How much power can a small wind turbine produce?
How much power can a small wind turbine produce?
Warning: a little bit of math follows – not to hard, bear with us. We will give examples for 2 small turbines, 
one with 2 foot blades and one with 4 foot blades.
Power AVAILABLE in the wind = .5 x air density x swept area x (wind velocity cubed)
Air density = 1.23 kg per cubic meter at sea level. Swept area = pi x r squared. Our 2 foot blades = 0.609m, 4 ft = 1.219m. 10 mph = 4.4704 m/s, 20 mph = 8.9408 m/s.
How much power is in the wind: 2 ft blade, 10 mph winds = .5 x 1.23 x 3.14×0.609squared x4.4704 cubed
= .5 x 1.23 x 1.159 x 89.338 = 63.7 watts
With 4 foot blades and 10 mph winds = .5 x 1.23 x 4.666 x 89.338 = 256 watts
With 4 foot blades and 20 mph winds = .5 x 1.23 x 4.666 x 714.708 = 2051 watts
That’s the MAXIMUM power in the wind. However, it’s impossible to harvest ALL the power. The Betz Limit tells us that the maximum percentage of power we can harvest from the wind is 59.26%.
Thus our maximum power from these turbines would be:
2 ft blades, 10 mph wind = 37.7 watts
4 ft blades, 10 mph wind = 152 watts
4 ft blades, 20 mph wind = 1,215 watts
Keep these numbers in mind when a sales person starts to tell you their small wind turbine will meet all your needs….
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Filed under: Wind Turbines
Would these systems be more effective by using a multi-blade unit, similar to the old farm windmills?
Could you use more generators?
Thank you for any information that you may provide.
Lewis A. Morris
Multi-blade wind turbines, like the old wind mills on farms that pump water, provide more torque. They generally start turning sooner and because of the torque, they can move something that’s providing a lot of pressure.
Triple blade turbines, are easy to balance and provide more rpm, thus more voltage.
If you want to pump water, use several blades. If you want to generate electricity, use fewer blades.
I’m not sure what you mean, when you ask if you could use more generators. We use different sized DC generators (motors), based on the power you want to produce. Bigger motors required bigger towers and blades (more money) and produce more power.
Did that answer your questions?
Best regards,
Bruce.
Great information!
What is the time frame?
Hour? Day?
These calculations are for watts per hour. Thus the maximum possible power a wind turbine with 4 foot blades in a 10 mph wind, could produce, is 152 watts. Homemade turbines will certinly produce less than this maximum hourly wattage.
Hi Bruce im a student and im doing a report and i was hopeing that i could use some of this information in the report and i was wondering if you could tell me how you came across this info and what your title is are you an expert in the feild or what thank you
Jeremy
Hi Jeremy,
The math, (Power AVAILABLE in the wind = .5 x air density x swept area x (wind velocity cubed), is public domain information. You are free to use any information you find on this site as part of a student report. If you post any info on another website, please make referrence to our site.
My backgound is via hands on – the wind turbine in the picture on this site, is one that I built from scratch and now is running at my cottage.
Note that the power numbers listed are the maximum available. In practice, a homebuilt turbine will likely get about 30% of this maximum.
If you need more info, you can email me at:
feedback @ greenterrafirma.com (take out the spaces).
Regards
Bruce (publisher – Greenterrafirma.com)
Using a 1kw generator as an example, what is the relationship of torque to rotor speed to the torque required to turn the generator or (put another way) How much torque is necessary to turn a 1kw generator and how does that equate to wind speed?
Thanks,
Mark
Mark, there are several 1.5 horsepower dc generators out there with a wide variety of specs. Does your generator produce 10 volts at 60 rpm or at 600 rpm? What is the max amps that the generator will support? Maybe you can provide the specs for your dc motor…
Thanks,
Bruce.
Actually this is a due dilligence question in advance of me puchasing a 1kw turbine. The goal is to purchase a turbine that has a low startup speed with a rated speed at around 300 rpm. Hope this helps.
Thanks,
Mark
Mark, what’s most important to you then, is the power curve for the particular wind turbine that you are looking at. And of course, the average windspeed for the location where it will be installed. However, given that the average windspeed will be the same, you’ll want to compare the power curves. These curves will show you at what wind speed the turbine starts to produce power and at what windspeed they achieve their rated output. There could be very significant differences in the power curves of different turbines.
You can check the wind speed maps on the internet. Some will show your average windspeed at several different tower heights. You could also try the nearest airports to see if they have average windspeeds that are more specific to your site.
