WindSave: All Betz Are Off!

Update, May 2005: While it’s true that WindSave appear to have made their device obey the laws of physics (at least according to their published spec), I’m keeping this posting intact.

If the data on their website are to be believed, WindSave cannot generate the figures they claim. There’s much geeky theory involved, but basically, they’re claiming efficiencies that cannot be attained.

In short, they are claiming coefficients of performance of 0.724 and 0.887 for their turbines. Unfortunately, the theoretical maximum efficiency for a wind turbine — the Betz limit — is 0.593. So something, somewhere, is screwy. I’m pretty sure it’s not my sums, as they’ve been verified by an external source.

I worry that the UK energy minister, Brian Wilson, has been taken in by this. Five minutes with a calculator and a wind energy primer shows that these things are too good to be true.

Wind Turbine Power is given by:

P = ½ ρ A Cp v3 Ng Nb


ρ air density (~ 1.225 kg/m3 at sea level)
A rotor swept area, m2
Cp Coefficient of performance
v wind speed, ms-1
Ng generator efficiency
Nb gearbox/bearing efficiency

(from the American Wind Energy Association’s wind power FAQ,

Rearranging this:

Cp = P / ( ½ ρ A v3 Ng Nb )

If we assume perfect gearbox and generator efficiency (Ng = Nb = 1):

Cp = P / ( ½ ρ A v3 )

Since A = π d2/4, where d is the rotor diameter in metres:

Cp = P / ( ½ ρ π ( d2/4 ) v3 )

Plugging in the numbers:

Cp = P / ( 0.48106 d2 v3 )

WindSave‘s website gives the following performance figures for its machines, both at 25mph (12.07ms-1):

  • 1m diameter, 750W
  • 1.4m diameter, 1200W

which gives for the 750W machine:

Cp = 750 / ( 0.48106 × 12 × 12.073 ) = 0.887

and the 1.4m diameter machine:

Cp = 1200 / ( 0.48106 × 1.42 × 12.073 ) = 0.724

Betz’s Law, a proof of which is given on the Danish Wind Energy Association’s site,, states that the maximum Cp is 16/27, or 0.593

There’s also the small problem of turbulence. No renewable energy practitioner recommends putting a wind turbine on a roof, as the turbulence from structures causes the machine to run rough, increasing wear and noise. Plus you will very seldom, if ever, see the rated wind speed of 12ms-1 on a domestic rooftop.


  1. Not that I didn’t trust your calculations, but I like an occasional chance to practice my physics, so I checked them. And I came out with the same results.

  2. Your math looks okay to me. I don’t have the address of the web page any longer, but the only thing I can think is that someone may have confused radius with diameter, and that the 1m diameter is in fact a 1m radius. This would put the coefficient in the right range.

  3. The value of rho for air density that you use assumes standard temperature. Perhaps they did their tests at absolute zero (-273C) when the wind would be much denser and rho higher.

  4. Maybe Windsave are not assuming constant wind speed. If the average wind speed is 25mph/12m/s (given on this site: then the wind speed might be 24m/s for half the time, and 0m/s for the other half for example. Putting 24m/s in gives Pc=0.177 for the 1m turbine which is still high but more realistic. Maybe the turbine generates an average 750W in some periods when the average wind speed is 25mph. Their website doesn’t give any data at the moment, maybe they are revising it.

  5. Yes it is all a lie, a ripoff, whatever you want to call it. These turbines are the least efficient type known, and would never even approach the power coefficient of a regular turbine, let alone exceed it.
    Don’t waste your time with this nonsense.
    -Doug Selsam

  6. And what about the potential to rip away the masonry of a domestic structure? Isn’t the average semi’s roof likely to loose the bit attached to any turbine capable of harnessing such forces?

  7. I don’t think the minister, Brian Wilson, was taken in, it’s just a fact that Tony Blair and his team wouldn’t know the truth if it was printed in letters ten feet high. False performance figures of this sort give renewable energy a bad name, and they should be exposed.

  8. Brian Wilson taken in? According to web sources he is a paid consultant for Windsave. So that OK then isn’t it.

  9. Necesito información para posible comercialización en España

  10. I’ve been questioning these products since I first saw them and still think they’re living in dream world.Where are all the installations if its that good? The UK responsabilities under Kyoto would be covered by this equipment if its that marvellous

  11. David Gordon has a history of promising more than he delivers. His company ICC went bust in 1992

  12. If you look at the specification, the diameter of the blades are 1.75m, running the Betz formula again, you’ll see that it does what it says on the tin. Furthermore, the real ‘added value’ is in the plug and save box, it connects straight into the ring main with a standard plug…..

