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Solar panels – are they really a clean energy technology?

I bought a couple of solar photovoltaic cell (PVC) panels last year.  They’re generating electricity off-grid, so I’m not eligible for the Feed-In-Tariff (FIT).  Which is perhaps as well, since this article isn’t about solar subsidy.  Instead, it’s about a question often raised when the subject of solar panels comes up: does manufacturing a solar panel use more energy than the panel produces over its lifetime? 

It’s an important question, because solar PVC is often referred to as a “clean” technology – one that generates electricity without producing environmentally-harmful greenhouse gas emissions.  Solar panels do this at point of use of course, but they are complex pieces of kit with many components, and energy is required in their manufacture: mining the raw materials, processing, assembling etc.  As the world’s electricity system is still primarily fossil fuelled, the energy required to manufacture the panel will probably have come from burning fossil fuel, which means greenhouse gas emissions – the very thing a solar panel is supposed to avoid.

The easiest way to determine whether there is a clean energy payback from solar PVC is to work out the “Energy Yield Ratio” (EYR, the ratio of energy delivered over a panel’s lifetime to the energy required to manufacture it).  If the panel’s EYR is greater than 1 (or “unity”, the break-even point), the panel will generate more energy than was required to manufacture it and is therefore an environmentally sustainable energy source.  But if the EYR is less than unity, more energy was required to manufacture the panel than it will produce, and it cannot be labelled as a sustainable energy technology under current production methods.

So what is the EYR of a typical solar PVC panel in the UK?  To make this calculation, first you need to know how much energy was required to produce the panel.  Then to calculate how much energy the panel itself will produce, you need to know the average sunlight intensity at the point of use and the conversion efficiency of the panel (how much of the light energy hitting the panel can be converted into electrical energy).  You also need to know how many years the panel is expected to last for.  Fortunately this information is readily available.  The only thing to bear in mind is the units: power is the rate at which energy is generated and has the unit Watts, which is joules per second.  Energy produced or consumed is generally measured in kilowatt hours.  For example, if a solar panel has a power rating of 100 Watts, over the course of an hour at maximum output it will generate 100 watt hours of energy, which is 0.1 kilowatt hours (kWh).  Over 10 hours at this output it will generate 1000 watt hours of energy, which is 1kWh.

To treat each of the above points in turn:

  • To produce typical silicon solar PVC panels requires around 420 kilowatt hours of energy per square metre (kWh/m2).  (1) This is roughly the same amount of energy an old-style 100W incandescent light bulb would consume in 6 months.
  • Average light energy intensity in the UK on a south-facing roof is around 110 Watts per square metre (W/m2).  (2) 
  • The conversion efficiency of typical solar PVC panels used in the UK is about 12%.  (3)

So 110W/m2 of light energy at a conversion efficiency of 12% means a solar panel has an energy rating of 13.2W/m2 – over an hour, it will produce 13.2Wh/m2.  Average day length in the UK is about 12 hours – average over the year – so in a day it will produce around 158.4Wh/m2 of electrical power.  Over the course of a year this adds up to just under 58kWh/m2.  Over the 25 year life of the panel it will therefore generate around 1450kWh/m2 – around four times as much energy as was required to produce the panel, giving an ERY of 4.

So even in the UK, which receives sunlight intensity of only around 60% of that at the equator and where average daylight hours over the year are comparatively short, solar PVC is still an environmentally sustainable source of energy.  And it’s expected that production processes will continue to become more efficient in the future, so it’s probable that the EYR will continue to increase.

(1) http://www.nrel.gov/docs/fy04osti/35489.pdf.

(2) http://www.inference.phy.cam.ac.uk/sustainable/book/tex/sewtha.pdf

(3) http://www.inference.phy.cam.ac.uk/sustainable/book/tex/sewtha.pdf

25 Responses to “Solar panels – are they really a clean energy technology?”

  1. Charles Mossman says:

    I wonder what the energy yield ratio is for Fracking? I think the DECC should do a calculation to see if there is actually any energy benefit to be had once all the multitude of factors discussed above are taken into account. I suspect the EYR may well be less than one.

