This is a very important point! And it is one that is often overlooked when comparing alternative energy systems that generate electricity directly, such as photovoltaics and wind turbines, with electricity generation via fossil fuel-fired heat engines running generators.

When a wind turbine puts out the equivalent of 1 million BTUs of energy in the form of electricity, it is not displacing 1 million BTUs of fossil fuel, but more like 3 million BTUs of fossil fuel (based on an overall powerplant efficiency of something like 30 - 35%). This, of course is a very crude comparison, and doesn't take into consideration the energy input of construction, energy payback period, etc., but I think it illustrates the point.

It is also well to keep this difference in mind when considering electric cars. The energy content of a gallon of gasoline is roughly 36 kilowatt-hours. But if you have a battery-motor system with an overall efficiency of 80% , you will be able to drive that car the same distance as you would on 4 or 5 gallons of gasoline, all other things being equal (such as vehicle weigh, rolling friction, aerodynamics, etc).