The total power you can expect for that 1 kw wind turbine, will be determined by reading the power produced, at the average windspeed for your site. A turbine that starts soner, but produces less power at your average site windspeed, will still produce less power for you over a yearly period.
One last note: if the people trying to sell you a wind turbine don’t have a documented power curve, one that shows the power DOWN at your average windspeed, then RUN, don’t walk, to the next dealership. I hate it when someone produces a power curve that starts at 10mph when my average windspeed is 6mph.
what would be considered ‘best conditions’ for a wind turbine?
if you could always control (i.e. it only blows in one direction) what direction the wind is coming from and its speed how much power could be produced?
Cody,
If you could control the wind, you would want it to always blow at the “rated speed” for the wind turbine that you have. Every wind turbine lists the power that it produces at it’s rated (highest) wind speed.
A wind turbine that is advertised as a 2 kw wind turbine, will produce 2 kilowatts per hour at it’s rated speed. Rated windspeed for “small” wind turbines is usually around 20 mph.
For example, the Skystream 3.7 is “rated” at 1.9 kw and only produces this much power at wind speeds of 20 mph. In many parts of North America, the average windspeed is much closer to 12 mph. At 12 mph, a small wind turbine produces about one tenth of it’s rated power level. Big drops in windspeed result in big drops in power output – or big increases in average windspeed mean big increases in power output.
Hope that was clear.
Here’s a link to the Skystream 3.7 data sheet:
http://www.skystreamenergy.com/documents/datasheets/skystrea_%203.7t_datasheet.pdf
Hey thanx for the info.
Does generator size matter to the size of the turbine.. i.e could u have a small turbine with large generator or vise versa or do they need to be relatively the same?
Would it be possible to make a turbine where the rated speed is variable.
I need something than can be flexible.
I’m actually thinking about a special kind of wind turbine where air direction and speed are always controlled. Any advice on patents?
I wish I could go into more details but its too risky..
Thanx again for your help.
There is a formula for the maximum amount of power that you can get out of the wind. The 2 biggest variables are the sweep area and the speed of the wind. You can’t get any more power out of the wind, than what this formula states. Large commercial wind turbines can get very close to this theoretical maximum. Most wind turbine manufactures, will match the sweep area (blade length) to the generator in order to get the maximum amount of power. If either one is mismatched, you get less power output.
Yes, in theory you could make a wind turbine with a variable rated wind speed, if that turbine’s sweep area was also variable. I would be concerned with the complexity of such a unit and cost, vs overall power output. Of course, new and improved models are designed everyday. Afraid I can’t comment on patents.
thanx man youve been alot of help
A question – since energy it being taken from the velocity of the air, how much does the wind speed slow down, downwind of the turbine?
I’m wondering on the effect of a large number of turbines spread worldwide on the weather. Are we taking a miniscule amount of energy and hence no effect? Remember, 100 years ago the small amount of carbon dioxide from the internal combustion engine was not considered to be a major problem. With a few engines that’s true, but multiply that by millions of them over a longer time and we get global warming….
Bill,
You are correct, that wind turbines slow down the speed of the air that passes through them and is used to turn the blades. However, if you consider that a tree does the same – slows down the air that comes in contact with it. Then by cutting down millions of huge rain-forest trees, we have already more than offset the effect of present and future wind turbines.
In addition, very small increases in the overall temperature of the earth (global warming) increase the amount of wind and severity of storms.
Bruce K.
How many volts and amount of amps will a windturbine produce
what percentage of a houses electricity needs can a small turbine produce
If you are asking about do-it-yourself, small wind turbines, a DIY wind turbine with 3 foot (1 metre) blades, will not replace much of your household power. You really need to build a fairly large 9 foot (3 metre) wind turbine to replace a significant amount of your purchased power. Of course, good access to a smooth flow of air is also very important.
What can you tell us about vertical axis wind turbines? Are they better, more compact or just a new thing?
Ben,
You can find specific info about VAWT on these pages:
http://www.greenterrafirma.com/vawt-designs.html
http://www.greenterrafirma.com/diy-vawt.html
At GreenTerraFirma, we believe the vertical axis wind turbine DOES have a place… If you have access to smooth wind, then a horizontal wind turbine would likely be your first choice. A HAWT will produce more power than a VAWT, given “clean” air…. HAWT is more efficient.
The VAWT however, will still produce power and some people may find it more visually appealing.
