# The Wind Power Facts That Will Help You Knock The Half-Truths On The Head

Wind power information is readily available on the internet but you may not be getting wind power facts – unless you know what to look for. The whole truth is often somewhat obscured, to put it mildly, so we are outlining the really important wind power facts that you need to have at your fingertips.

Let's get right to the heart of the matter...

A wind turbine captures the power in the wind and then generates electricity for your use. Simple enough. However, you need to know how to calculate the power that's available from the wind itself and then calculate how much of that energy you can capture for your use. This where it gets a little technical but it's A1 important.

There is a formula for doing this: (it's a little scary to look at but don't worry we'll walk you through it).

**P= 0.5(d)(A)(Cp)(V3)**

P= the power in the wind which is a measure of the rate at which energy flows and is measured in watts. To put this in practical terms – an 11 watt light globe consumes energy at the rate of 11 watts.

d = air density (which is 1.225 at sea level and 15°C(59°F). The air density decreases with increased height above sea level. If you live in an area above sea level consult the table on air density in our **measurement tables** page.

A = the area in square meters swept by the turbine blade. In the case of horizontal wind turbines the blade is sweeping a circle and the area of a circle is calculated by multiplying 22/7 (known as Pi) by the radius squared. Therefore A=(22/7)(radius2). In the case of a vertical axis turbine it is simply the length of the turbine multiplied by the width of the blade.

Cp = the coefficient of performance – the percentage of the wind's available power that is actually extracted. In other words, the number of watts the turbine extracts from the available watts in the wind (expressed as a percentage).

V3 = the wind speed in meters/second(cubed)

So let's take an example...

A wind turbine has a blade of length 1.8 meters The average wind speed is 5.5 m/sec (14.5 mph)The Cp is 35% (more on that later)

So the calculation is as follows:

Power in the wind (watts) = 0.5 x 1.225 x 0.35 x 22/7 x (1.8) x (1.8) x (5.5) x (5.5) x (5.5) = 363 watts

Therefore in one year you could obtain 363 x 8760 hrs/1000 kilowatts = 3,181 kilowatt-hrs

To put that information in a practical context. The overall average US household consumes around 1000 kilowatt-hrs of electricity per month or 12,000 kilowatt-hrs per year. So such a turbine might reduce your power bill by about 25%. You begin to get excited!

However, before you get too excited, you need to be aware that there are some other factors at work in the turbine operation that will reduce its efficiency below the 35% level we used in our example – perhaps to a level of 25% or less. Therefore the reduction in your bill will need to be adjusted accordingly.

The real point of all this was to explain to you how the power in the wind is calculated and what factors are involved. So remember this formula and use it all times to evaluate those sales pitches for turbines that you will undoubtedly encounter.

So let's move on to listing the key wind power facts...

## The Maximum Power That Any Turbine Can Extract From The Wind Is 59.3%

The formula for this key wind power fact was developed by German physicist, Albert Betz, and also independently by others. To understand this, imagine for a moment, the wind blowing directly onto the blade of a wind turbine. If the turbine could extract 100% of the available power in the wind then the blade would stop turning, as there would be no power left to turn it – the wind would have been stopped! You don't need to understand all the laws of fluid dynamics - just accept this fact. The turbine needs to have a fresh flow of air past the blade in order to keep operating.

A little earlier on this page we were talking about Cp, the coefficient of performance and this is in reference to the Betz formula. In practice, wind turbines don't achieve the 59% optimum figure. There are a whole host of other factors which reduce that performance figure, quite often to somewhere between 15-25%.

The real point here is you now have enough information to spot the shysters who claim that their wind turbines produce results either beyond this figure or that are quite unrealistic. Simply apply your formula and the shysters will be revealed for what they are.

## Location Is Paramount.

This wind power fact is very much like that old saying about buying real estate - “location, location, location”. Where you locate a wind turbine is critical to the energy outcome. This wind power fact applies in all cases, whether it be a turbine as part of a large wind farm or a turbine on an individual basis on your own property.

Exposed areas usually attract more wind than sheltered areas and commercial installation of wind farms means extensive testing to get the best location, bearing in mind that with wind farms they also need to be located away from major pockets of population.

When it comes to individual installations this is even more critical as you don't have the same freedom to move around to find the best location as with bigger installations.

The rooftop of the family home is not a good location as you will see if you visit our page on Rooftop Wind Power

Any turbine placed on your property needs to be well clear of obstructions such as buildings and trees. Height is an important consideration as the wind at higher levels is more consistent. In the family home situation this may pose problems due to local height restrictions. For more detailed information on the location of wind turbines at your home, please visit our Wind Power For Homes page.

