The power curve describes a unit's behavior with different wind speeds. Some important points revealed by the curve are:

• Start-up speed. This refers to the wind speed as the rotor and blade components start rotating. A low start-up speed is not important as it does not give usable power output.
• Cut-in speed. This refers to the wind speed where usable power is generated but it is often misunderstood. Any wind speed less than 3 mph (5 km/h) generates no power; hence a low cut-in speed is not a purchasing point.
• Rated speed/power point. The manufacturer's manual lists the unit's nominal power rating at certain wind speeds. The Bergey Excel-S has a rating of 10,000 W at a wind speed of about 31 mph (50 km/h). In reality, this rated speed/power point has little value because the assembly seldom functions at this wind speed. Moreover, every manufacturer offers different turbine ratings at various wind speeds which tend to skew data values and make comparisons difficult. Remember that the area covered by the rotor and the blade diameter are of primary importance.
• Peak or maximum power. This refers to the unit's maximum generated power. The Excel-S can produce 12,000 W at a wind speed of 36 mph (58 km/h).
• Furling speed. This refers to the wind speed that causes the unit to switch to a "self-protection" state. The turbine has to reduce its rotational speed and power output when wind speeds are very high. For most small wind turbines, the tail and nacelle "hinge" such that the blade/rotor component turns away from the wind. Other models point their rotor upwards like a helicopter until the wind slows.

It is important to have ratings, but more important is determining the functioning point of your turbine according to the power/yearly energy curve. You must find out how much wind is required to make your investment worthwhile.