The first step is to estimate the energy you expect to produce annually from available wind at the installed height and the unit performance characteristics for units you are considering. This is very sensitive to local conditions, performance characteristics and height. Your vendor may be able to assist in estimating the output.
A term commonly used in the industry is Capacity Factor which is expected output divided by theoretical output. For example if you were considering a 5 kW unit and site and unit conditions indicated it would produce 6,570 kWhr per year, the capacity factor would be 6,570 kWh/ (8760 hours *5kW) or 15 percent.
The next step would be to total the amount of electricity that you use from your last twelve electric bills to get your annual use and divide that number by 8760 to get your average hourly use. For example if your 12 electric bills showed you used 14,450 kWh per year that would be 1.65 kW per hour on average.
The next step would be to determine the size needed by dividing the 1.65 kW/hour use by the capacity factor of 15%. This would indicate you would need to install an 11 kW unit to produce your annual needs.
The sensitivity of output to location and unit specific characteristics is demonstrated by units in the net metering program in place from late 2007 through mid 2009. Wind units in that program have operated at annual capacity factors ranging from 4 to 24 percent. A 5kW unit operating at these extremes would produce 1,752 kWh per year at the low end to 10,512 kWh per year at the high end. Consequently, to properly size your unit it is essential that your specific conditions be considered.
The exact same procedure would be used to size a solar installation. However, solar units are less sensitive to site conditions (assuming all installations would be in unshaded areas with the proper inclination and directional placement).
The lack of sensitivity for solar installations is demonstrated by units in the net metering program in place from late 2007 through mid 2009. Solar units in that program have operated at annual capacity factors of 14 to 15.5 percent. A 5kW unit operating at these extremes would produce 6,132 kWh/year at the low end to 6,789 kWh/year at the high end.
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