So the prospect of generating electricity for your home from free wind energy has become too irresistible, and you’re ready to start looking seriously at the different wind turbines available. You don’t know it yet, but soon you’re going to be awash in marketing claims, engineering specifications and industry jargon without the proverbial paddle.

To help you understand the differences between the dozens of home wind energy systems you’ll be comparing, here are the essential concepts and terms you need to understand.

Horizontal vs. Vertical

Horizontal axis wind turbines (HAWTs) resemble a child’s pinwheel toy, or the water-pumping windmills seen in the open pastures of the plains. Driven by blades that look like an airplane propeller, the rotating axis is horizontal.

Horizontal Axis Wind Turbine

Vertical axis wind turbines (VAWTs) resemble an anemometer – the instrument with cups attached to spokes that spins and measures wind speed. Spinning in the same direction as a child’s top, the rotating axis is vertical.

Vertical Axis Wind Turbine

The difference between the two is more than just a matter of appearance. Both designs have been around since the 1920s and many variations of each have been thoroughly tested over the past 80 years. For the most part, the HAWTs are far more efficient at converting wind energy into electricity than VAWTs.

The greatest attraction to VAWTs seems to be the visual appeal of various novel designs compared to the old-fashioned propeller blades on HAWTs.  But because of lower efficiency, a VAWT typically generates less electricity than a HAWT of comparable cost, and is therefore a poorer investment. Recent advances in engineering and manufacturing, however, are helping to reduce cost and improve efficiency of some VAWT designs, making them more competitive.

Many companies marketing VAWT designs claim they are more durable because they don’t have to spin around to face the wind, and they cause fewer bird deaths, but these claims have not been substantiated. Some may also claim quieter operation than HAWTs, but that’s not true across the board. That comparison is only meaningful on a model-by-model basis.

The Effect of Average Wind Speed

Obviously, wind turbines require wind to work. The biggest problem wind-energy shoppers encounter is over-estimating their local average wind speed.  This is critical because a wind turbine’s electrical output is related to the cube of the wind speed.  Refer to the following chart for examples.

Wind Speed  Units of Power

5 mph             5 x 5 x 5 =     125

10 mph          10×10x10 =  1000

15 mph          15×15x15 =  3375

Referring to the chart, if you’re guessing your actual local average wind speed is 10 miles per hour, and it’s actually only 5 miles per hour, you think that means your power output is cut by 50%. Actually, that miscalculation cuts your power output by 87.5%.  Where you might have seen a return on investment at 10 mph, you could possibly see a negative return at 5 mph.

Although you could have a wind study performed, it’s typically expensive and requires monitoring throughout a full year to develop the seasonal wind profile. As an alternative, you may be able to locate wind maps for your area through resources such as the National Renewable Energy Laboratory or the Dept. of Energy’s Energy Efficiency and Renewable Energy website. You might also find local wind maps by typing “(your location) wind resource map” into your favorite Internet search engine.

Roof Mount vs. Tower Mount

Now that you know how important wind speed is to your return on investment, consider this. At any average wind speed, the wind energy at 150 feet above the ground is twice as powerful as the wind energy 30 feet above the ground.  Slower wind nearer the surface is due to friction with the earth and turbulence created by trees and buildings.

This is important to keep in mind because many wind resource maps display wind data at 50 meters (164 feet) altitude. If you intend to mount your wind turbine on your roof, you can expect to receive half the energy output compared to mounting the turbine on a much taller tower.

Though mounting a wind turbine on your roof or a low tower sounds easier than raising it up on a very high tower, it can lead to a negative return on investment unless you live where there is a stiff and steady breeze at roof-top level.

Battery vs. Batteryless

Batteryless designs are wired directly into your home’s electrical system. In wind energy systems with batteries, the turbine is first connected to the batteries, then the batteries are connected through a DC-to-AC inverter to your electrical system.

The advantage of a battery system is if your local grid loses power, you can keep powering your home for a while using the backup batteries. The disadvantage of the battery system is the extra cost, and the extra electricity that is consumed in the battery and inverter system. The higher price tag and lower net output reduce your return on investment.

Energy Output Rating

Perhaps the most important wind turbine specification you want to compare is the kilowatt-hour output. Most homes use between 500 and 1,000 kilowatt-hours per month. Wind turbine specifications include the kilowatt-hour output per month at a given wind speed, but manufacturers use different wind speeds making comparison difficult.

Remember that you can’t do straight comparisons at different wind speeds because the power output differs by the cube of the speed, plus other factors unique to the turbine affect power output at different velocities. If you’re serious about comparing a few selected models, ask the manufacturer for specs showing power output for a range of wind speeds.

Rotor Diameter and Swept Area

You may see specifications for rotor diameter and “swept area.” The swept area is just the square footage of the circle “swept” by the blades, and is calculated using the rotor diameter.  These measurements affect the turbine energy output, but there’s no real point in comparing these specs. What you really want to compare are the energy output ratings.

Peak Power Output

Peak power output is the maximum power rating in kilowatts (kW). Although every manufacturer features this specification in their literature, it is a relatively meaningless point of comparison. The wind turbine’s value is in its sustained energy output, not its peak capacity.

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