There are a lot of factors that effect you access to wind energy. Much of it has to do with your particular site. Trees, hills, buildings, etc, all effect the microclimate. You need to get a minimum of about 30ft, and preferably 70ft above close features, to avoid turbulence, which will rob you of your efficiency. Of course, that is not always possible, or cost effective. Beyond those site related factors, is your more general geographical location wind characteristics. This will give you an idea of how much wind you can probably expect, if you can overcome you site features, and get a generator up into the good winds.
I salvaged a 40ft triangular truss tower a couple of years ago, and plan to run a 40ft pole up the center of it, which can be wenched up to a total height of 70ft. Since I will be installing it on a hill, at the end of a small NW-SE valley, that lets the predominant fall-thru-spring winds in, I think I will have acceptable exposure, even in the forest. So, looking at the wind maps at: http://rredc.nrel.gov/wind/pubs/atlas/maps.html#2-6 I can get a rough idea that I might expect the following average seasonal winds.
After averaging in those figures to about 10.5mph annually, I looked at the graph for a little 13 pound Air-X generator at: http://www.windenergy.com/PRODUCTS/airxland.html They claim that I can expect somewhere around 26.5 kWh per month (318kWh/year) or about 850Whrs/day average. However that will be strongly weighted toward the winter, when the weather may interfere with my other generation. There will be very little output in the summer, since the generator will not put out anything until the average summer wind is exceeded. I can use wind in the winter, and my tower can get my PVs up above the trees in the sunny times, when the forest is green. This is not nearly as economical as a 12 month/year PV system, but that is just part of the climate. We have great water, nice cool summers, and lots of wood, just laying everywhere, for free heating, and building materials.
There is also a performance curve showing rough instantaneous output for the Air-X, which like any wind generator performance curve, expresses that basically, storms are great, and sunny weather is usually bad for wind generation, in my location. Just as a Solar sunspace lets you smile about cold winter days, a wind generator, can give you something to look forward to, in a winter storm.
What this all means, is that since I already own my tower, and will have batteries, and the rest of my off-gid cost included in my house anyway, that adding the little Air-X, at only $500-600, especially if it is the final factor that allows me to get free of the monthly grid base fee, will pay for itself in as little as 5 years.
Everyone’s site and circumstance will differ, and as they say; “YMMV” (Your mileage may vary). There are a lot of those variables, and many ways to approach the problem, so a high degree of accuracy is not really a reality in these calculations. However, they can give us a fairly good general feel for what is going on. There are some who question the Air-X performance data, but SouthWest Wind Power is the world’s largest producer of small wind energy system in the world, and the Air-X is probably the best selling. I may build my own, but I like the idea of a very light weight system, that hopefully will need less servicing than a homemade job, so that I do not need to spend very much time, up on top of that 40′ tower.