This leads us on to the next of our wind power facts...

## Wind Speed Is A Critical Factor.

Every wind turbine needs a certain amount of wind in order for the blades to turn and generate electricity. This factor is referred to as " wind speed" and there are various levels of wind speed which are used.

- Some times you will see turbine specifications that refer to “start-up speed” which is the wind speed at which the blades begin to turn. Don't worry about this figure – it is an irrelevant piece of information as there is no energy being produced at this speed.
- More important is the wind speed necessary for the unit to start producing electricity, usually called the "cut-in speed" and for most turbines it will be in the range of 7-10 mph (3.1- 4.5m/sec).
- Beyond the “cut-in speed” the wind speed at which any turbine reaches its maximum power is called the “rated wind speed”. Usually it is in the range of 25-35 mph (11.1 – 15.6m/sec)
Beyond that speed the power curve usually levels off.- There is also a “cut-out speed” which can range from 40- 80mph (17 – 35m/sec) which is designed to protect the turbine in high winds which may otherwise cause the turbine to self-destruct. Various mechanisms are employed by manufacturers to bring this about. The process is often referred to as “furling”.

It is worth noting that if the wind speed doubles, then you produce eight times the energy because of the “cubed” effect in our formula. The converse also applies of course – if you halve the wind speed the energy produced drops by a factor of eight.

However, the most important part of this key wind power fact is the average annual wind speed in your area. If you are considering the purchase of a wind turbine the first thing you should do is consult your local government authority or the central energy agency in your country. They will have wind maps showing average wind speeds in your city or region.

These wind maps are useful background knowledge but, in our opinion, you also need to do your own measurements. To do that you will need an anemometer – a device for gathering wind data. The best solution is to buy one on eBay for a very reasonable cost. Some local authorities will lend or rent them out but it's better to do long term measurements, so one of your own is better.

We say, long term measurements because you really need to get a year's worth of data if you have the patience. If you are serious about wanting to get a good wind turbine working effectively, then you need to do your homework. - it's as simple as that. If you are impatient to get started then certainly refer to local wind maps as they do provide useful wind power information.

## The Energy Production At Average Wind Speed Is What Really Counts

All turbines have a rating as to their maximum output, usually expressed in watts, kilowatts or megawatts, depending on the turbine size.

Usually, the manufacturers and marketers of the turbines will supply a graph known as the “Power Curve”.

However, turbines never operate constantly at that maximum level due to varying wind speeds, periods of no wind or shut downs for maintenance or repair. In practice, efficiency is in the range of 15-30% of rated capacity.

The way to look at the maximum energy your turbine is likely to give you is to see the rated power as a bonus that will boost your energy total from time to time as the wind gusts to higher speeds. Your focus always has to be on what your turbine will produce at annual average wind speed.

Then you apply the formula and that will give you a much clearer idea of the energy that will be generated for your use. This key wind power fact is absolutely fundamental.

## The Diameter Of The Rotor Is Your Best Guide To Energy Generation

We know from our formula that the area swept by the rotor is a critical part of the overall equation. In this case, size matters. The bigger the rotor diameter, the greater the area that is swept and at any given wind speed, the more energy will be generated. So when you are comparing turbines, bigger rotor blade is better. Just bear in mind when you are reading brochures that the terms “rotor diameter” and “blade length” are not the same thing. The blade length gives you the radius and you use (Pi)(r^2) to calculate the area. In the case of “rotor diameter” you use (Pi)(d^2)/4.

## You Can Establish Your Turbine For “Off-Grid” or “On-Grid” Use

In the case of “on-grid” use you are using power in your own home but you can not only sell any excess to the grid you can also benefit from feed-in tariffs where you get paid for the amount of electricity generated. This is an attractive option for many people as it obviously helps defray the costs of your system.

“Off-grid” use usually applies where the location is not connected to the grid. This makes it necessary to have a battery bank for storage so that you have electricity you can draw on at times of little or no wind.

## Calculate The Payback Time On Your Investment.

If you are going to invest in a worthwhile wind turbine then you are going to outlay a fair amount of money. Visit our **wind power costs** page for more information. You not only have the turbine cost, but the cost of installation, which can sometimes run into as much as the turbine itself, or even more. You need to factor in maintenance and then balance it with feed-in tariffs and energy savings to work out the time after which your investment is paid for. After that time you know you will be saving money on a regular basis and at very little cost, apart from maintenance.

Remember this important wind power fact as many people fail to do this calculation and fail to get a proper understanding of the return on their investment.

Energy from the wind offers an excellent opportunity to reap substantial benefits by cutting down on your energy bills but you need to bear all these wind power facts in mind.

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