The Bottom Line About Wind Turbines

wind turbine sunset

The wind is perhaps the most unpredictable and unwieldy source of power we can use. The 'fuel' is the wind, and there is no controlling it, you need to take what's there and hopefully protect the system when things get out of hand. Imagine a gas powered generator with no throttle control--one moment it stops, the next moment its running full speed--and you need to figure out how to use the power that's coming out, and figure out how to keep the engine from blowing up when the throttle gets pushed up a little too high! All good wind turbines have some system to protect themselves in high winds. We'll discuss that in more detail later.

Do I really want a wind turbine?

For people who live off the power grid, wind and solar power can compliment one another nicely. Often if the sun isn't shining then the wind is blowing. Having a wind turbine in your off-grid power system can allow you to have fewer solar panels and a smaller battery bank. The cost of solar versus wind depends a great deal on your available resources. In some places (like Northern Colorado where we live) the sun shines over 300 days a year, so solar power makes a great deal of sense. We're also fortunate to have a reasonable wind resource, so the two complement one another nicely. In some places the sun doesn't shine for weeks at a time and wind power might be the most cost effective solution. We live 11 miles from the nearest utilities and for us wind power is very cost effective compared to our other options.

Can I get a wind turbine and unhook from the power grid?

Yes, you can -- but it's not likely to be cheap or easy or cost effective if you already have grid power. Lots of companies selling wind power systems and plans will tell you only what you want to hear. Do your research -- look for information provided from objective sources who have nothing to sell. There are many good books and some good websites on the subject, and it is wise to educate yourself before you spend your money.

It almost always costs more to generate your own electricity than it does to buy it from the power grid. Just the cost of batteries alone can often break down to a monthly cost greater than you'd have paid had you bought the same power from the grid. These costs can vary, and if you buy surplus/used equipment that can change your overall cost per kilowatt-hour (kwh) dramatically in an off grid system.

There does tend to be an economy to scale with windpower though, and with larger machines it can become cost effective with a reasonably short payback period. Before considering generating your own power to reduce your power bills--do everything you can to conserve! Some estimate that one dollar spent towards conservation is better than 10 dollars spent on generation.

Wind Power Facts

We cannot think of another 'industry' where we see more misleading information out there than with small household-size wind power. Because accurate wind speed is difficult to measure and many customers are not well-informed, it becomes easy to make unrealistic claims about a machines performance and it seems unlikely that those claims will ever be contested.

Power in the wind(Watts) = 1/2 * rho * A * V^3

This is perhaps the most important formula to in wind power. It tells you how much energy is in the wind (not how much you can extract) where:
rho = 1.23 (this is air density, 1 cubic meter of air weighs 1.23 kg at sea level)
A = swept area in square meters. If your wind turbine has a blade diameter of 3 meters (about 10 feet) then swept area is a bit over 7 square meters (or about 80 square feet). Because of the formula for area, it's important to keep in mind that doubling blade diameter will cause the machine to sweep 4 times the area. Power available from a wind turbine is related to the *square* of the blade diameter.

V = velocity (wind speed, in meters per second). Notice the V is to the 3rd power (cubed). This means if you double wind speed you have 8 times the energy. This is very important to think about. If a machine is designed to produce usable power in a 10 mph wind then it has to deal with 8 times that much energy in a 20 mph wind, 64 times that much energy in a 40 mph wind, and 512 times that much energy in a 80mph wind, etc... For the sake of the wind turbine and its tower, there must be some system by which it protects itself.

So let's look at the formula above and consider a small 5 foot diameter (60 inch) wind turbine in a 10 mph wind. This is a bit larger than the popular 'Air' series of wind turbines built by SouthWest Wind power.
Diameter = 5 feet = 1.524 m
Swept area = pi x r^2 = 1.8241 square meters
Wind speed = 10mph = 4.4704 meters/second
Power in the wind = 1/2 * 1.23 * 1.8241 * 4.4704^3 = 100.22 Watts.
And remember, if you double wind speed you have 8 times the power, so in a 20mph wind there would be over 800 watts available--but you can't harvest anywhere close to that amount in practice.

