My understanding of Vawts is that unlike Hawts they cannot exceed the speed of the wind - ie if the wind is 20mph the outer edge is turning at 20mph.

So you need to calculate the circumference of the rotor......

The 125s model (from your link) has a 32cm diameter rotor

so using our friend Pi 3.141 x 32 = 100.53 cm circumference .

1 mile = 160934 cms

therefore theoretically a 5 mph wind =160934 x 5 = 804670cm /hr or /60 =13411 cms /min

13411 / 100.53 =133.4 rpm

Taking it farther - 1800rpm (you stated) x 100.53 =180954cms /min x60 =10857240cms /hr

/ 160934 = 67.46 mph !!!

does anyone else agree ??

George