  2. Abhijeet Nayak, EnU Consultant at Cognizant says:

    We all know about the concept of Opportunity Cost. So, a different perspective to this would be the Opportunity Carbon Cost of a PV. Imagine if the consumer had not installed the PV, what energy would he have gone for? Thermal?Biomass?Nuclear?
    Let’s say if the community had instead set up a coal fired plant, the lifetime carbon footprint would have been un-comparable. So apart from the carbon footprint of the Manufacturing we should look at it in light of the operational emissions as well.
    Then another perspective may be the geography & the weather. Take the case of Hawaii(where solar grid-aprity has been achieved) or any other tropical/equatorial country with low wind density. Their best option would be solar. State of Gujrat in India is a shining example. (But a blessing in disguise for Gujrat was that it abounds in non-arable barren land which can be best suited sites for Solar Farms)..

  3. [...] Department of Energy & Climate Change blog. Read the original article and join the discussion here. (function(d, s, id) { var js, fjs = d.getElementsByTagName(s)[0]; if (d.getElementById(id)) [...]

  4. alan says:

    hi jonathan, like the article, looks like you may of missed out the energy used to produce the inverter, additional wiring and meters. all these things are required in a solar pv installation. so surely this needs to be included in the calculation.

  5. World is under “transition and transformation phase” of energy; we are doing research to discover “GHG emission free energy”. It will take some time but our scientists will be able to achieve this goal at cumulative rate. Solar Panels provide safer green energy. This technology is boon for rural areas of those parts of world where there is no single source of power supply. In future, solar power will be the mutual part of life.
    Thanks Jonathan for this well written, well discussed and EXCELLENT article.
    Regards
    Prabhat Misra
    http://www.twitter.com/redtapemovement

  6. Alan P says:

    As Matt F states, there is a lot more to consider than just the panel. Do any of these calculations take into account the energy/carbon for the production of cables, batteries, control panels and the frequency of maintenance visits? Also, if Matt F is installing PV ‘farms’ (which from the material list it looks like) has anyone calculated in the loss of natural habitat for flora/fauna etc?

    How do the figures produced for PV stack against the likes of small-scale hydro projects?

    Sorry to be asking all these questions, but it is good that Jonathan and other folk are considering life cycle analysis of such products.

    • Matt F says:

      Alan – not a solar ‘farm’ a 40 panel system on unimproved grassland but I take your point – there is some impact on the field the panels are in. However, when set against 10,000 units of electricity produced by the panels (not from the dirty grid) the carbon offset is vast.

  7. I think it is misleading to say that “If the panel’s EYR is greater than 1 (or “unity”, the break-even point), the panel will generate more energy than was required to manufacture it and is therefore an environmentally sustainable energy source.”

    Whilst it is true it will generate more than the embodied energy in manufacture, it is not that simple to say it is an “environmentally sustainable energy source”. The reasons for this are:

    i) The solar panel still relies on fossil fuels which are a finite resource and create pollution.

    ii) Environmental sustainability is not just about energy payback, it is also about other environmental impacts such as land use change, acidification, resource depletion, etc.

    iii) Most studies also take no account of disposal at the end of a products life, as for most Solar PV in the UK this is largely unknown. End of product life can have large environmental impacts depending on how products are disposed or or recycled, i.e. can be energy intensive.

    Furthermore one final issue with net energy analysis is that it does not take into account the energy consumed in labour, e.g. people installing.

    From a net energy analysis perspective though I like this simple analysis and the fact it demonstrates if you take into account one environmental impact (energy use) using Solar PV should reduce the rate of resource depletion.

    • Matt F says:

      It is certainly true that many kilocalories were consumed by us when installing the system I have used as an example – not to mention the:

      • 10 tonnes of shingle (dredged, stored and delivered)
      • PVC mounting buckets
      • Timber edging
      • Weed control textile
      • Module shipping from Singapore and onward delivery
      • Balance of system component manufacture
      • Daily journeys to the job
      • Disposal of packaging
      • Heat and light used in the office in planning the installation

      It is very difficult to quantify all of the environmental impacts accurately since we will never know exactly what has happened before we arrive on site to install so having some kind of benchmark is useful. A universal calculator would be great but is probably impossible to produce with any accuracy.

  8. Matt F says:

    For a start – PVC – what are you talking about? Polyvinyl Chloride?

