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December 28, 2007



I was wondering, how is a PV watt defined, considering all the variables involved?


They define PV watts as peak power, so you can actually reckon on getting around 20% or so of the rated power out of the cell, allowing for night-time and so on, a fact which fools a lot of commentators, but not our esteemed host, who correctly says here that 12.4GW installed is enough power for around 2.4 million homes, not the 12 million a lot of commentators would wrongly give

Cyril R.

To be technically correct, that would partially power 2.4 million homes (or whatever). Storage for PV is still vastly uncompetitive right now, and geographical distribution is just not enough, so you'd need backup for nightime and bad weather.

You could of course place the things in the desert. But then that would not solve nightime power needs and if you have a desert you'd better build a solar thermal plant which is more competitive on the utility scale and has better storage with current state of technology. Keep the PV for end use consumer (retail) markets, and prey a good cheap storage solution comes around soon!


Cyril, I think the way you phrase that may mislead some people, as to be even more technically correct what the PV will do is at noon on a hot day totally power around 12million homes, and won't power any at night time, with various points in between.
Although there is no storage, at the level of penetration of solar this does not make much difference.
So the simplest and most informative way of looking at it is that this is enough, on average, to power just over 2 million homes.

Al Fin

Likewise, it is misleading to speak of "world PV production" in terms of how many "US homes" it would power. Spread out over the entire world as it is, the level of production is still not high. Given the diversity of uses for PV--industrial, agricultural, street lighting, third world off-grid residential, etc.--we should understand that referring to PV production in terms of "number of homes" is not realistic.

Most of these installations in developed world countries will be grid-intertie, meaning the storage issue will be moot. The PV will supply a daily portion of the 24 hr power load.

Still, I like to see the exponential growth curve for PV.


PV watts peak is defined as how many watts the PV (cell or panel) will generate under standard test conditions which include a junction temperature of 25C, 1000 watts per square meter of solar spectral irradiance and reference air mass of 1.5

gory details available at


Al Fin,
Any way you talk by analogy rather than with precise details is inaccurate, for instance 'as big as a football field' is pretty vague compared to the number of square meters you want to talk about.
Since most of the readers of this blog likely hail from the developed world, it is reasonable to put it in some terms which mean something to anyone who needs to ask how much energy we are talking about.
It is a common mistake even in supposedly highbrow publications to look at an installed capacity of say 12GW and imagine since the average use in the Western world is around 1kw energy flow per person per year, that you can cater for around 12 million people.
That is a mistake that the author of this blog did not make, but got his numbers of just over 2 million in the right ball-park, as I attempted to explain when someone asked.
And BTW, most installed capacity is in the developed world, not the undeveloped.

Cyril R.

Although there is no storage, at the level of penetration of solar this does not make much difference.

So what you're saying is that solar won't make much of a difference without storage.

My point exactly.

I'm still waiting for something like Eestor's ultracaps. Been waiting for a long time now. Maybe bio-energy as backup for PV is more realistic for now.


Cyril, what I am saying is that solar can go on expanding quite happily for the foreseeable future without hitting any barriers due to lack of storage options, and by the time it becomes a matter of engineering concern there may well be a range of totally different options available.

Stephen Boulet

It might come to pass that, in the next 10-15 years, solar panels are integrated with nanotech capacitors. Given the flat solar panels, a stacked planar supercapacitor might be a good fit.

In the meantime, solar panels have a great role to play generating power at times it's needed most right where it's being used.


Cyril R.

Most of these installations in developed world countries will be grid-intertie, meaning the storage issue will be moot

No it won't. Grid-tied systems decrease system reliability so, in the absence of sufficient storage, more backup will be necessary to maintain the same level of system reliability. Right now this effect is infinitesimal but if grid-tied PV will continue to grow this fast it will become more severe. Waiting for more high-tech options to become available? That's called 'faith' and is usually reserved for more religious folks. It's generally not very constructive to base arguments on religion. I really hope some really good storage solutions will become available but again, not holding my breath. More flexible combined cycles running on biogas or -syngas might be a good investment here. Solar thermal with large amounts of storage could also be flexible enough to respond to variations in PV by throttling down or up.

BER Assessor Ireland

Solar energy is becoming more and more popular in Ireland. The efforts of our country are sadly miles ahead of the big guys like the US...it will be interesting to see the decisions the next US president makes with regard to energy saving.


Solar still has a few years at current growth rates before storage (or demand management) becomes an important issue. If one or more of the seemingly wild claims I've seen become true (coolearthsolar $.29/watt) we could find ourselves with lots of cheap, but time variable power.


