You've seen them: a 5 kW wind turbine at a bargain price, in a box you could carry to the car. The hope is that it'll run the house, or at least keep the batteries up overnight. Here's how to tell before you pay whether it can, with a tape measure and one sum.
The Short Answer
✖A turbine small enough to carry to the car does not make 5 kW, even in a strong wind.
✔The power a turbine can make comes from the size of its rotor.
✔A real 5 kW turbine has blades several metres across.
✔You can check any claim yourself with a tape measure and one sum.
Size Is the Whole Story
✔A turbine only catches the wind that passes through its blades, so what matters is the area they sweep.
✔Double the area and you double the power.
✔Wind speed counts even more: when the wind doubles, the power in it goes up eight times.
Power (watts) ≈ ½ × 1.225 × area (m²) × wind speed (m/s)³ × efficiency
✔1.225 is the mass of a cubic metre of air, in kilograms.
✔Area: for a horizontal turbine, 0.785 × the rotor diameter (tip to tip) squared. For a vertical turbine, rotor width × rotor height.
✔Efficiency: good horizontal turbines catch about 35–45 % of the wind's power, vertical H-type turbines about 25–35 %, and small twisted "tulip" types about 10–20 %.
✖No turbine can beat 59.3 %. That's the physics limit (the Betz limit).
✔Then allow about 10 % for losses in the generator and controller.
A Worked Example
✔Take a vertical rotor 0.9 m wide and 0.9 m tall, bigger than small vertical turbines we have seen sold online as 2 kW and more.
✔In a strong 12 m/s wind (43 km/h), the air passing through it carries about 860 W.
✔A good vertical design catches 25 to 35 % of that. Take off about 10 % in losses and you get roughly 190 to 270 W.
✖In an ordinary 6 m/s breeze (22 km/h) the wind carries one-eighth as much, so the same turbine makes a few tens of watts.
What a Real 5 kW Turbine Looks Like
✔To make 5 kW at 12 m/s, a horizontal turbine needs a rotor roughly 4 to 4.5 m across.
✔Our G-5000 is rated 5,000 W at 11 m/s (40 km/h). This is what that takes:
✔A rotor 5.4 m across, with blades 2.6 m long.
✔230 kg of turbine.
✔A ground mount on a 10 to 15 m mast. It is not a rooftop unit.
✔A matching controller and a 48 V battery bank.
✔$5,390 for the turbine, with delivery quoted to your address.
✖If a "5 kW" turbine weighs 20 kg and fits on the back seat, it isn't 5 kW. Whatever the reason for the number on the box, the rotor size tells you the truth.
What Small Turbines Are Good For
✔Topping up a 12, 24 or 48 V battery bank at night and through winter, alongside solar.
✔Boats, caravans, cabins and sheds with modest loads.
✖Running a house on their own.
✖Replacing solar. A turbine assists solar; it does not replace it.
✔Our X-300 is rated 300 W, honestly: a vertical turbine that weighs 20 kg and starts turning at 2 m/s. It's a battery top-up, and we sell it as one.
Check Before You Buy
✔Find the rotor diameter (tip to tip), or the width and height for a vertical turbine. If the listing won't tell you, ask.
✖Run the sum above at 12 m/s. If the answer is ten times smaller than the badge, walk away.
✔Ask for a measured power curve, showing the output at each wind speed. We publish one on every horizontal turbine we sell.
✔Size the turbine to your battery bank and your site, not to the biggest number. The Wind Turbine Calculator does that for you.
More on picking the right size in Choosing a Wind Turbine. Not sure? Ring 0400 333 898 or email Enquiries@BushLine.com.au with your postcode, your battery voltage and a line about the site.