The size of a wind turbine does matter; but the turbine blade diameter is of primary importance. For kinetic energy to be converted to electrical energy, as much "moving fluid" must be captured as possible. It is the blades that catch the wind's kinetic energy. A smaller turbine has smaller blades to capture smaller amounts of energy. It all comes down to mathematics: as the blades spin, the area covered can be calculated.
If the turbine blades are 10 feet (3m) in diameter, they would sweep a 5ft (1.5m) radius (half the diameter).
Area covered by blades = Pi x Radius squared = 3.1415 x 5 ² = 78.5 ft ² (7.3 m²)
As the power ratings used by machine manufacturers are not consistent with those used in PV modules, comparisons are difficult. Different machines with different spin sizes can only give estimated value comparisons on power output. Remember that the area swept by the rotor is in direct relation to the power generated, so if the sweep is doubled, the power output is doubled.
The best way to compare the manufacturers' figures is through a comparison of power curves using similar wind speed and rotor-swept area. The two ratios should be similar: a machine with double the sweep area will have double the output power curve.
The power curve shows other interesting wind turbine facts; low wind speed (under 8 mph or 13 km/h) will give near-zero turbine output. Though many dealers emphasize low-wind start-up or earlier power generation in selling their turbines, this is not a factual support of wind energy physics. The turbine is not impacted by "starting up earlier." Even if the unit spins in a low-speed wind, it does not produce any energy. Strong wind is needed to produce energy.
This reveals another important point about wind energy: doubling the wind speed does not double the power. Example: a 16mph (25.5 km/h) wind gives about 2,200W power output. If the wind speed is doubled to 32 mph (51 km/h), the expected 4,400W output is not generated, but a whopping 10,500W!
This is the typical non-linear relation of wind speed to wind energy; it is a cubic relationship. Hence for a doubled wind speed, you get an eight times increase in power (2 cubed = 2 x 2 x 2 = 2³ = 8).
This point is crucial for turbine installation, especially at a lakeside where "there is always a breeze". It may not be the ideal site. Where the wind is concerned, the general rule of thumb is: if it is always too windy outside your house to stand too long, that might just make it right for a turbine installation. It is crucial to have a proper assessment of the wind site for a worthwhile wind turbine installation.