Windmill Power Basics and FAQ

Windmill power is a good, affordable choice for many people aiming for energy self sufficiency.

Whether your interest is in camping, boating or residential wind mills, there are a few basic things you have to nut out before you can identify the right wind powered home generator for the job.

Want to buy a serious home wind turbine? If you're in Australia you can buy a 2K turbine from our online Store.

Or you can build your own for next to no money, even if you're not that handy.

How Much Energy Will I Need?

Affordable, successful off grid living depends on being conservative with energy.

If you insist on being an energy hog, then you’re going to have to pay handsomely for it in a bigger and more expensive renewable energy system.

Otherwise you’re going to find that you consistently use more energy than you can produce (or afford to produce!).

There is little point in establishing a renewable energy system if you aren’t going to use energy wisely in the first place!

It is amazing how wasteful the average home is with energy.

But that’s good news! It really means there are lots of opportunities for improvement!

The average household in Australia uses almost 17 kwh of electricity energy each day.

This does of course depend on a lot of factors, like the number of people living in the house (each uses an average of 6.4 kwh a day), as well as whether other forms of energy (e.g. solar, gas, solid fuel) are used for major energy guzzlers such as space heating, water heating and cooking.

If you are serious about energy sufficiency, you should aim to limit your daily household energy consumption to 10 kwh i.e. a level that can be reliably supplied by windmill power.

Hey, it’s not that hard!

But it will demand the development of energy conservative practices by ALL family members, and often requires investing in more energy efficient appliances.

You can

learn how to be a great energy trimmer here!

What Size Wind Powered Home Generator Will I Need?

The amount of watts of windmill power that wind powered home generators can produce depends on three things:

• Efficiency of the system

• Swept area (size of turbine head)… measured in square meters

• Wind speed… measured in meters/sec

System Efficiency

The physics involved when turbines harvest the power of the wind to produce windmill power sets the maximum theoretical efficiency limit of any wind turbine to 59%.

When real world engineering requirements are also factored in, even the best turbines only attain 35% to 45% efficiency.

And other essential system components such as the generator, bearings, and power transmission add further inefficiencies.

As a result, only 10-30% of the power of the wind is ever actually converted into usable electricity.

The best design depends on the situation.

Theoretically… the efficiency of vertical axis wind turbines is not as good as that of horizontal axis wind turbines.

However, in low-wind speed, high turbulence sites (e.g. low-set residential wind mills in urban situations) they tend to perform better.

Plus vertical axis models are safer for birds and easier to mount.

In open, low turbulence sites with a good wind resource power output of the horizontal axis turbines is often better.

Swept Area

The size of the area swept by the rotor of a turbine directly influences the amount of windmill power it will produce. So if you triple the swept area, you’ll triple the potential power output of the system.

Wind Speed

Wind speed has a dramatic effect on wind mills elec power output: power output is proportional to the wind speed cubed!

So when wind speed doubles, eight times the power is produced (2 x 2 x 2 = 8).

This is why turbines are placed up on towers – wind tends to move faster (and with less turbulence) above the ground than it does nearer to the ground.

500W Wind turbine power output chart


This is a power output chart for a 500W turbine.
The lower values on the output line correspond to turbulent sites. Note that power output cuts out completely in this turbine when wind speed exceeds 13.5 m/s. Turbines have these built in close down mechanisms to prevent damage in high winds,

While wind turbines are given a rated power output by manufacturers, this is misleading!

They peg these values to a particular wind speed (which is not the same for each, which makes it difficult to directly compare different models).

And, as we’ve seen, output varies dramatically with wind velocity.

So it is best to ignore the peak output and power curve graph, and do your own sums based on your local wind energy resource (as we’ll show you how below).

Power Output Formula

For these units:

• wind speed in m/s

• Swept area in m2

• Power in watts (joules/second)

Power = 0.615 x Swept Area x (Velocity)³

To get figures for your wind speed, you can monitor it yourself over a year, or get an idea of your wind resource by accessing free data from a weather monitor near you. You can source free Australian data here.

People in other countries should check their national Bureau of Meterology websites for data.

Bear in mind that, since it varies so dramatically with changes in wind velocity, average wind speeds only give you a very rough idea of likely windmill power output.

But let’s do the math anyway!

In our case, our average wind speed over the year is about 4 m/s, which is pretty reasonable [tops in January at 5 m/s (17.8 km/h), bottoms in July at 3 m/s (10.4 km/h)].

The swept area of our turbine can be worked out as follows:

Swept area = 22/7 x radius² (i.e. Area of a circle=¶.r²)

= 22/7 x (1.8)² = 10.2 m2

So Power Output = 0.615 x 10.2 x 4³

= 0.615 x 10.2 x (4 x 4 x 4)

= 401.5 watts

Multiply this by 24 hours gives us 9635 watt hours or 9.6 kwh a day.

The "average" is a pretty good match for our household energy needs with practical conservation, with a winter low of 4.1 kwh and a summer high of 18.8 kwh.

So it makes sense to also have another renewable energy source to cover the winter low (we will be using photovoltaics).

When it comes to residential wind mills, in practice it might be better to have a few smaller wind powered home generators that one really big one.

Each needs a considerably less substantial tower that the biggies do, and there is the added benefits of being less visually intrusive as well as allowing maintenance to be carried out without shutting the whole system down.

Other Things to Consider…

Will your system be grid connected?

Before investing in wind powered home generators, you should decide if you are going to connect your residential wind mill to the grid or not.

This is because it is relatively easy to fit a special converter to the larger wind turbines that will convert its output (typically around 120V) to mains power.

But with the smaller wind turbines, windmill power output is typically 12 to 48 volts. Such low voltages are more suited to trickle charging battery systems – connecting them to the grid is possible but not really practical.

Is the system direct drive?

Most larger wind powered home generators are direct drive: the rotor is connected directly to an alternator.

In smaller systems, an intervening gearbox is used to ramp up alternator speed when the rotor is turning slowly. These gearboxes require maintenance, a disadvantage to such smaller non-direct drive systems.

Where to Put Your Windmill Power Turbine

Wind turbulence


Obstructions to wind flow create turbulence

The best wind turbine sites are where the wind has a clear pathway to the turbine.

Intervening buildings, trees and even hills create turbulence which reduces the efficiency of the windmill for a distance of about 10 to 15 times the height of the obstruction to wind flow.

If you don’t have a lot of choice about siting, for example with urban residential systems, you are best off going for a vertical axis turbine which handles turbulence and low wind velocity better.



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