Page last updated Dec 3, 2008 @ 05:33pm

Wind Power Information


Wind Power

Do you have a good wind site? Wind powered battery charging systems can be cost effective if the average wind speed is nine miles per hour (mph) or more at the location of the wind generator. If you are using wind in combination with photovoltaic power, it may be cost effective if you have good wind only during part of the year. The power available from the wind is proportional to the cube of the wind speed. When the wind speed doubles, the power delivered is eight times as great. Most wind generators are designed to deliver maximum power at a wind speed of 30 mph. At 15 mph, they will deliver about 1/8 their rated power. A wind generator should be mounted at least 20 feet higher than any obstruction within 300 feet to avoid turbulence.


Measuring Wind Speed

You can use one of the measuring devices in our online catalog to determine wind speed. The NRG Wind Totalizer acts like an odometer in a car, giving you the total wind passage over time. Average wind speed is calculated by the user by dividing the miles of wind passage by the number of hours the machine has been running. This unit can be installed and checked weekly or monthly. If you are patient, you can measure for a year or two and get a good idea of your potential power availability. The Dwyer and Kestrel wind speed indicators are both like a speedometer, displaying wind speed at the time you are looking at it, but it does not record any information for future reference. Both units can be mounted on a tower to give you an idea of wind speed where the generator will be located.

If you measure wind speed at ground level, you can expect about 1.5 times the wind speed 30 feet up, which equates to about three times the power. At 120 feet above the ground, wind speed will be twice what is measured at ground level and power output will be more than twice the output at 30 feet.

If you do not have a wind gauge, you can get a rough idea of wind speed from the table below.

Wind Speed

Wind Effect

0 - 1 Smoke rises vertically.
2 - 3 Direction of wind shown by smoke drift, but not by wind vanes.
4 - 7 Wind felt on face; leaves rustle; ordinary wind vane moved by wind.
8 - 12 Leaves and twigs in constant motion; wind extends a light flag.
13 - 18 Raises dust, loose paper; small branches are moved.
19 - 24 Small trees in leaf begin to sway; crested wavelets form on inland waters.
25 - 31 Large branches in motion; whistling heard in power lines; umbrella use is difficult.

Another good source of wind power estimates is the Wind Energy Resource Atlas of the US.   Or click here for our own page of links to Wind Maps.   This very detailed document assigns classes to the levels of available wind energy as defined in the table below.  The atlas contains maps covering the US and its territories showing these classes.  It suggests that class 3 regions and above are suitable for wind generator application.  However, it also states there can be certain sites even in regions shown as class 1 that are not captured in the detail on the maps.

Table 1-1 Classes of wind power density at 10 m and 50 m(a)
Wind Power
10 m (33 ft) 50 m (164 ft)
Wind Power Density (W/m2) Speed(b) m/s (mph) Wind Power Density (W/m2) Speed(b) m/s (mph)
  1 0 0 0 0
100 4.4 (9.8) 200 5.6 (12.5)
150 5.1 (11.5) 300 6.4 (14.3)
200 5.6 (12.5) 400 7.0 (15.7)
250 6.0 (13.4) 500 7.5 (16.8)
300 6.4 (14.3) 600 8.0 (17.9)
400 7.0 (15.7) 800 8.8 (19.7)
1000 9.4 (21.1) 2000 11.9 (26.6)

(a) Vertical extrapolation of wind speed based on the 1/7 power law.

(b) Mean wind speed is based on Rayleigh speed distribution of equivalent mean wind power density. Wind speed is for standard sea-level conditions. To maintain the same power density, wind speed must increase 3%/1000 m (5%/5000 ft) elevation.


Each wind power class should span two power densities. For example, Wind Power Class = 3 represents the Wind Power Density range between 150 W/m2 and 200 W/m2. The offset cells in the first column attempt to illustrate this concept.

For most purposes an assumption about the distribution of wind speeds around the average is made.  This allows the single figure for average wind speed at a site to be used to predict a turbine output over a period of time.  This distribution for a 13 mph average wind is shown below.

wind-speed-distribution.gif (8048 bytes)
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Griggs-Putnam Index

The Griggs-Putnam Index is one of several methods of estimating the prevailing wind speed at a site by observing the growth patterns of trees.  Strong winds will deform trees and shrubs so that they indicate an integrated record of the local wind speeds during their lives.  The effect shows up best on coniferous evergreens because their appearance to the wind remains relatively constant during the year.  Deciduous trees shed their leaves in the winter and thus change the exposed area tremendously.   If average wind speed is high but still below some critical value, above which deciduous trees cannot survive, they will not indicate relative differences in wind speeds very well, although they do show distinctive wind damage.

griggs-putnam_index.jpg (34104 bytes)
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The average wind speed at which these effects occur will vary from one species to another so calibration is necessary.  This is a long, laborious process so refinements can be expected for a number of years as new data are reported.


