Solar Panels - verifying the numbers

I guess that this is off topic but I suspect that I will find some experts here.
My sister has had a man around to quote for this and I've been tasked with checking out if what she was told makes sense.
She has a bungalow with a shallow sloped roof and the quote was to install 14 panels on an Easterly facing roof because the Southerly facing one is shaded by a tree. So the figures are:
Expected output 2928 kWh per year.
Returning 41.3 feed in tariff plus a saving on her electric bill of 50% of the output at say 12p and the other 50% output gains 3p from the electric company.
The prices are:
a) A "leasing" scheme whereby the householder pays an "installation" fee of 2610.00 and receives the "free electricity plus the "3p" but the company receive all of the 41.3 feed in tariff.
b) Outright purchase costing 18237.00 whereby the householder keeps all of the returns.
I calculate that (a) returns around 220 pounds (8.4%) and (b) returns 1430 pounds (7.8%).
So the question is, are these figures actually achievable and is the cost price competitive?
Does anybody know?
tim
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tim.... wrote:

also ask yourself how long the output will stay at those levels with dust, birdshit, and general wear and tear.
And how long the government will tolerate buying micro amounts of electricity at 20 times the bulk price from a nuclear set.

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They need cleaning once per year. I don't think that there's any "wear".

They "promised" 25 years, but this is a risk!
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We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "tim...."

Output falls from day one. Ten-year-old PV panels can be producing half of what they were when new, but it depends on the solar tech involved. As always, the technology is steadily improving, but I really don't know if it's worth it yet. Fine, if you live in an off-grid cabin in the woods, but otherwise, probably not.
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The Natural Philosopher wrote:

This particular subsidy doesn't count in the government spending because it is funded but those people that buy fossil fueled electricity isn't it?
AJH
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On Thu, 1 Jul 2010 21:23:55 +0100, tim.... wrote:

How much extra would be gained by chopping the tree down? i.e. how much is that tree worth.
So far as calculating the returns, did you take into account the "lost" income from having the difference - £15,600 in the bank earning interest? Even at 3%, that's nearly another £500 / year.
My inclination would be to let the supplier take the risk of not meeting the expected output, or of a future govt. reducing / removing the subsidy (as happened in Spain recently).
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wrote

And any money "made" by selling the electricity back would be classed as taxable income.
--
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Is it?
(I really don't know, but it would seem to be too complicated if it were because you'd be allowed to deducts all the costs first)
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It's not her tree!

Obviously one looks at this in totally. That's 6-7% instead of 2-3 %

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Ask on what basis they arrive at the expected output. I think you will find its a back of an envelope calculation based on theoretical maximums which are never achieved in reality. Not using a South facing aspect is going to approximately halve the available energy so either the tree goes or forget it in my opinion
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wrote:

Ask on what basis they arrive at the expected output. I think you will find its a back of an envelope calculation based on theoretical maximums which are never achieved in reality. Not using a South facing aspect is going to approximately halve the available energy so either the tree goes or forget it in my opinion
---------------------------------------------------------------
It seems to be 10% of the theoretical maximum.
The panels are rated at 230 Wp (whatever the p is) and I assume that's per hour.
230 W* 14 * 24 * 365 is 23000 kWh per year. (of course that includes night time!).
I assumed that the supplier took the orientation of the roof into account. It is quite a shallow slope it will still get the sun for most of the day, it just wont be direct. It is in his interest to do this if she goes for the "lease" option which I think is the favourite ATM, the ROI on the 18,000 doesn't look good enough given the risks.
tim
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On Thu, 1 Jul 2010 23:03:45 +0100, "tim...."

p is "peak", in other words the maximum the panel can achieve at the correct angle pointing at full sun and artificially chilled . It is in practice never achieved .

With a non-tracking system on an easterly roof the figures quoted are wildly optimistic. Typically an optimally sited solar PV panel will achieve a peak output roughly half its "p" value. Face the panel east and that nearly halves again.
Most greenwash wind/solar companies are out and out frauds run by people the double glazing industry cast out as undesirables.
As a rough guide 1 sq m of panel, optimally aligned will receive 1mW of energy per year. Solar panels are typically about 15% efficient at the most (many sold in the UK are about 10% efficient) so 1m2 of solar PV facing south with a clear sky cannot produce more than 0.15mW in one year. Assuming each panel is 1sqm that is an absolute maximum of 2.1mW per year (which is probably where the shysters got their 2.9 mW from with a bit of imagination).
Facing east and at a less than optimal angle takes at least 50% off that so 1mW per year is probably closer to what will be achieved in reality.
The leasing scheme means she will probably get nothing back but be lumbered with "annual maintenance" costs. She needs to read the contract (probably printed in 5pt gray on gray) with a good magnifying glass.
The outright sale simply means she will lose a lot of money straight away.
In your calculations of return don't forget that the capital is lost with the solar panels. After somewhere between 10 and 20 years they will be worthless and need replacing. If the house is sold the panels will add a small amount to its value but nowhere near their original cost.
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Peter Parry wrote:

