Well the answer is simple, VERY.
We had a power cut yesterday and i could do nothing while the power came back on (about 35mins). We couldn't ring out on the Landline or VOIP line, watch the tele, do my day-to-day business on the interweb, i couldn't even hoover(yes i was forced to tidy up, and i still haven't finished)
But just think how much of our infrastructures are dependant on electricty? Increasingly Phones, Traffic Lights, Safety systems (these should have back-up power) Even the bare essentials are effected, showers, heating/AC, Cook Appliances. Practically everything goes off, i would be interesting to see how we could manage without power. The big power cut in New York a few years back highlights out dependancy how many millions did it cost and how many people were trapped or unable to commute in that 14hour stretch.
No moving to focus on the UK, with the amount of redundancy in the National Grid dropping as more and more pwer stations are taken offline we are going to face more and more power-cuts, which means we have to get big on renewables or nuclear, or even both. I have to say i arent a fan of nuclear but the fact is there is that many people who object to the current viable renewable means that nuclear is the only option.
N.B. I couldn't be bothered to link to sources for the information but it is freely available have a quick google.