Nov 13

# Wind Turbine Power Ratings Are Useless

The power ratings most manufactures give for their wind turbines are almost useless. They use wind speeds that are much to fast! Speeds that are much higher than the annual average here in Canada or the US.

For example the average annual wind speed in Canada in most places is between 4 and 5m/s or 9 to 11mph. Most wind turbines rate their power for wind speed of 20 to 25mph.

Here is a example of how far you would be off.

If you google up 2kw wind turbine for sale the #1 entry is 2kW system with 10.5′ blade diameter.

They got there rating using a wind speed of 9m/s or 20mph. So if you calculate the power of this system for 4.5m/s or 10mph you will find out your don’t have a 2kW system at all but at 110Wh system.

They are advertizing it for 18 times what it really is. Sure it will do 2kW when there is 26mph winds but the large majority of Canada and US has average annual wind speeds of half that.

Conversion

1 square foot = 0.09290304 square meters
1 mile per hour = 0.44704 meters per second
1 kilometer per hour = 0.277777778 meters per second

Here is The Calculation

10.5′ Diameter Blades (8.04m2 ), 10mph average wind (4.47m/s)

HWP = 110Wh

HWP = 0.5 x 1.23 x 8.04 x 89.31 x 0.25

HWP = 0.5 x 1.23 x SA x WV^3 x 0.25

(HWP) Harvestable Wind Power (w/hr) = 0.5 x AD (air density kg/cu m) x SA (sweep area m^2) x WV^3 (wind velocity m/s) x cP (coefficient of performance Betz limit 59.26%, R^1 or Copper loss, linear power curve to cubic curve matching — 0.25 for most systems)

So make sure you do your own calculation. Here is a shorter form for power. It is off by about 3% but it is very easy to remember.

Power = 0.15 x SA x WV^3(m/s)

Power = 0.15 x 3.14 x 5.25 x 5.25 x 0.09290304 x 4.47 x 4.47 x 4.47

Power = 107Wh

Here’s another one. Canadian Tires AirX 400W Wind Generator that will only do 52Wh at 12mph. Sure it will do 400W at 45km/h but how often is that going to happen?