Solar Panels - verifying the numbers

[Chris J Dixon]

Only if you are not interested in being paid the Feed-in tariff. To be eligible, you have to use both approved components and approved installers.

Chris

[Arfa Daily]

I really don't know a lot about this subject, other than what I've read in general, but just to fling a curved ball in for a moment, if the OP's sister is intent on having panels on a less-than-ideal-facing roof, wouldn't she do better to make them water-heating panels ? I seem to recall reading that the energy saved on fuel for heating the same water, is quite significant, and that the installation costs are not that high. If the fuel saving *is* significant, then I would have thought that with the very high energy costs that we suffer now, the 'effective' returns might be quite good, without the hassle of having to sell energy back via a government scheme of dubious longevity, and without the inherent progressive efficiency reduction that comes with PV panels. Anyone here clever enough to do the comparative sums ? I'd be quite interested to know this for myself.

Arfa

[The Natural Philosopher]

My my calculations the exact same applies. I.e. no ROI of any real validity.

F in law did this, and the numbers make no financial sense, and after 6 months the builder next door had covered them in cement dust, they barely work and there is argument about who is going to pay to have them cleaned/glass replaced etc etc.

[NT]

[e-mail]...

Its possible to make solarthermal pay its way and some, but not easy, and commercial installs, as a rule, don't. Since the op was asking about basics I cant see a good diy design/instal being likely.

On the upside it would waste a lot less money than PV.

NT

[Tim Streater]

I'm told that heat pumps are cost effective - even in lowish outside temps. A quick google shows Danfoss and Hitachi, at least, make them. Anyone considered these?

[Andy Dingley]

I keep my expert in a book:

MacKay (and if you're a geek, yes it's the same David MacKay) has written what is by far the best guide to energy policy and alternative generation approaches. Sums too. It's even there as a free download. This book is very highly recommended.

Time to break out Excel and do some real modelling of it. For this money, you can even afford to buy a bottle or two of something for someone who does understand financial modelling with Excel.

There are broadly four issues here:

• Does the tech work, and does it last? Read MacKay, but the stuff is now pretty good and pretty predictable.

• Does the sun work? Site surveying isn't the black art some make out and you can make good predictions. Some years will be better than others, but over a few years' lifecycle, it's good enough. Read MacKay (and some of his further reading - try ther Navitron website too)

• What price will you get? This cannot be justifiable for "real cost"(sic) electricity. However the massive porkbarrel of the feed in tariff does make it close enough that you have to check the numbers to know which side, you can't simply assume. However what's the political future of inflated feed-in tariffs? (IMHO it's good - it's so small a market that the cost to Whitehall is tiny and the press reaction to cutting it would be bad).

• What does money cost? It's a big capital outlay. However if you're like many of the bungalow owners I know, there's capital available, no bank interest to do much else with it, and a wish to create a return on it.

The rest is numbers and you have to do this with care. However don't decide something like this (risking a large investment) based on an estimate, calculate three estimates (mean, worst, best) and do your real decision making on the worst case assumpitions and your risk assessment of how likely this is to happen.

There's also the question of getting a second supply installed, as a source of electricity - because you won't want to use any that you could have sold for 3x market price instead! However in practice, you don't use that much domestically when the sun's out, unless you're at home during the day.

[Peter Parry]

It was probably all that was left at the time I posted that :-).

[The Natural Philosopher]

yes.

The calculations are really the ONLY eco fluff things that made sense.

Heatpumps work.

Sadly, not with conventional radiators and hot water tanks by and large.

I was faced with a sort of £15k-20K plus re plumb and rip the garden up prospect to save a grand a year on heating (maybe).

The the oil price came down.. and I gave up on it.

[js.b1]

GSHP do involve "a bit of work" (cough, angle grinder).

ASHP R410a heat pumps do work, but are technically subject to PD re still yet unagreed noise regs. ASHP CO2 heat pumps work very well, cost a lot more as yet re dual stage DC compressor, but are the future.

