kWp, probably well within the tolerance band of the panels ...

Think about an ideal day's generation curve being compared to an equilateral triangle with capping the generation to 80% (your 3010/3780) being the equivalent of cutting the top 20% off the height (kW) ... the effect on the area (kWh) remaining is simply subtracting the area of the triangle removed ... ie, area if height=1m=0.577sqm, area if height=0.2=0.023sqm, so (0.023/0.577) describes a reduction in area (energy) of ~4% ... similarly capping by 10% results in a reduction of ~1% (0.006/0.577) .... not strictly the same as a bell curve, but the principle and results are similar, so even on a 'perfect' day undersized inverter capping really makes little difference ...

HTH
Z

Sterlingtimes wrote: »

Thank you, Zeupater, for taking the time to explain. I am still learning so I have read this with an open mind.


My best ever recorded 15 minute output is 789 Wh. This is a fluke but if it were sustained it would be 3,156 Wh.

Hiya Sterling, those micro's are rated for 225W peak, which is almost exactly 789Wh. But as you say 215W continuous.

Some installers suggest undersizing a string inverter in the UK by up to 20% to maximise low level production. But this would depend on location and orientation. Not sure how that relates to micro's.

It might be completely correct, but 20% sounds a tad high to me for a south facing system. Crucially it will be interesting to see how your annual generation compares to an unrestricted PVGIS estimate.

As Zeup says though, the capping will only affect generation when in excess of the cap. My system is currently going great guns at about 90%, but that's unusual for this time of year, when I'd be happy with 75%. The high sustained gen is down to the extremely cold wind blowing around the panels (for time of year).

Mart.