The first step is to estimate the energy you expect to produce annually. For wind generators,
the estimate is determined from available wind at the installed height and the unit
performance characteristics for units you are considering. This calculation is very sensitive
to local conditions, equipment performance and installation 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 (kilowatt) unit and site
and unit conditions indicated it would produce 6,570 kWh (kilowatt hours) per year, the
capacity factor would be 6,570 kWh divided by 8,760 hours (5 kW) or 15 percent.
The second step would be to total the amount of electricity that you use from your last 12
electric bills to get your annual use and divide that number by 8,760 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 third step would be to determine the size needed by dividing the 1.65 kW per hour on
average use by the capacity factor of 15 percent. 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 5
kW 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 same procedure would be used to size a solar installation. However, solar units are
less sensitive to site conditions (assuming all installations are located 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 5 kW unit operating at these
extremes would produce 6,132 kWh per year at the low end to 6,789 kWh per year at the
high end.