In Urban areas, you may find that a VAWT will produce more power than a HAWT. It should be noted however, that most locations where a VAWT will out perform a HAWT, would not be consider “good” locations for a wind turbine.
I have a greenhouse that I am setting up and trying to be off the grid. I will have 3 large fans (48″) that will run all most nonstop. My question would be if I could set up a turbine on the exhaust side and recapture the energy, or would it cause more drag on the fans and or slow the air flow in the greenhouse? If this would work, which style be the most productive? I am guessing that the wind speed would be around 10-15mph and would be constant. Thanks for your time.
I have not heard of anyone who has this type of setup – exhaust fan, with a wind turbine to capture some of the lost energy. As you pointed out, I would expect the wind turbine to cause some drag/feedback which would make the exhaust fans work harder…which could shorten the life of the fans.
If only you had another use for the warm air – preheat water???
hi,,,,, i ve been looking out to do a small project upon small wind turbine… lik what using dis mechanism upon the trains… where turbine wil rotate for 100% when train moves at even in faster speed…….. plz ten me how output we will get though it n it wil be useful or not…….
I’m getting about 100 watts in a strong wind. Are you mounting this on a train? I doubt that it would take the speed.
Hey I have an idea for a wind powered car (double wind turbine, one turbine on a side) just a quick question if I were supose to have a wind turbine 2 ft in diameter going on wind about 30mph how much watts will it produce?
Great email id Rick… I believe it says it all….dumb idea.
[...] of the circle that the blades of the turbine make = the swept area. (To see full equation go to: http://greenterrafirma.com/wordpress/how-much-power-can-a-small-wind-turbine-produce/) The bigger the swept area the more power you can produce – hence the need to know both the [...]
Thanks for the great info! I’ve included a link to your site on my blog.
I’m working on a novel design for a VAWT. Do you have a similar formula for a the power you can get from a VAWT so I can compare my result with the standard?
Also how (if at all) would the formula change if the VAWT was slow, with high torque?
[...] information about wind power and how effective it actually is for home use. I found a great page http://greenterrafirma.com/wordpress/how-much-power-can-a-small-wind-turbine-produce/ . The guy had a formula for figuring out how much power one could generate and advise for talking [...]
Hi Joe,
A VAWT is never as efficient as a HAWT as half the time the blades are turning INTO the wind. There are designs that block the wind on the returning side, or cause the blades to go limp, but these designs just add complexity.
Here’s a link to one VAWT I built:
http://greenterrafirma.com/diy-vawt.html
The beast turns great without the motor, but my gearing was to great – add the gearing and you need a wind storm to make it go around. If you want to try this design, I’d recommend using fins that are twice as long, and adding another bicycle rim to the middle.
Let me know how your system works out… love to see pics.
Bruce.
Hi Bruce,
I understand why a HAWT is more effecient than a VAWT. I was wondering if there was a different formula for VAWTs. Something like replacing “diameter x height” with “sweep area”. It seems that the formula above is based on wind facing area and using this replacement would work for VAWTs if there isn’t another one. To get too much farther you’d have to start calculating wind directional changes. And how would you measure the power comming off if its high torque and low speed?
Hi Joe,
Unfortunately there is no single simple formula for a VAWT. As the shapes for a VAWT can differ, (Darrieus,Savonius,Giromill – lift or drag) the formulas can become very complex.
In this study of a Darrieus VAWT, the study broke the blade into 50 separate segments, as each segment has a different angle of attack on the wind. ( http://www.reuk.co.uk/VAWT-Vertical-Axis-Wind-Turbine.htm ) .
As you’re make a VAWT as a hobby, why not simply post the power output you get, to share with others.
Bruce.
Thanks Bruce. Ill keep everyone updated.
Hi Bruce,
The formula prescribed above -> Power AVAILABLE in the wind = .5 x air density x swept area x (wind velocity cubed) i understand gives the thoeretical max power generated.
if i’m not wrong, wouldnt i be more sure of the power output of my homemade windmill if i use W = V x I?
using the voltage and amperes from my windmill?
For my generator, what i know is that it puts out about 18 V with a quick twist of the rotor axle.
Can i use this voltage reading for calculating the windmill power?
Thank you…
Sheldon,
The formula for maximum power available, helps people understand that you can’t just keep putting a bigger and bigger motor on the blades and expect more power – there is a maximum that’s available, given the size of the sweep area (blades) and how strong the wind is blowing at the moment (velocity). It also points out that WIND SPEED has a huge impact on power output as the power available is CUBED by the wind speed.