    Secondly, 12% efficiency – you mean 15%. SEWTHA is a great publication but on this subject, it is out-of-date.

    To get a true reflecton of the ‘carbon payback’ of PV modules, actual yield is key not theoretical efficiency:

    Each of our monitored, south facing; 25 degree inclination, modules produces 250kWh (units) per year in the south of England. Each module is approximately 1.5m2 so we divide 250kWh by 1.5 to find the per m2 value, which gives us 167kWh/m2 per year.

    If we then conservatively say that the life of each module is 25 years, that gives us 4,167kWh/m2. Even using the outdated 2004 figure of 420kWh/m2 to produce the module, that gives us 10 times the production energy cost. In your terms an ‘EYR’ of 10.

    Of course, the wise among the PV aficionados will note I have not factored in the predicted 1% per year performance drop for PV modules but then this is just a fag-packet calculation – like the one above?

    If you want a clear explanation of what you attempt to explain try the Centre for Alternative Technology: http://info.cat.org.uk/questions/pv/what-energy-and-carbon-payback-time-pv-panels-uk

    • Peter Hill says:

      The figure of 110 Watts/m2 used in the article is per hour per day – based on 24 hours a day (immediately doubling the EYR in the article as Jonathan has used an average of 12 hours sunlight a day not the actual figure of 24 hours a day). Worked out another way – the average solar irradiance received by the UK is 1000kWh/m2 per year – divided by 8760 (the number of hours in a year) = 114W/m2. Also, that’s per m2 of flat land – a tilted array would receive more per m2 than that. The David Mackay book (page 40) references a 25m2 system that produces 12kWh/day – 0.48kWh/m2/day – that equals 175kWh/m2/year – almost 3 times the 58kWh/m2/year in the article.

      • Matt F says:

        It doesn’t matter what any calculations based on theoretical irradiation for a country wide area give us. We can’t predict the future climate either so nothing on this page is indisputable but we can at least use empirical data.

        The figure of 250kWh/year/panel I refer to is actual data, which must be considered a better basis for future prediction than any purely speculative numbers.

  9. Danny Buckley says:

    Feed-In Tariff payments
    All renewable electricity generated by an eligible installation can receive payments under the Feed in Tariff for every unit of electricity produced. This is true for off-grid systems as well as on-grid ones – you will get the same generation tariff, though obviously you can’t get export payments.

    Quote from EST WEB SITE

    • Matt F says:

      As long as you have an approved Ofgem generation meter fitted and your panel and installer are MCS accredited. You will receive 7.1p/kWh until October 31st when DECC will tinker again!

  10. santosh says:

    Dear Jonathan,

    is their any reference for 420KW/m2 for manufacturing of panels?. I think this is too much. On weight basis it will be more than a ton of steel produced.

  11. Danny Buckley says:

    My understanding – check Energy saving trust web site – is that you are eligible for FiTs. If this is the case it is worrying that someone at DECC doesn’t know this !

  12. Chris Wright says:

    Good to see someone have a go at this sort of analysis. Presumably also depends on actually being able to use/store all of the rated output (demand is variable within a day and across a year and doesn’t necessarily correlate with periods of daylight)

  13. Martin Sherring says:

    One more correction – the actual life of the panel is likely to be well in excess of 25 years, in fact most of them are warranted to generate 80% of rated power after 25 years.

    An EYR of 4 (or say 6 if we increase the life by 50%) is at least positive, but comparing it with the early on-shore oil wells (EYR of about 100), we are obviously going to need to be more careful with how we use electricity.

  14. Arthur Akinyemi says:

    I hope this report will help manufacturals
    to continue driving the production cost
    of cells down.

  15. Craig Siddons says:

    Hi Jonathan,

    Sorry to have to point it out, but I think you may have mixed up your definations of energy and power:

    Energy is generally measured in Joules (J) or kilo-watt hours (kWh).

    Power is generally measured in Watts (W).

  16. James Hastings says:

    Hi Jonathan,

    Thanks for your analysis of the likely EYR for solar pv panels in the UK. However, just as a point of correction, average daylight hours per year are pretty much the same the world over, i.e. 12 hours, give or take a few minutes, given the slightly elliptical shape of planet earth.

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