No Fossils in MY house...part 2 - wind power

As part of the ongoing research into how we might lessen our dependency on the grid I looked at windpower as a possible option.  There might even be a possibility of selling any excess electricity back to the grid and becoming part of the micro-renewables community.

There are a few companies that will sell you turbines - A company called Renewable Energy Devices offers Swift Turbines for example.  On the face of it these look like nice devices, with an option for battery storage which is probably essential if you're going to get the best out of them.  The brochure claims about being quiet and bird and bat safe are impressive.

They estimate that a turbine that is capable of generating 1.5 kW of power will cost about 7000 pounds to buy and install.  That can be reduced to the end user if grants are available as they typically are in the UK.  It's not clear if that costs includes planning permission.  They further suggest that that could generate about 2000 kWh of electricity per year.  That's less than a sixth of the theoretical maximum (24 * 1.5 * 365 = 13140 kWh) so may be realistic for a typical UK residential site with minimal obstructions such as up on a roof.

Just for comparison, the biggest industrial wind turbines are now capable of generating 5 Mega Watts - and 2.5 Mega Watts is typical.

Given that we pay just over 10 pence per kWh (and we use about 7500 kWh per year,) it would seem we could reduce our grid electricity usage by about 25% and save 200 pounds a year.  That would suggest a payback period of 35 years!  Perhaps that could be reduced to 20 years with maximum grants.  Their brochure somewhat misleadingly claims that on a good site payback could be as little as 5 years.  This can only be achieved if maximum output can be maintained unbroken for that 5 years. (13140 * 5 = 65700 kWh @ 10p per kWh = approximately 6500 pounds.)

One has to dig through the brochure small print to discover that 1.5 kW is from a rated wind speed of 12 metres/second.    Unfortunately there is no table available on the site showing the performance under different wind speed conditions.  They do claim that it will work in extreme wind conditions - although they don't explain what that means.  I happen to have a remote weather station on my roof.  I haven't been monitoring it very closely recently, but between October and December 2008 the average wind speed, from snapshots taken every 15 minutes, was 3 metres/second.  Even today, which is fairly windy, the average is under 4 metres/second.  That seems to fall a long way short of the number needed for efficient generation from this turbine.
Update - 8th September - a very windy day and the average is still only around 8 m/s.  Strongest gust was just over 15 metres/second.  (1 metre/second = 2.237 mph).

I am therefore lead to conclude that investing in personal wind power at this stage doesn't seem like a good option for our household.  A payback time of 5 to 7 years would not seem unreasonable but 35 years strains my sense of credulity.