In the late 90s Greenpeace published an estimate that computers used 13% of the nation's energy supply. The study was debunked, but it represented one of the first attempts to sort out just how much energy was being used. At the time it wasn't too much (under one percent of electricity being generated), but it was significant and rising. The most recent estimates I've seen date to 2010 and are about two percent.
The New York Times has published the first in a series of articles on the physical infrastructure that makes up the Internet cloud. It notes some, but not all, of the inefficiencies. (not mentioned is that the programs themselves are often enormously inefficient - low quality programming using the wrong tools because it is inexpensive to throw more servers at the problem).
The Times piece claims about 30 gigawatts of power are being used internationally, a third of that in the US.
Ten GW ...
For scale the million households in San Diego are responsible for an electricity power demand of about one GW (note this is an average - late at night it may be very low, while on a hot day in the late afternoon it can be much higher).
Total electrical power consumption in the US averages about 480 GW (note this is power - how much energy is used per unit time - so it represents a rate of energy use), which is about 42% of total generating capacity (we need that capacity for spikes in demand - it is common on hot average days in the Summer to be at more than 80% of capacity nationally). This is about 2.4%. It would be interesting to see if that is a current 2012 figure or from the 2010 study period (I suspect the latter). But the point is it is high, it exists during periods of heavy load (which can tough full capacity at times), it is increasing rapidly, and there are great inefficiencies.
It probably is the case the need for reliability is stressed and the cost of power is relatively low compared with other costs. Since over 70% of our electricity comes from burning fossil fuels (the average is higher during high demand periods when auxiliary, usually petroleum based, power must be used) like coal and natural gas, this is a serious problem.
It will be interesting to see the full series... as well as reaction. Many of us have been aware for some time, but the generation reaction is "electricity is really cheap.." These guys are not paying for externalities like moving new generating capacity or its distribtion or the pollution consequences.
There is some hope - Apple and Facebook have both been chided about this and some steps have been taken to improve things, but these represent drops in the bucket. Getting infomation is enormously difficult in this sector - perhaps some transparency is required and the players who worry about corporate image can be motivated.