  13. January 3rd 2005 – This has been a very informative discussion. As I have just heard a discussion on BBC Rasdio 4 where the Windsave system was mentioned. (( – see Renewable energy to listen Again.) It caught my attention as many many years ago I was involved with a little tinkering with windpower as an undergraduate at Imperial College in London, – it seemed that what Windsave was capable of seemed too good to be true so I have spent the last half hour googling the subject, and it seems that even after 1 year since their launch they are still not available on the market and have not yet got their ‘grant status’ worked out. It is a long time since I used any complicated maths that you have discussed, so I just worked on a few simple figures from their web site – £1000 to buy and approx pay back in 5 years – They claim the machine has a design life of life of 10 years so the total saving will be £1000 total in the life of the machine: if we assume that it can save £1000 every 5 5 years so for the fisrt 5 it pays back £1000 and last 5 years of the machine’s life it saves £1000,or £100 per year if all goes well for 10 years and there are no accidents of servicing costs. HHMMMM. I think I will just buy a computer that runs on less wattage and other savings to acheive £100 saving per year. I live in Cornwall in the UK, near the windiest lake in England so I would get the good winds but I doubt the grade 2 listed Georgian building I am in will stand the forces. As I type it is howling outside !! and my tiles are rattling!!


    Jonathan Stedman

  14. I agree with your premise about windsaves’ gobbldygook. and having seen the turbine recently up close and personal.. it is less than impressive, judging by the diameter of this thing (somewhat less than 2metres) it will struggle to accomplish its rated output in a hurricane let alone in a 12m/s wind. I have a bergey xl1 (a company of long standing in the field) installed on an open site, it has a swept area some 30 % bigger and struggles to generate a kw in similar windspeeds….believe me I would not have it or any other turbine bolted next or near my house, cause when it furls in medium to high speed winds it sounds like a chinook and the force on the guyed steel support column is enough to deflect it some 100mm from the vertical (think house insurance).
    Personally I’m not sure whether this is an attempt to hitch to the clear skies initative or just a technically flawed concept ..time will tell.

  15. please will some one revise this website/entry – it’s two years out of date and not only has Windsave made massive changes to the product but also made sure that published data is accurate, valid and current.

    For those folks keen to check claims, I recommend that other micro-wind systems be looked at in close detail. One firm is claiming 4MWhr/year from a unit of 1.5kW rated output!!

    — “Richard” appears to be posting from somebody called “Graham” at WindSave. — scruss

  16. Interesting to find this site. I also did the calcs and together with the experience of part owning two larger turbines came to the conclusion that the claims are total fantasy. a realistic output from a turbine of this size is about 200 -250 watts at rated speed given a good clear site and some decent clear height in the tower. On a typical roof top installation the turbine will be lucky to produce 250 kWh/pa-far short of the 30% of a typical household requirement [3000-4000kWhpa] that is being suggested. I believe that the synchro-inverter is a sunny boy designed for pv use though only trace do a plug straight to main inverter that i know of. What is particularly worrying is that there seems to be a lot of pre selling to organisations who obviously have no clue as to what is real or not. The product has been recently featured in two broadsheets-virtually identical words indicating a press release rather than a researched article. As luck would have it I have access to good comparative testing facilities including computer monitoring re wind speed and direction so if windsave feel agrieved by the comments, I would be more than willing to participate in field trials of the product

  17. I am a civil engineering student undertaking a dissertation in ‘energy efficient housing’. I am considering domestic turbines as an alternative electricity source. I am particularly interested in how you come up with figures for annual savings and how the system works with regards to connecting to the existing grid. I would much appreciate it if anyone could provide me with any relevant information.

  18. Ha sanyone done a recent review on this product? I would love to get a small domestic turbine but am confused. Can someone help me

  19. I see that B&Q (UK diy store) is to sell this product starting in October 2006.

    Does anyone have an up to date review of this products capabilities – I’m very sceptical about the thing being attached to the wall of my abode. I would much rather have a stand alone arrangement.

    My main problem with this type of device is the fact that I live right under the flight path to the local airport (1/2 mile from runway) and am concerned about downdrafts (turbulence) from some of the aircraft flying overhead.

    I really wonder if this product can cope with that sort of turbulence.

  20. Many people seem sceptical about the whole turbine thing, however, not one person here seems to have taken on board the rationale behind the whole concept! i.e. reducing carbon emissions! Everyone seems to be focussed on saving money by having “free” electricity. And this is a very selfish attitude to have! Shame on you all!

