Wind Basics

It all sounds so easy. You just put up a wind turbine and never see another electric bill. Free energy from the wind! Put it on your rooftop, in your backyard, anywhere and you'll be energy independent.

The three most important things to know about wind siting:

There is more wind at higher elevations.
You need good exposure to the prevailing wind.
Wind doesn't go through or around trees or buildings - it just stops.

Wind Math

The energy (kw-hrs) extracted from the wind increases with the third power (cube) of windspeed.
Doubling windspeed increases the energy by eight times.
Small increases in windspeed cause large increases in energy. Change in windspeed with elevation can be small or large depending on terrain. In one case, a doubling of tower height will result in an increase in energy of only 33%. At another site, the energy will increase by 130% ! So it's critical to know this effect at your site. It will make or break your project.

Wind Turbine Math

Bigger machines produce more energy. The energy increases with the square (2nd power) of the rotor/blade diameter. So doubling the diameter will produce four times the energy. The "swept area" of the blades is the most important single factor in wind turbine output. None of the new "miracle" machines have managed to break this law of physics.

Where Does the Energy from the Turbine Go?

Most systems tie to the electric grid and feed either to your home or back to the grid depending on demand at your home. It's all automatic, safe and reliable. See Net Metering sidebar to the right.

Wind Turbine Costs

Larger machines cost less per kw-hr of energy production.
You make your money back by avoiding electricity costs.
The payback period is often very long. 15-20 years is not uncommon and some installations may never pay back. Taller towers usually have good pay back if you have a site with rough terrain or obstructions.
Here's a table of approximate costs for several popular small residential systems.
There is a 30% federal tax credit for residential wind energy systems.
Farms or rural businesses can also get a 25% USDA REAP (Renewable Energy for America Program) grant. Details.

Wind Turbine Power Rating

This is approximately the maximum power the machine produces, usually specified in kilowatts. This rating is usually taken at 25-30 mph so you very seldom see this output.
The more useful specification is kw-hr production at various average windspeeds. This can also be misleading because different locations have different distributions of windspeed. Some distributions will produce more energy and some will produces less for the same average windspeed. Coastal areas generally "outproduce" the average windspeed prediction.
You'll often see quoted a monthly production for average windspeed of 12 mph. There are very few sites in Michigan with this average windspeed - only coastal areas and high exposed ridges have this wind resource. Once you move 10-20 miles inland, the wind resource drops dramatically.

Net Metering

This is the system by which you connect to the electrical grid and "feed back" any excess electrical energy you produce to the utility company. As your wind turbine produces energy, some or none of that energy will go back to the utility, depending on the electrical demand at your home. In high wind conditions, as much as 80 -90% of production could go back on the grid. In low wind conditions, all of the production will go to your home. The utility company then deducts this "net excess generation" (NEG) from your bill at the end of each month.