just want to summarize the typical efficiency of electrical power sources:
Coal (http://de.wikipedia.org/wiki/Braunkohlekraftwerk) (Germany): 38% (typical) to 43,7% (maximum brown coal), 46% (max. stone coal)
Gas (http://de.wikipedia.org/wiki/Gas-und-Dampf-Kombikraftwerk): up to 60.7%, about 55% typical (http://de.wikipedia.org/wiki/Well-to-Tank)
Nuclear power plants: 33% (although not really applicable due to nuclear reaction)
Water, Wind & PV: 100% (as I regard these sources as endless)
Biomass and auxiliary: 90%
Energymix in 2013 (Germany):
So, for every kWh that comes out of the outlet, 2.02 kWh of primary energy was necessary. This means that 454g of CO2 per kWh electrical power have been issued.