    It would be very narrow-minded of anyone to assume this thing could solely give you an alternative to paying your electricity bill. But the broader picture should consist of:
    1) electricity prices are increasing – prices of turbines are decreasing – long term it can only get more and more cost effective
    2) fossil fuels used in power stations are running out, demand for something becoming scarcer means higher prices – hence point 1) is reached again
    3) climate change is being taken seriously (at last!) and it is inevitable that inefficient users will ultimately be taxed/penalised higher in the future, therefore the more energy efficient the consumer becomes, the less his energy bill, indirect or otherwise, will be.

    In summary, it might not physically save you money on your energy bill, but it will certainly contribute to it not getting any higher in years to come!

    DIY stores and independent companies are indeed promoting, selling and installing them, and grants and finance options are available subject to conditions.

  21. I wonder what I’ve done to get this flurry of responses. I’m actually in Glasgow, and saw my first WindSave installation yesterday. Let’s just say that where it was installed isn’t going to save much of any emissions.

  22. The keyword in relation to emissions is “reduce”, “save” relates to the planet, which is what we should all be aspiring to do through combined effort, however great or small. Windpower is not the only sustainable energy source available to us, and quite obviously may not necessarily be the most effective in certain areas. Just as solar may not be very wise six months of the year in the Arctic.

  23. Yes, but a wind turbine installed in a place where it sits idle 95% of the time may never recoup the energy it took to make it in its lifetime. That would consitute a net increase in emissions.

  24. With regards to David Atkins’ comments, Windsave aren’t doing this out of the goodness of their hearts or to save the planet. They’re in it to make a healthy profit and it seems they are willing to lie and bend the truth to Joe Public to make that happen.

    There is a serious shortage of information regarding this kind of turbine (not just Windsave’s), their TRUE performance in a typical urban/rural environment, their effect on buildings, the noise they create and pretty much every other feature.

    My opinion is that all this is withheld or blurred by the manufacturers because truth is pretty negative. I’d certainly like to know whether David Gordon of Windsave has one installed on his own house.

  25. Whilst I fully appreciate Windsave Inc. (or whatever ther corporate entity is entitled) isn’t a volunteer organisation and probably has mouths to feed, they have at least gone one further than debating the issue to actually providing an option, viable or otherwise.
    Other organisations are also offering their solutions, albeit some of them being in the prototype stages with little or no results yet other than theory.

    Changes in the rules and regulations of what is an acceptable amount of energy consumption will mean these organisations will already be “on board” and be at an advantage when the sh*t hits the turbine and heavy penalties for excessive consumption start to become enforced.

    British Gas, for instance, now insist on all their engineers and contractors being educated to be able provide best practice advice to consumers when new or replacement installations are undertaken by recommending appropriate control gear to make their boilers work as efficiently as possible. SEDBUK Band A & B boilers (>86% efficient) are also the only ones now acceptable in England & Wales in the majority of situations.

    Local authorities provide incentives for home owners to insulate their homes to recommended minimum requirements.

    New house builds incorporate water saving devices such as rainsavers to use for flushing toilets and washing cars so less water being processed at water treatments works, and therefore yet more energy, is being wasted.

    ALL of these actions serve to fulfil Part L (Conservation of energy & power)of Schedule 1 of the Building Regulations 2000 and which we all will at some stage have to comply with.

    The First Law of Thermodynamics states “Energy can neither be created nor destroyed, merely converted from one form to another” so no-one should be mislead by companies advocating massive unrealistic energy savings.

  26. The whole argument ref turbines etc will no doubt rage on for some time until elec gets so expensive that we will have no alternative but to go down the solar/tubine route. I have a house in Spain and I have just been offered a connection to mains electricity, the problem is they want 6000euros connection fee plus 4000 euros as my share of the pylons. This will give me upto 3.5 kw. if I go above that I will be cut off. If the cities require more power then it will be at the expense of rural properties and we will be cut off. Apparently that does occour quite frequently.
    I have opted to install a solar/turbine system which is sufficient to run a small house including a washing machine. We currently have solar power and that is brilliant, the addition of a turbine will make it even more reliable. The new turbine and 24 x 2.2v batteries will cost me under 4000 euros, its a no brainer for me.
    I think that we should continue to challenge the manufacturers ref their claims of performance and maybe even lobby for some controlling legislation but what we can’t do is dismiss the concept of household generated elec. Spains problem with power ie demand exceeds their capability to generate it, will be our problem in the not too distant future.

  27. It’s all very well to say, “don’t think of how much money you’ll save; you must think of saving the planet”, but from Windsave’s own specifications, the device doesn’t even generate enough wattage to power my electric kettle.

    Get real! To “save the planet” we need alternative energy sources which added together will supply the energy needs of a very energy-efficient household. Nobody will bother with them unless they do, especially if they cost more to buy and install than the average householder thinks they are worth.

