I looked at the specification of the Scirocco turbine here http://www.eoltec.com/English/scirocco_en.htm and came up with an approximation using our page here http://www.rensmart.com/Information/Betz%27Law to find the efficiency of the turbine.
It works out at about 45% efficient which is pretty good.
I also looked at the wind data recorded at your nearest Metar station here http://www.rensmart.com/Weather/Metar
The latest full archive data we have near you is Kirkwall Airport in 2008. It shows that the wind measured wind speeds were close to the NOABL prediction.
When I adjusted the turbine hub height to 15m, the turbine diameter to 5.6m and the efficiency to 45% I get 16,600 kWh/year which is a bit closer to your value.
However, the current live version has inverter losses of 30%. We are about to revise that figure to 15% based on better data. This makes the value 20,100 kWh/year which is a pretty good match to your estimate and measurements.
Is your turbine the ROC project named Kirkwall Squash Club which we display here ?
. We would like to combine the data entered on the Google spreadsheet with the location. Any photos would be nice as well :-)
Would you be interested in giving quotes through our service when it goes live in the next couple of weeks ?