The cost effectiveness of producing electricity from the wind has reached its peak for the time being with turbines rated at about 600 kW. So claims a study from German wind institute DEWI. It reports that 600 kW units can operate economically at winds averaging 6.3 m/s at 30 metres, while 300 kW machines need average wind speed of 6.8 m/s and 150 kW turbines as much as 7.7 m/s. Furthermore, the new class of 1.5 MW turbines require average winds of 6.7 m/s to break even under current market conditions, say the study's authors, Knud Rehfeldt and Bärbel Schwenk. They add though that specific investment costs for 500-600 kW machines with rotor diameters of up to 45 metres have dropped considerably since they were introduced in 1990. This would indicate the same pattern could emerge for megawatt class machines, especially with an increased volume of turbine sales. But the Electricity Feed Law's guaranteed premium payments for wind energy are vital if this is to happen.