Here's a back-of-envelope calculation

germany's current electricity consumption is 1.6  billion kWh/day  To cut CO2 emissions, you need to shift most transport to electrified rail (as Jerome advocates) and to plug-in electric road vehicles.  At a minimum, this will double electricity consumption to 3 billion kWh/day.  

Onshore wind is the only renewable source likely to make much contribution to this.  Wind farms have an average output of about 50,000 kWh/day per sq km.  So you'd need 60000 sq km of wind farms - one seventh of germany's land area - for average wind output to equal average consumption.   For peak capacity, you'd need far more than this, even with pumped storage and demand management.  Does anyone really think that allocating this much land to wind farms is feasible?