The two most important factors in designing a wind system are the blade length and the tower height. The longer the blades, the more swept area of wind will be caught. The area is increased dramatically with an increase in blade length (Area=3.14xRadiusxRadius. The radius being the blade length) The higher off the ground the rotor is the faster the wind is that turns it. The power created is proportional to cube of the wind speed, so even a minor increase in wind speed will dramatically increase the electrical output. If the wind is blowing too hard the wind turbine will automatically turn sideways to prevent destruction, but it will still turn, creating power. Another reason to raise the turbine as high as possible is to avoid turbulance from the ground or surrounding impedements. The turbulance will at least decrease the efficiency of the turbine and, at worst, can destroy the turbine. The tower should be at least 30 feet higher than the highest point within a 500 foot radius. This being said, it is not unusual for the tower to cost more than the turbine itself.
Financially speaking, the cost to create a unit of electricity goes down as the size of the turbine increases. For example, say a home sized, 7 meter diameter turbine on a 100 ft tower that makes 14,000 kWh per year costs about $35,000 and would pay itself back in about 15 years. Now lets say a small business that uses a lot of electricity puts in a 29 meter diameter turbine that makes 480,000 kWh per year and costs $300,000. This turbine would pay itself back in about three years. This is a simple comparison, but the general rule is the larger turbine you can afford (for your needs), the better return on investment.
More and more areas have energy buy-back programs, known as net metering, allowing owners of wind systems to sell excess electricity back to the power company. In general, the utilities will not credit you at retail rates for electricity that you produce over your typical demand. So it makes sense to size a system for what you use, but not larger.
Some owners choose to remain off the grid if the cost of extending electrical lines to the house is not cost-effective. Some decide to become independent of all utility companies or want to go completely green. Those who choose to remain independent of the grid can still generate power for themselves in the event that the grid power cuts off, as in the event of a storm. The ideal off grid system will combine wind power with solar power and store electricity in batteries for when no power is being generated. These hybrid systems allow each component to be sized smaller and the wind could be blowing while the sun is not out, or the system could be charging on calm sunny days. Every location will have its own characteristics of average wind speeds and average hours of sunshine, and will be sized accordingly.
Wind Power Facts (from "The homeowners Guide to Energy Independence")