With a solar isolation of around 0.14, then PV panels are a long, long way from being economic in Ireland.
Can't work out why the various governments in northern countries don't put their subsidies and our money into better insulation and so on, rather than trying to argue with the laws of physics.
Incidentally, at the latitude of Ireland, in the winter you are talking about an isolation of 0.07 - so to get out 1kw of power you would need around 15kw installed.
Multiply that by the price of PV panels, installed on a roof.


You would clearly get more carbon saving per unit of expenditure with conservation. I guess its just not very sexy.
Now I'm personally happy to have your government provide subsidies to develop solar, so from a nonresident perspective go for it! I just hope the effect of those subsides advances the technology, instead of just making it profitable to keep using the older less efficient techniques.

Ronald Brak

Davemart, your figures for insolation in Ireland seem too low for solar panels. While each square meter of land in Ireland recieves considerably less sunlight than places closer to the equator, solar panels will be angled to compensate for the high lattitude and a 1,000 watt system should be able to produce an average of at least 2.6 kilowatt-hours a day, which is only half as good as in most of Australia, but it's not as bad as your figures suggest. But for the moment at least, wind and wave certainly look like better souces of clean energy in Ireland.


Actually, in the UK we do the same as the States about PV, and subsidise it intermittently, presumably to lead to maximum uncertainty in the market!
It's the 33GW of off-shore wind we are promised which sounds expensive at this stage to me!
If you want to thank someone for PV, thank the Germans, who seem seriously confused about the latitude of their country, and are putting huge subsidies into PV - mind you, Deutsch World inform us that a lot of Germans are getting fed up with having it stuck to them, but in the meantime I have no doubt that Angelinos are duly grateful!


Good point, I can get sloppy when the basic figures look silly.
Taking account of the sloping you refer to, you might be talking about 10kw installed to give you 1kw in the winter.
Now of course you can install less and suppliment the power, but efficiencies go down if you are just using sources for off-peak power.
PV in northerly locations, together with urban rooftop windmills, are my least favoured sources of power.

Ronald Brak

I'm not quite sure what you mean by needing 10 kilowatts installed to get 1 kilowatt in winter in Ireland. On a crystal clear day in Ireland, a panel that is optimally aligned will produce almost as much wattage as an an identical panel at the equator. The greater amount of atmosphere the sunlight passes through will not make a huge difference. As a rough guess it might reduce wattage by 10%. But what Ireland does have a problem with is rain and clouds. This results in an optimally aligned solar panel only generating about half the kilowatt-hours on average as an identical panel in most of Australia.


I think the bigger problem for Ireland/Germany PV is cloudiness not latitude. High latitudes do quite well during the summer, if clouds don't get in the way. And of course the seasonal difference is very large as well. We are agreed about rooftop windmills, many people seem to want their homes to be self-sufficient or carbon neutral. I'm all for carbon neutrality, but in many cases that will be better accomplished by utility-scale installations, not backyard projects. I suspect many/most smallscale wind turbines are net energy consumers.

A more intersting question (to me at least). I'd been thinking that thinfilm and concentrating PV are the likely future of the technology. Is that really true? Currently polysilicon is expensive. Is the expense largely an artifact of the supply/demand imbalance? Or is it intrinsic to the technology? Likewise CPV power is a bit less reliable than diffuse light PV, as clouds will totally stop it. I imagine a significant part of total PV in unfavorable locations such as yours comes from diffuse light.

I hope Germanies overkill (in terms of PV subsidies) doesn't generate a backlash. The theory
of politics as being like a pendulum swinging between extremes seems to have some validity.


Ronald, it is because of the way maximum's are calculated - and that would be for a panel, optimally inclined, at the equator, at noon.
So even on the sunnyest day of the year, with no cloud cover, on the equator, a panel rated at 10kw would only yield an average energy flow of 5kw, as it is dark half the time.
Actually, it would not do even that, as of course you do not have the maximum solar incidence early in the morning, or late at night.
You also do not get the maximum when there are clouds, or whatever.
So it all boils down to that if you get the absolute maximum, with motorised dished to make the most of the sun when it is low or high in the sky, you get around 0.25 of the rated capacity as a 24 hour energy flow, or for a 10kw inatallation around 2.5kw.
bigTom , actually the latitude is critically important, as you get less hours of daylight in the winter at northerly latitudes, quite apart from the fact that the angle of incidence to the mirrors is likely to be wrong for the low winter sun for anything except a motorised array.
For the UK, averaged between north and south, and averaged over the year, for a flat panel you would normally get around 0.14 of the total installed capacity.
The variation is by around a factor of two between winter and summer - hey guys, it is light a lot longer in summer.
So Tom, yep, latitude is a major problem for PV - but almost everywhere with large populations is a long way south of Northern Europe - for instance Japan, with it's major PV program, and New York, on the same latitude as Madrid.
That's what is so weird about Germany going big on PV!