Also, as a DIY wind turbine, you’ll likely find that your output is half of the maximum. Do it yourself wind turbines are not as efficient as large commercial units.
Regarding your generator, you need to know what the output is at around 400-500 RPM, and then you can use W=VxI to estimate the output in a strong wind.
Hi Bruce,
Thanks for the reply. It will surely help me.
Hi Bruce,
I have some very specific, yet general, questions.
1) What would be the most “efficient” Electric producing Generator, to produce enough electricity to power the average home. Say approx. 40 KW-hr per Day!! i.e. requring the least amount of energy, wind, RPM of shaft, “force” upon shaft to rotate it.
2) What would be the required RPM of the Shaft of said Generator to produce the required electricity for an average home?
I just saw one video which showed a HAWT. That at 89.6 RPM it produced 101.4 Watts, at 268.9 RPM it produced 2,738.3 Watts (30.0 mph of wind). Clearly there is NOT a linear relationship between RPM and Watts produced.
3) What is the “resistance (mechanical)” or “friction” in the shaft? Or more specifically, what would be the “force” required to rotate the shaft at the required RPM to produce the required electricity, i.e. approx. 40KW-hr?
In other words, is the shaft of a generator “stiff” and difficult to rotate, requiring alot of “force”. Or is it “looser” requiring little “force”.
This would seem to be important to me. Because if you have a Generator which “can” produce alot of electricity, but even in a 60 mph wind the shaft won’t rotate because it is so “tight”. If you see my point?
To relate this to your discussion and information on wind turbines, this would be equivalent to how much “force” is required by the wind, upon the blades, to turn the shaft at whatever RPM to produce the desired electricity.
Maybe you use the term “torque” upon the shaft to rotate it, whereas I am using the term “force”.
Thank you.
Why do I ask all this?
Cuz I still don’t get how this all works. With all the different terminology – watts, KW,KW-hr, volts, amps, etc, “ratings”, “averages”, etc., etc, etc, ..
I just want to know, if you want to produce enough electricity to power an average home, approx. 40KW-hr per day, what Generator will do the job, how many RPMs does the shaft have to rotate, and how much “force” is required to rotate the shaft at that RPM speed?
All at a “continuous” operation of shaft and Generator?
Also, I just watched a video on how a wind turbine works………they have the Generator up top, by the blades, in the “box”.
Why are their 2 different shafts and why are their gears? It said “low speed shaft” and “high speed shaft”?
Is it “boosting” the electricity produced thru the use of gears?
Hi Todd,
Personally, I would not try to build a DIY wind turbine to produce 40kWh per day! A production quality 5kW wind turbine, mounted at a site with average wind speeds of 5.4 m/s (my location) would produce around 33kWh per day. And the blades would each be over 10 feet long – over 20 ft sweep diameter.
If you want to look at production wind turbines, your first step is to determine the AVERAGE wind speed at your site. There are wind maps that contain this information – some maps show the average wind speeds at different heights. Your closest airport may have some average wind speeds for your area as well – even TV station weather departments.
Once you know your average wind speed, you can look at the production charts for various wind turbines to find out how much power they produce at that wind speed. A wind turbine may be rated at 5kW or 10kW, but it only produces that power at very high wind speeds. I’ve found that most wind turbines produce power at 1/10th to 1/5th of their rated power level, at 5.4 m/s which is the average wind speed for my location.
And don’t forget to price in a professional tower that is at least 30 feet higher that the tallest item in a 500 foot radius from your tower. You can expect to pay as much for the tower as you pay for the wind turbine.
Hi Bruce,
I have had a solar system for about 6 years and it has been fine but have had to use a diesel generator to top up the system.
I have just bought a turbine and have set it up, trouble is when I put a voltage meter on the output and found that in a 25kph wind it was producing 130 volts! As my system is based on 24 volts I am concerned that this will damage my batteries..Am I right?
Thanks
Colin
Hi Colin. Sounds like you are producing AC rather than DC. Was the wind turbine you purchased, made to connect directly to the grid (via fusing etc)? What is the make model?
hi, i wanna make a generator that works on simple wind power but its not dependent on the wind speed i wanna introduced an exhaust fan to rotate the rotor like in hydro power at constant speed .. will it definitely work?
This falls under the “no free lunch” category. Yes, you can make this work, but you won’t get “much” power out of your wind turbine and it will be considerably less than the energy required to run the exhaust fan.