So above you can see how much power exists in the wind in theory--and this is assuming we start with a certain wind speed and stop the wind completely. Of course... we cannot stop the wind, then it would pile up around our wind turbine and the machine would quickly stop making power. In 1919 Albert Betz proved mathematically that at best we can only slow the wind by about 1/3 and he calculated that the most power you can take from the wind is 59.26% of the total power.
So the very best a wind turbine can do is 59% of what the formula above tells us. This means that in theory, a 5 foot diameter wind turbine could produce 59 Watts in a 10 mph wind. In practice no wind turbine blade is perfect and a good blade on a small wind turbine might be around 30-35% efficient. A good small 5 foot diameter wind turbine blade will likely produce about 30 Watts in a 10 mph wind, and this is just mechanical energy into the shaft -- it does not account for alternator/generator inefficiencies. Most small wind turbines have good electrical efficiency in low winds and then tend to drop off in efficiency in higher winds. Electrical efficiency of 80% or so should be expected in low winds and in higher winds (25 - 30 mph) it is often down around 50%, unless there are power electronics involved to improve the matching of the blades to the alternator/generator.

It's easy to use the above formula to see how much energy is available in the wind with any known wind speed. Any claims of output greater than 59.2% of that are definitely breaking the laws of physics, and any claims about a machine greater than 30% or so are probably unrealistic. This is important to keep in mind, because we frequently see people making impossible claims about wind turbines.

Swept area and rated output

'Rated output' of a wind turbine is fairly meaningless compared to swept area. We often see 'light duty' small wind turbines rated for 1000 watts or more in high winds, and then see another manufacturer with a nice big heavy slow machine thats rated for about the same output but in much lower wind. Remember the power in the wind is related to the *cube* of the wind speed. There is twice the power in a 25 mph wind as there is in a 20mph wind. So the small 1000 watt wind turbine that claims to produce 1000 watts in a 30mph wind is not comparable to the larger 1000 watt wind turbine that produces 1000 watts in a 20mph wind.

Swept area is also important because of how the wind comes to us. Slow winds in the 10-16 mph range are by far the most common, and winds over 25 mph are extremely rare. This varies only a very small amount between sites. In fact, the difference in *average* wind speed between an excellent site and a poor one is less than 5 mph! The chart below shows how wind speeds are distributed at almost every site worldwide.

distribution of wind speeds
The chart below shows windspeed across the top and blade diameter/swept area(square feet) down the left side. The numbers in the middle suggest what sort of power output you might expect from an 'efficient' wind turbine of such size at such windspeed. This is assuming the wind turbine captures 35% of the energy available in the wind -- it is possible to do slightly better (especially in low winds), but most machines will not do nearly as well in higher winds as the chart suggests. This stuff is interesting to look at! We sometimes see machines for sale on Ebay and other places with 7 foot diameter rotors that claim to be 2kw machines. Perhaps in hurricane conditions they can produce 2kw, but a potential buyer you need to understand what is, and what is not possible. We've actually seen machines of less than 8 feet in diameter on Ebay that claim to produce 25 watts in 'very low 2 - 4 mph winds'. It's unlikely most wind turbines can even spin in winds below 5 mph - and if they could, then an 8 foot machine could not possibly make more than about 5 or 6 watts in a 4 mph wind.

When shopping for a wind turbine it's much more important to look at swept area than it is to look at the manufacturers rated output.


Wind speed (mph)  2      4       6       8      10      12      14      16      18      20      22      24      26  


4'     12.56          .18     1.4    4.6    11     22     38      60       90      128    176    234    304    386

6'     28.26          .39     3.1    10     25     49     85      136     202    288    396    526    684    868

8'     50.24          .70     5.5    19     45     88     151    241     359    512    703    934    1215  1542

10'   78.5            1.1     8.6    29     70     137   237    377     564    800    1099  1460  1900  2410

12'   113             1.6     12     42     101   198   341    524     809    1152  1582  2102  2735  3469

14'   154             2.2     17     57     137   270   465    739     1103  1571  2156  2864  3727  4728

16'   201             2.8     22     74     179   352   607    965     1439  2050  2814  3739  4864  6171

18'   254             3.6     28     94     226   444   767    1219   1819  2591  3556  4724  6146  7798

20'   314             4.4     35     116   279   533   948    1507   2248  3203  4396  5840  7599  9640