Sanyo(-Denki) for one do ASHP CO2 with very high CoP, but I would wait for economy of scale because it will reduce price very substantially. The price of the compressors is a bit stupid really because it is akin to making the first fax machine very expensive, exactly what you do not do even if selling at a huge loss - you ensure you have capacity to make it cheap so once 1000 people get one 1000000 people want to get one and so on. Most likely of course they do not want to upset supply chains and existing proven, low cost & relatively profitable products at present.

[andrew]

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

[Gareth]

Which is about right for a SOUTH facing roof in the UK

from

"The power of raw sunshine at midday on a cloudless day is 1000W per square metre. That?s 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.

[Gareth]

David Mackay makes some realistic estimates of the energy you are likely to get from solar panels here:

to the link below, good panels are ~20% efficient and cheap ones about 10%. With the good ones David Mackay estimates an average of

22W per hour per m^2 for a SOUTH facing roof in the UK. 22*24*365 = 192.72 kWh per year per m^2 of panel 2928/192.72 = 15.19 so to get 2928 kWh per year you would need ~15m^2 of high efficiency panels on a south facing roof.

don't know how much you would have to reduce that for east facing, but I imagine it could be a significant difference.

[pete]

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.

[The Natural Philosopher]

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,.

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.

[Peter Parry]

Most cells supplied in the UK are the Chinese made polycrystalline sort with efficiencies of around 10-15%. These claim to maintain 90% of initial efficiency for 10 years, dropping to 80% after 20 years. Whether these panels will last 20 years is unknown. The only people who have carried out accelerated aging tests are the likes of BP on well made monocrystaline panels. Given the volume of panels being made and the poor quality control on much of this type of production from China it seems unlikely. Many invertors are also (rather surprisingly) proving to be very unreliable.

One thing is for sure, it is highly unlikely your supplier will be around in 20 years - "green" companies fitting solar hot water and PV seem to be keen to practice what they preach on recycling and regularly go out of business one day to emerge the next under a slightly different name as a Phoenix company having shed all warranty liabilities.

[Gareth]

He also gives a value for a south facing roof:

"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."

From Mackay's numbers:

110 x 24 x 365 would give 963 kWh of sunlight per square metre per year (sunlight not electricity).

From someone selling PV,

"The total annual energy available varies from approximately 1,259 kWh per metre squared per year in the South-West of England to 850 kWh per metre squared per year in Scotland."

The average of those two figures is 1054.5 which is within 10% of Mackay's numbers so I don't think he is out by a factor of two.

[The Natural Philosopher]

I must have made a mistake in my calcs somewhere then. But that's the princely sum of £100 per sq meter per year at a 10p unit price.

For maybe 15 years?

Cripes, there are so nay cheaper and less ugly ways of saving 100 quid a year.

I averages to what.. 120W continuous?

The average sort of capital cost of 120W of *reliable* 24x7 nuclear energy is around £200-£400 ..that needs no inverters or battery backups to do 24x7,.

An advert on ebay sells such a system with inverter - about 1 sq meter, for £16,0000

I rest my case

[Gareth]

Either that or we are not talking about the same thing (or I'm wrong). The numbers I quoted above are available energy from sunlight not the actual electricity generated. After allowing for the efficiency of the PV cells you only get 1/10 to 1/5 of that.

Well 10 to 20 quid after allowing for the efficiency of the cells.

Yes but with the added bonus that you get the most energy when you least need it.

I don't think that is an option for the OP though :)

[js.b1]

To add one other point.

NSandI offer Index Linked Saving Certificates. They are of 3yr & 5yr duration and provide RPI+1%. You can withdraw, if you withdraw in the first year you forfeit interest.

Obviously RPI will vary.

2010-2011 we may see another recession and deflation, so the return may be just 1% tax free (ING offer 2.75% gross). 2012-2020 we are likely to see inflation, indeed central bankers aim for 2-3% or tolerate higher if it means avoiding deflation. So if RPI rises to 3.5% per year then your return becomes 4.5% tax free, your capital is government guaranteed.

Just another investment to compare solar panels to.

[geoff]

In message , The Natural Philosopher writes

One hundred and sixty thousand ?