Well one milli watt is not even a unit of energy.
Its a unit of power, and even a fly can generate that much.
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On Fri, 02 Jul 2010 01:01:04 +0100, The Natural Philosopher

It was probably all that was left at the time I posted that :-).
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Crazy scheme. Solar PV on south facing roof on a grid connected house makes no economic sense at all. If it did they'd be going up all over the country. Putting them on east facing and pretending it makes any sense is simply a disingenuous sales pitch.
There is a blip in the picture, which is that a govt scheme is giving high payback currently, but its hard to see how that will last long.
Run, run away from the con artists.
NT
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On Thu, 1 Jul 2010 23:03:45 +0100, tim.... wrote:

If it's any help, the weather records for my part of the sunny south-east shows that 2009 got 1695[1] hours of sunshine. Presumably these panels can generate some (how much?) current in overcast conditions - and even when the sun has gone past south, which would put it out of sight from your proposed panel positions.
[1] Last year was considered quite sunny. A better average is 1500 hours
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On 01/07/2010 23:03, tim.... wrote:

Which is about right for a SOUTH facing roof in the UK
from http://www.inference.phy.cam.ac.uk/withouthotair/c6/page_38.shtml :
"The power of raw sunshine at midday on a cloudless day is 1000W per square metre. Thats 1000 W per m2 of area oriented towards the sun, not per m2 of land area. To get the power per m2 of land area in Britain, we must make several corrections. We need to compensate for the tilt between the sun and the land, which reduces the intensity of midday sun to about 60% of its value at the equator (figure 6.1). We also lose out because it is not midday all the time. On a cloud-free day in March or September, the ratio of the average intensity to the midday intensity is about 32%. Finally, we lose power because of cloud cover. In a typical UK location the sun shines during just 34% of daylight hours.
The combined effect of these three factors and the additional compli- cation of the wobble of the seasons is that the average raw power of sunshine per square metre of south-facing roof in Britain is roughly 110 W/m2, and the average raw power of sunshine per square metre of flat ground is roughly 100 W/m2."
Of course that 110W figure is the power falling on the panel you only get 1/10 to 1/5 out as electricity due to the poor efficiency.
I wonder if they have made any adjustments for the east facing roof? An interesting experiment would be to get someone else to get an estimate for a south facing roof and see if it's any higher.
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On Fri, 02 Jul 2010 22:28:10 +0100, Gareth wrote:

But does that mean the other 66% of daylight hours produce no electricity at all. Does anyone have a figure for the output from PV on a cloudy day? I'd be very surprised if it was 0 - which is what MacKay's calculations shown here imply.
What's worse is that MacKay is looking at the power produced per sq. m. of FLAT land, whereas PV _is_ tilted towards the sun, even if it's not on a tracker. So the figures he calculates are too low. I would be generous to him and assume this was a simple error or misunderstanding and not pursuit of his own agenda. Since he has used ths tilt to only include 60% of the available energy, we can simply double his result for a close enough correction to this mistake.

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pete wrote:

well my old West0om expiosure meter says that acutial intensity is two stops down which is 1/4 the output for 'slight overcast' or 'shade'
once into proper murky cloud,, its 8,16 or 32 times less..
Before exposure meters the rule was 'one stop for haze, two for shade or cloud, three for deep cloud, another stop for within an hour of sunset.. and so on. each stop being half the light intesnity, and presumably power.
At some level its so low the elecrtonics to convert the power will stop.

Mackay is going 'orders of magnitude' here. 1/16th of the power is effectively 0, to a first order.

He was looking to understand the total output of a *land area*. Tilting helps the panel by essentially shading a much BIGGER area of land with a smaller panel.
Any way, you may quibble, but there are figures for TOTAL insolation of the UK available averaged over months and years. You can use those to derive figures,.
http://www.contemporaryenergy.co.uk/solarmap.htm
I did and it was essentially pants. You need somewhere with steady continuous sunshine:The UK is too cloudy to often. And or covered in snow!
Bearing in mind that whilst straight south helps during the midday sun, its gonna to sod all for when the sun is rising or setting. you aren't going to see much more than 400Wh/sq meter/day at 20% efficiency..for every square meter of panel. Now I suppose that's then at a buy price of say 10p a unit, a net return of 4p per day, or about £15 per year.
I can grow more than £15 a year worth of potatoes on a square meter every year. At a lot less capital outlay.
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The local potato merchant will sell me a 25kg bag for 6. I reckon in 1 m^2 you would get at the best 12 plants, so that works out at 5kg per plant. I think not. Your selling is about as imaginative as the PV panel man.
Rob
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