Power from the wind

If we take the wind speed distribution readings of any location taken over a year we can derive the average wind speed. ( New Zealand average wind speeds - windfinder ) 

Wind Speeds: Wind speeds conversions.
KPH
 1
 5
 10
 20
 30
 50
 70
 90
M/S
 .28
 1.39
 2.78
 5.56
 8.33
 13.89
 19.45
 25.00
Knots
 0.54
 2,70
 5.40
 10.8

 16.2
 27.00
 37.8
 48.60


Power in the wind : Total watts per meter at various wind speeds. Turbine efficiency determines poer extracted.

1
 2
 3
 4
 5
 6
 7
 8
Power /SqM
 0.6
 4.9
 16.5
 39.2
 76.2
 132.3
 210.1
 313.6
Wind Speed 9
 10
 11
 12
 13
 14
 15
 16
Power /SqM 446.5
 612.5
 815.2
 1058.4
 1345.7
 1680.7
 2067.2
 2508.8

Doubling of the wind speed increases the power available by a factor of eight.

Wind Turbine Efficiency : The Efficiency of a wind generator is calculated by taking the output at each wind speed divided by the power available. 
Wind Speed 4 M/S
 5 M/S
 6 M/S
 7 M/S
 8 M/S
 9 M/S
 10 M/S
Efficiency
 0.01
 0.15
 0.25
 0.35
 0.44
 0.40 0.38

( For Auckland airport with average wind speed is 5.1 M/s, a turbine of 0.15 efficiency and swept area of 3 sqm we will get a potential output of about 300 Kilowatt a year. )

Number of Blades : A wind turbine output voltage increases as the RPM increases. The ideal is to get a turbine that generates higher output voltage at the low wind speeds. This will enable it to start charging batteries at low wind speeds.
Higher number of blades optimises performance at lower wind speeds.

Generator Size : The higher the rated generator output the more power is needed to turn it. Conversely the smaller the generator the easier it is to spin in the wind. The faster you spin the generator the higher the output voltage. Most generators have braking mechanism to stop overspining and damage to the generator.

Rotor Size : The larger the rotor the larger it's swept area and greater the amount of power it can harness from the wind. The smaller the rotors the faster it can spin in low winds enabling it to start generating in low speeds.

Voltages : In order to charge the battery the output voltage of the wind turbine have to be higher than the battery's voltage. For a given generator because of back emf from the battery, a 12 volt battery bank will charge up at lower wind speeds but will require higher wind speeds to charge at 24 or 48 volts. This is why small wind generators have 12 volt outputs only.

Rectifier : Most generators produces Ac current which is rectified to DC (the opposite of an inverter ).

Noise : Noise in wind generators are mainly from the gears. Further noises can come from the rectifiers and propellors.