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This is best analysis I've found if you are seriously considering buying a wind turbine.  This is a re-posting of a blog written by Amy Berry for Green by Design and can be seen here.  Amy can be found on Twitter at @wind2power and helps promote the Windspire Turbine by Mariah Power.  I'll comment and link to relevant Mapawatt posts in blue!

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Used to be if you wanted to put a wind turbine up at your house you either had to live on a remote farm, or grow your hair long and pledge allegiance to an aging group of touring musicians. Thanks to major improvements in technology and a general awareness of the benefits of making your own energy from clean and free wind, small wind power is going mainstream. According to the American Wind Energy Association (AWEA), the US small wind market grew by 78% last year with many new turbines hitting the market. But more options don’t make finding the right wind power solution easy. If you are interested but not sure how to even get started, here are 9 things to know as you consider wind power.

1. Small wind turbines can be broken into two main technologies: Horizontal Axis Wind Turbines (HAWTs) or Vertical Axis Wind Turbines (VAWTs). HAWTs are propeller based turbines that are traditionally mounted on tall poles and are commonly used in large wind farm settings. HAWTs have blades which rotate vertically around a horizontal axis, similar to a propeller on an airplane. VAWTs include two main classes: a tall vertical airfoil style (Darrieus), and a solid winged style (Savonius). Darrieus Turbines come in a few varieties. Some have rotors with curved blades that look like an eggbeater and rotate about a vertical axis. Another variation uses straight-sided airfoils and is called a Giromill. Like propeller turbines, Darrieus turbines utilize some lift to capture wind energy. Savonius Turbines have rotors with solid vanes or “scoops” which rotate about a vertical axis.

2. There is no precise definition for “small wind” but it usually applies to machines with less than 100 kilowatt (kW) ratings. The “ratings” refer to how much power the turbine can instantaneously generate at a specific wind speed. There are no standards in the small wind industry, so manufacturers are able to set their ratings at varying wind speeds. It is not uncommon to find one turbine rated at 25mph and another one rated at 48mph. Obviously the higher wind speed used will result in a higher kW rating, so its not a completely useful figure to go by.

3. While kW ratings will give you a general sense for the size of a turbine, what really matters is how much energy it will produce over a period of time. Wind turbine companies provide energy curves that tell how many kilowatt hours (kWhs) you can expect to generate at specific average wind speeds. You can check your monthly electric bills to gain an understanding of how many kWhs you use. Electricity use varies by season and time of day, so ideally you should add up the kWhs of the last 12 months.

Points 1-3 are summed up (in much less great detail) in my short slide-show on the Basics of Wind Power.

4. This should go without saying, but you need wind to create wind power. All turbines have a minimum required wind speed at which they will start to generate power, this is also known in the wind world as the “cut-in” wind speed. It is possible for a turbine to spin at speeds below the cut-in speed, but those rotations won’t be fast enough to actually create energy. The majority of small wind turbines require a minimum of 10mph average annual winds to generate significant energy. Wind power is a cubic function of wind speed. For all you non-math people out there, this means that a little more wind can create a lot more power. When determining average annual wind speeds, a 10mph average annual wind does not mean it blows 10mph all day everyday. Because of the cubic function, a day of high wind can generate enough power to make up for multiple days of low wind. For you math geeks, average wind speeds follow the Rayleigh distribution curve http://en.wikipedia.org/wiki/Rayleigh_distribution)

5. So, how do you know if you have enough wind to make wind power a feasible option? The most ideal way to know is to install an anemometer where you want to place your turbine. You can get a very good anemometer for around $500 from www.madgetech.com. If you don’t want to wait a year, you can do shorter anemometer tests but you need to realize that wind speeds change with the seasons. Not ready to invest $500 in your research? Check out local weather sites which should provide data on average wind speeds. Local airports are also wonderful resources for this information. The DOD provides wind maps, but these are measured at 50 meter heights (for use by the big wind guys) and are not always localized enough for small wind installations which are very site specific. You can also call a local wind turbine dealer and request a site visit.

Points 4 and 5 are briefly covered in my recent post on the importance of using an Anemometer!

6. What about a site? A “site” is the place on your property where you install your turbine. Site location is a crucial element, and will have a major impact on which turbine you can consider. Turbines are best placed with enough open space to allow the wind direct access to the rotor. This does not necessarily require a specific lot size or a totally open and clear site. Many small wind turbines are designed to work in various settings, for instance HAWTs will work if you can put up a large tower and have consistent wind direction. A VAWT maybe a better option if your wind changes directions and you cannot put up a structure taller than 30 feet. Wind speed can also vary drastically on one piece of property due to structures and topography. Always choose the site with the most access to wind.

7. You’ve done your research and know you have a good source of wind. Now it’s time to pick a turbine or turbines. It’s very common for people to put up multiple turbines to meet more of their energy needs. Two big factors to initially consider are the expected power output and the cost of the unit fully installed. Consult the energy curve of each wind turbine to determine how much energy it is likely to create with your average wind speeds over the course of a year. Compare the kWhs at the same wind speeds across wind turbines, while keeping in mind total cost of the unit. A wind turbine that generates 400kWhs for $2,000 is a lot more expensive than a wind turbine that generates 2,000kWhs for $6,500. Also, don’t be fooled by energy curves that show amazing results at 30mph average winds. It is very unlikely that you live in an area with wind speeds of that level and will ever reach those energy levels.

Excellent point.  You have to compare initial costs vs. the expected power output.  And you MUST compare your expected wind speeds and the power output at those speeds! I'm looking forward to the Honeywell wind turbine for locations that require a small footprint and a smaller power outputs and learning more about the Windspire which requires more room but has a higher power output.

8. The other big factors to consider are the look of the wind turbine and the sound it creates while operating at moderate wind speeds. Try to visit the wind turbines that you are considering (or at least view on video) so that you can see and hear what they sound like when they operate. There are many designs on the market, all with varying looks and sounds. Find a local dealer or contact the manufacturer with questions. This is a big purchase decision, so you should feel comfortable working with the company and its local representative.

9. A final note on independent testing. As I mentioned above, there are no standards for small wind at this time. AWEA is currently putting these together, but it will be at least a year before they are finalized. Until then, we recommend focusing your search on independently tested wind turbines. Power curves, which turbine companies use to estimate power ratings and energy curves, can be supposed from complex calculations. But, the truest power curves are created from units being independently tested in real world scenarios. It is very easy for manufacturers to create their own power curves, so it is important to look for wind turbines with independently tested data. I recommend avoiding any turbines that do not have their data verified by an independent test facility.

Bottom Line:  It is extremely important to test your site and make sure you have enough wind.  Once you have the expected average wind speeds, use that value to compare several wind turbines.  Also, don't forget about which direction the wind comes from.  If it changes speeds constantly than you might want to consider a VAWT.  As Amy says, don't always trust the manufacturers power curves, because those were probably done under ideal conditions in a wind tunnel.  Do your research!  If you have any questions, don't hesitate to ask!