I have a friend who lives off grid. He changes the angle of his PV array twice a year, one angle for the winter and one angle for the summer. That way he can increase his winter generation without needing a motorized array.

Ronald Brak

Davemart, so you were refering to average kilowatts produced in winter, including night and day, rain snow and sun? Over November December and January, a thousand watt system in Ireland would produce an average of about 30 watts, so you'd need 33 kilowatts installed to get an average of 1 kilowatt. From May to July you'd need 5.2 kilowatts installed to get an average of 1 kilowatt.

Ike Solem

Take a look at how Germany is balancing three power sources to maintain a constant grid supply at


Solar and wind are intermittent sources, so the load balancing and energy storage are key. On-demand biogas plants can be used to supply any grid imbalance, and Germany is using a hydropower system to store excess wind/ solar energy.

The key thing for success is to have an efficiently integrated energy grid that is capable of taking energy from a number of different sources, some on-demand and some intermittent, and distributing efficiently to end users. The people at Kassel in Germany have the best approach I've seen.

As far as the number of homes issue, per household energy usage is a highly variable number. For example, the draw of ten 100W incandescent bulbs vs ten 25 W compact fluorescent bulbs makes a big difference, as does the amount of refrigeration, heating or air conditioning used. Generally, you'd want to present a range of households - x million inefficient homes to y million efficient homes - not some set number.

Cyril R.

Biogas is indeed one very good idea, considering they could respond quickly to fluctuations in solar/wind etc. And there's a large amount of fossil methane infrastructure already in place which could gradually mix more and more biogas (which is just methane but from biomass) with relatively few modifications.

I think after these posts here it may be better not to talk about homes at all, but just kWh's produced, maybe with a correlation with demand to show some degree of usefulness of a power source.

Expanding on that, it is wrong to state that even a nuclear powerplant powers x million homes because peaker/other powerplants have to provide extra power during high demand. So even a nuke doesn't fully power x million homes.


Ronald, for the winter that is even worse than I thought, by a factor of two.
It seems that at high latitudes for PV power we are talking about huge expense to produce power when it is least needed, whilst the contribution at peak periods is negligible.
Sure, nuclear also needs topping up, as it is expensive for non-baseload purposes, but it is available when you most need it.
Installation of PV would also make building nuclear less likely, as it is going to be producing power which would otherwise be nuclear in mid-summer.
None of this of course applies to areas like the American Southwest, where the power from solar comes in exactly when it is most needed, and anyway the reductions during winter are much less as you are further south.
Sensible strategies seem to me to vary a lot depending on where you are talking about, and if you are worried about CO2 emissions nuclear power is hard to beat for northern Europe, although dry-rock geothermal might fit the bill if costs can be reduced enough.


the claims of the german experiment are nonsense.
40% by 2050...and that only of electricity at current levels....
no allowance for heating and transport...no allowance for growing vehicle electrification.

germany is already the prime electricity importer in the world....which is probably why they this moonshine justification.

france is the world's prime electricity exporter...i suggest you wonder why! see p.27
of http://www.iea.org/textbase/nppdf/free/2007/key_stats_2007.pdf
germany's anti-nuclear stance is ludicrous.


Ronald Brak

Davemart, point of use solar power won't be profitable in Ireland until installed costs are down to about $1 a watt or less. If this does occur it will cut the use of gas, peat, coal and to an extent hydro power in summer while having little impact in winter. But currently wind seems a much better clean energy option for Ireland, particularly when the Irish grid is connected to Wales.


Thanks Ronald!
I wasn't aware that Ireland planned a link to the UK - that would improve the economics of both Irish power and provide another option for wind resource for the rest of the UK, rather in the same way as Denmark and Scandanavia are integrated.
I really don't believe in PV at the latitude of Ireland and the UK, but solar thermal panels are an excellent resource, and in my view should be mandatory in new builds.

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Solar still has a few years at the current rate of growth prior to storage becomes an important issue. If one or more of the seemingly wild claims I've seen it happen which could end up with a lot of money, but time-varying power.

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