    The nearest I have found to any wind turbine system supplying me with a worthwhile amount of electricity requires a rotor of 5 to 10 metres radius with an appropriate mast – for which I would probably not get planning permission in an Area of Outstanding Natural Beauty, and the cost of which I could not pay off in my lifetime.

  28. Anthony, I should add that electric kettles are a network planner’s nightmare. When I am installed as Ruling Technocrat #1 by a grateful world populace, they’ll be the first thing to be banned …

  29. The electric kettle was just clutched at as an illustration. Ban them, I agree! However, 500watts generated 24hours a day could only supply a worthwhile percentage of an energy-efficient household’s needs during the winter months if stored in batteries during low-use periods. At what environmental cost do these deep-cycle lead-acid batteries come? A large enough inverter would then be required to deliver the peak electricity requirement. We really need something that can make every household independent of the national grid without being too inconvenient for the householder. Whatever that is would be a real contributor to saving of the planet.

  30. Hi, Your calcs are very interesting. I have always been sceptical about the claims for these small roof mounted turbines for several reasons; payback times assume that you are able to consume the energy when it is produced, wich considering this system is off grid makes me sus. Also the mountings would be under serious stress and I question wether this would be structurally viable for 10 years. Further feeding into the ring main is legally questionable as this could on some meters cause them to malfunction if my research is correct. In addition the noise claims for the system are completely out of sync with the stats for other turbines. This would appear to be a case of a big company cashing in to the general publics partial knowledge of climate change and lack of interest in fully researching these technologies before spanding their money. The ols addige of you only get what you pay for appears appropriate to this product and I gather that the claims made will not be leggally binding or checked by any of the easily lead who will buy this generator. I personally see no problem with the national grid provided you are aware of the massive amounts of power lost in this massive system. I think that the answer is in efficientcy, and on grid small scale generation. Our family of 5 have reduced our electricity consumprion from 13.5 to 11.5 kwh per day and I think we can go further. We cook with elec, heat in the main with a wood stove, dry clothes in front of the stove and wash the clothes in a big and efficient machine. We do have two freezers and two fridges so we can save more there on getting more efficient models. Really the problem is with dish washers, and clothes driers, filament light globes, appliances on standby and a general lack of understanding about the massive amounts of energy we in the first world use in contrast with those in the developing world. I have three children all of whom have a right and a desire to inherit a world that is worth living in. They put much of the effort in to reduce our power use and we enjoy and celebrate our lack of consumption.

    Realy things like windsave do very little to help and just wrongly ease the consience of those who over consume.

  31. I have read all the comments with interest and agree that a vertical wind turbine attached to the side of a house does not perform as quoted in a wind tunnel. The vortices alone keep the machine spinning and the head out of the wind. We are at present testing vertical axis wind turbines in the uk specifically build for an urban environment. We are still doing the math on them but we are also monitoring there actual performance in real life (not in a lab!) will keep you posted.
    You will note that as a company we also consider energy loss as well as production and agree that saving energy is as valueble as making it !!

  32. Hello,
    We have a windsave turbine installed on our house and I am afraid we are not at all impressed, after some modification the noise has been reduced to minimal but the turbine is not efficient and it keeps developing faults just when it appears to actually be doing something.
    We are willing to let windsave get it right within reason and will keep you updated on their progress and how the machine performs.
    So at the moment the matter is in their hands and we await a speedy solution.

  33. hello i managed to purchase 2 windsave genorater motors which i intend to use for water heating they do not have the grid tie inverters but will wire one of them into water tank heating and try to use the other for batterry charging through a charger they seam to give up to 290v when charging so have to use the power on somthing that can take excess voltage will keep you informed if it works.

  34. Being a manager and entrepreneur I recently started to work on a project to promote a very efficient vertical axes wind mill technology. Developers claim that its efficiency factor is 0.82 that far exceeds 0.593 by Betz limit. So I was curious if there is any theory. It is! Citation: “Betz was a graduate student barely out of Advanced Calculus when he formulated the famous Betz Limit. His simplistic mathematical metaphor has been invalidated for decades by Euler based Navier / Stokes Computational Fluid Dynamic theory”.
    Sorry – I am not familiar with Windsafe but speaking about efficiency the most important characteristic of any wind mill is its “power curve”The real energy produced in your specific wind conditions depend on the specific wind rose. To got realistic estimate the wind rose data must be measured and collected by you using some whether station BEFORE you try purchasing a wind mill. Well, i ngeneral Tebbb might be right. Regards!

    c’mon, that Euler turbine site reads like dissociated press output. Show me a year’s output from a utility-scale prototype, independently verified to IEC standards, and I might start to doubt Betz’s theory.
    Oh, and that YouTube video you tried to link to shows a deep and abiding lack of understanding of wind resource basics. — scruss

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