This is brought out very clearly in a series of blog posts over at NALOPKT (that's Not a Lot of People Know That, to those who didn't guess it). This is the first and the previous two develop the theme further.
What it says, in summary, is that whilst electricity demand currently peaks at around 50GW, gas demand frequently peaks at over 300GW. If this demand for gas had to be replaced by electricity, as required to meet the demands of the CCA, it would not only need massive increases in generating capacity, it would also necessitate a complete rebuild of the grid and transmission network as the current system would be overwhelmed. All this is still a few years away, so our present lot of politicians can still say they will do it, but the questions they will soon have to answer is - how? and equally important - who is going to pay?