http://www.greenspec.co.uk/documents/energy/BWEAsmallturbines.pdf

Small wind scenario: 17 kW turbine,
initial capital cost £20,000
At 4 m/s average annual wind speed, 11,800 kWh is estimated to be produced
per year with a value of £1416, so a payback calculation suggests it would pay for
itself in 20,000/1,416 or 14 years. For a good 5 m/s average wind speed, 23,000
kWh is produced with a saving of £2760 therefore the turbine pays back in 7.2
years. On an exceptionally good site of 6 m/s - 35,700 kWh is estimated to be
produced with an annual saving of £4,284 and a payback of 4.6 years.
Note that for small wind systems, 6 m/s average annual wind speed is considered to be exceptionally good due to
the height of the mast and effect of local siting conditions. Within this annual average, wind speeds will vary through
a range of 0–20 m/s or more. The examples above demonstrate that the economics of a small/micro wind system
very much depend on wind speed, but sizes, prices and outputs vary considerably, so this is only a guide. Electricity
prices are unlikely to remain stable, and any increase in prices will shorten the payback period of a small wind
turbine. Grant funding may also contribute to the cost of capital outlay and shorten payback periods. A turbine with
a longer payback period may be less economic; however there are many good reasons for installing a small/micro
system in addition to potential savings