We've gone over two very important things that we often think about here at Otherpower. The power in the wind is related to the *square* of the blade diameter (double diameter and you get 4 times the power) and the *cube* of the wind speed (doulbe the wind speed and you get 8 times the power). You have a great deal of control over blade diameter -- you can choose what size machine you buy. What about the wind speed? It's often suggested that the most economical choice is to put up lots of machines on short towers, or mount your wind turbine(s) to the roof. Manufacturers say this stuff because it sells machines -- people do not like the idea that a tower costs money, and they love to hear that once they buy the wind turbine very little further cost/effort is required. However, the wind is your fuel and wind turbines need good clean non-turbulent fuel -- and unfortunately you don't find that on the ground or on your roof!
It is usually suggested by most reputable installers and manufacturers that the most cost-effective arrangement will be to get your wind turbine 30 feet above anything withing 300 feet. If you have turbulent 10 mph winds at 30 feet up and smooth 13 mph winds at 70 feet up then it makes sense to go a bit higher, get out of the turbulence and get into a slightly higher windspeed where you might have *twice* the energy available! This is not the sort of talk that sells wind turbines, but it is reality. In most good installations the cost of the tower is much greater than the cost of the machine itself and there is a good reason for that.

Can I build my own wind turbine?

homebrew 20 foot diameter wind turbine

Sure you can! Pictured above is our home brew 20 foot diameter wind turbine that powers our home and shop. It's an axial flux machine built very much along the same lines as you'll see in Hugh Piggott's books. We think building wind turbines is great fun, and there are some good books, plans and parts available from our online store. It does get a bit tricky to properly match a given blade set to a given generator/alternator, so you either have to be prepared for a good bit of trial and error - or - get good plans. Some plans are better than others. A good bit of our plans for building a turbine are available free online, and will be published in great detail in our upcoming book. Building a wind turbine is not a cheap or easy project -- you still need a tower, batteries, and rest of your power system -- so it makes sense to do your research as you would if you were buying a wind turbine. I expect if I was just getting into that hobby I would buy a few sets of plans and read over them all before investing any time or more money.

And, be sure to check out our book Homebrew Wind Power for more small wind power information!

Buyer beware!

As we explained above, the small household-sized wind turbine industry is plagued with misinformation and hype. The following is snipped from an excellent article, and is used by permission of the author, Paul Gipe, his Copyright 2006.

Fantasy Wind Turbines or If It's Too Good To Be True . . .

April 24, 2006
by Paul Gipe
Scams, Frauds, & Flakes--Tell-Tale Signs

German engineering professor Robert Gasch calls them fantasy wind turbines. These are the inventions--or contraptions--that bedevil serious wind turbine advocates. They are the "revolutionary" inventions that periodically rise up from the dead whenever the price of oil goes up or there's a "power crisis" somewhere in the world.

Very few of these ideas are new, and certainly none of them are "revolutionary". While some of these inventions may spring from well-intentioned inventors, others are the brainchilds of fast-buck artists of--shall we say--questionable reputations.

It's often difficult for the unitiated to tell the difference and therein lies the problem. How to separate the real from the imaginary, the fraudulent from the worthwhile.

Below are some tips for spotting these questionable products. The most important tip to keep in mind is that from Robert Gasch. If there is a new wind turbine, no one should pay the slightest attention to it until they "build it, measure it, and publish" the results. Until then, it's just hot air--and nothing more.

How can you identify a "questionable" wind turbine design?

  • Hype High--Experience Low
  • Aggressive Marketing--Look for Multi-Level Schemes
  • Pyramid Schemes (see above)
  • "Get in on the Ground Floor Now"
  • New Design-- "Not Like Those Others"
  • "New" Patents
  • Targeted at Unsophisticated Market (People who don't know a Wind Turbine from Shinola)
  • "Works @ 2 m/s!
  • Drag Devices
  • Ducted Turbines!
  • It is Silent (No Wind Turbine is Silent)
  • It Does Not Kill Birds or Bats (This is one of the Most Disreputable Tactics)
  • Fancy Web Sites--Are Always Cheaper than Hardware
What lessons have we learned from 30 years of modern wind turbine development?

There are:

  • No Panaceas
  • No Cheap Solutions
  • No Breakthroughs--No Miracles
  • Numbers Matter (Wind Energy is Always about Numbers)
  • Experience Matters (If They Haven't been Building these things for Years then How Do You Know that it Works)
  • Size Matters (You can't get Blood from a Turnip nor a lot of Electricity from a Small Rotor)

In sum always be wary of "New" Designs. There's rarely anything truly "new" under the sun--or in the wind.

Good Advice from Paul Gipe! Check out his excellent website where you'll find lots of informative articles and lots of good books on the topic.

Wind power has it's place. It should be quiet, reliable and easy to live with. It is not always cheap or easy, and if you're thinking of having a windpower system you really need to inform yourself about what's available, what's realistic, etc. So... Conserve energy, inform yourself, and have fun with it all!