Are you wondering if the claims of a wind generator manufacturer, designer, or salesperson are legitimate or realistic? Apply a simple, generalized formula to get an idea of whether you’re listening to a shyster or a reliable source.

Take the average wind speed used for the claim, cube it, divide it by 240, and that will equal the approximate kilowatt-hours (kWh) per month per square foot of rotor, at the Betz limit. Cut that number in half and you’ll have a good number for a well-designed residential turbine.

For example, in a 9 mph average wind speed, a turbine operating at peak theoretical efficiency (the Betz limit) would give you 3 kWh per month per square foot of collector area [(9 × 9 × 9) ÷ 240 = 3]. If someone is claiming that a 15-square-foot turbine will deliver 90 kWh per month, they are claiming twice what is physically possible, and about four times more than the best turbines on the market.

For a different method that comes to similar conclusions, see Mike Klemen’s page at www.ndsu.edu/ndsu/klemen/Perfect_Turbine.htm