Vertical axis portable wind charger with battery and USB output

Typical efficiency of a vertical axis Savonius rotor which this looks like is very roughly 10% The theoretical power available in the wind is 0.5 x rotor swept area (m2) x density (kg/m3) x velocity3 (m/s). So guessing 12 inch by 4 inch (0.3 x 0.3 m2), swept area and 20m/s we have 0.5 x (0.3 x 0.1) x 1.225 * 20^3 = 147W so at 10% efficiency that gets the 15W mentioned on Kickstarter. But 20m/s is rather a lot of wind, Beaufort 8, fresh gale (http://en.wikipedia.org/wiki/Beaufort_scale#Modern_scale). The generator would certainly need to be lashed down. In a more moderate wind, say 10m/s, Baufort 5, fresh breeze you would only get one eighth part of that output, say 2W.

Unfortunately, as with many Kickstarter projects, the promoters make no mention of the actual specifications and operating conditions of the device. Looks like either a con, or perhaps just wishful thinking. Or someone can correct my calculations.

Of course such machines work, they have been used on boats for many years, but this particular one looks like a very expensive way of getting a trickle charger.

For comparison here is the spec. of an existing 3 blade Savonius with comparable output (12W at 8m/s): http://www.leturbines.com/products/le-v50-wind-turbine/. Note that it weighs 9kg and has a rotor 450mm x 270mm, that is four times the area! Even if we assume that the Kickstarter device is for less harsh conditions it still seems a stretch to get the weight and size down so far.