-Efficiency is determined dividing instantaneous power produced by the turbine by the theoretical power available in the wind at that instant, and multiplying by 100%: 
Efficiency = [ (Turbine Power )/(Theoretical Power) ] X 100%

-Click this link for a discussion of theoretical power:  Theoretical Power or Potential Power

-The cut-in wind speed for the Skystream is 8 mph: That is, the turbine wont produce power at wind speeds under 8 miles per hour.

-Below 8 mph wind speeds no power is being produced, but since some potential power exists the efficiency will be zero.

-Unrealistic efficiency values sometimes occur in gusty conditions.

- Sudden changes in wind speed are picked up by the anemometer measuring speed, but the turbine doesn't react as quickly.

- It is difficult to perfectly synchronize anemometer values used to calculate theoretical power and turbine power data

- A 'running average' gives the best estimate of overall turbine efficiency