Micro wind or roof top wind, as they're alternately described, generally get a bad rap; and deservedly so. As far as return on investment goes, roof top wind is probably one of the worst renewable energy investments you could make. Unfairly lumped in with this unproductive class of wind power is the home built wind generator.



To illustrate this point I'm going to compare the cost per kilowatt hour (kWh) for electricity produced by the Swift small wind turbine to a typical home built generator about half the size of the Swift. Regarding wind turbines, size is determined by the length of the propeller. The length of the propeller determines the "swept area" which is akin to the collector area of a solar panel. The larger the swept area, the more energy is produced.

Once you know the average wind speed at a given location and the propeller length, you can estimate monthly kWh production. Multiply that by the number of months the turbine is expected to last, and divide the result into the cost of the turbine. The number you end up with is the cost per kWh over the lifetime of the turbine.

Determining wind speed

The height of an average one story home is around 15 feet, or 4.5 meters. Using this information I looked through the Wind Energy Resource Atlas of the United States for average annual wind speeds at that altitude in Massachusetts. The average wind speed I came up with was 8.2 mph.

Determining monthly kWh output

The Swift turbine: Based on wind speed of 8.2 mph and the 7' prop of the Swift turbine, its monthly output would be around 32 kWh or 7,680 kWh over the course of its estimated 20 year life. The projected life span of this turbine comes from the manufacturer. Personally I think it's a little optimistic, but for the sake of simplicity I won't quibble.

(A quick note on how I came up with the 32 kWh figure. Hugh Piggot, a well known wind power professional, developed a formula using wind speed and prop length to estimate the monthly kWh production for ANY size turbine. But rather than put you to sleep with his formula, I've created a simple monthly kWh production calculator anyone can use. You can access the calculator from the link at the end of this article.)

Now, on to the home built turbine. In 8.2 mph winds with a smaller 4' prop the home made turbine would generate around 10.6 kWh per month or 2,544 kWh over 20 years. Since there are no statistics for the estimated life of a home made turbine, I'll adjust for the discrepancy in the next step.

Cost of the turbine

The cost basis for this comparison is for the turbine only. The Swift turbine lists for $10,000 to $12,000 so I'll settle on $11,000. Subtracting for the 30% energy tax credit leaves a net cost of $7,700.

The example wind turbine used for this article costs around $100 to build. As I just mentioned, there aren't any life span projections for home built, so I'll assume replacement every six or seven years for a total cost of $350.

The final answer

With a little simple division we can find out the cost per kWh over the life of the turbine.

Swift turbine: $7,700/7,680 (kWh) = $1.00 per kWh.

Home built turbine: $350/2,544 (kWh) = $0.14 per kWh.

Remember, the home built model is equipped with a 4' prop. If you increase the prop length from 4' to 5', the magic of physics increases lifetime kWh production to 3960 kWh and reduces the cost per kWh to $0.09. Now that's eco-nomical.