Site Location

Proper site selection is very important to the performance and longevity of your wind turbine. A poorly sited wind turbine will bring nothing but poor performance, maintenance issues and frustration. For a wind turbine to operate effectively, it needs to be placed in good clean wind above obstructions which cause turbulence.

SWWP-WhisperLink-location.gif (20996 bytes)
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Your site should:
1) Be free from trees and other obstructions within a 250 foot radius of the site.

2) Be able to locate a tower where the top of it would be at least 20 feet above any surrounding object within that 250 foot radius.




We do not recommend mounting wind generators on roofs. Though it is possible with wind generators with 500 watts or less output, it can be noisy at times. Larger wind generators could cause severe damage to the building. Freestanding towers, guyed towers or guyed poles may be used with wind generators.

Freestanding towers designed for antennas can be used. They require a large, engineered concrete base for support, but since they do not require guy wires, they can be installed in the smallest space.

Guyed steel truss towers, also designed for antenna mounting are less costly, but require a large area for guy wire placement.

A tilt-up pole tower is the most economical and the easiest to install. Wiring and mounting of the wind generator are done before the tower is erected. No climbing is necessary. Four or five inch steel tubing can be bought locally to save freight.

Do not underestimate the effect of tower height on wind generator output.   For best wind capture your turbine should be at least 20 feet above the tallest trees or structure within 300 feet of the tower location.  A taller tower will capture higher speed winds.  Even just a fraction of a mph improvement can significantly increase the KWH output over a period of time.  See more below under Wind Generator Output section.  The graph below shows the wind speed increase that can be expected for towers taller than 30 feet.

wind-speed-vs-tower.gif (5260 bytes)
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This graph shows that a 13 mph average wind speed at the top of a 30 foot tower would increase to 15.1 mph at 80 feet, 15.5 mph at 100 feet and 15.9 mph at 120 feet.



It is important to avoid excessive loss of power from voltage drop in wire from the wind generator to the batteries. It is not necessary to use a wire size that minimizes voltage drop for maximum generator output. It will be more economical to choose a wire size that gives a 2% voltage drop at the average generator output for your site.

The Air403 and Windseeker models have two wire DC output. Use a wire loss chart to decide on wire size. The Whisper wind generators have 3-phase AC output that is rectified to DC at the control box. Wire size can be smaller on these machines. Use a wire designed to carry 1/2 the rated current of the generator you are using, but remember, three conductors are required. If wire size required is too costly, compare the price of a whisper HVT machine, which has a 220 VAC output that is stepped down to battery charging voltage. This results in significantly smaller wire use, which will usually more than offset the extra cost of this machine.

Click here to go to our simple online voltage drop calculator.


Wind Generator Output

The actual output of a wind generator depends on wind speed. The chart below shows manufacturers specifications for watt output of two of the machines we sell at different wind speeds. It is important to get a machine that will deliver the power required at the wind speed available In your area.

whisper-h40h80-kw.jpg (43984 bytes)
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But instantaneous watt output at each wind speed does not tell the whole story about a wind turbine.  A more meaningful picture is gained by looking at the total ENERGY output over a period of time.  Electrical energy is usually measured in kilowatt hours (KWH) and this is the unit used on your electric bill.  If you know the average wind velocity for your site either by measurement or from wind maps you can estimate the KWH output per month using the manufacturer's chart for the turbine of interest.  The following shows the KWH per month for the same two turbines mentioned above.

whisper-h40h80-kwh.jpg (43272 bytes)
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Wind Generator Costs

Based on the wind generator outputs shown above and the purchase cost for each it is possible to get a rough idea of the cost per watt.  This is approximate because some wind turbines include more features than others and the wind speed assumed is never constant.  A better measure is to place a value on the KWH generated over time and compare that to the purchase and installation cost of the wind generator system.  Be sure to check your state incentives when determining overall cost for a system.


Links to Other Wind Info

Danish Wind Energy Association - excellent source of detailed information

Remember to check if your state offers rebates for wind power installations that can offset a substantial portion of the purchase price.  Also consider the net metering laws for your state to see how any excess generation is billed.



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