It's a good idea for companies to have a contingency plan, but it's also useful to have one at home. For instance, I know someone who's had 4 power cuts in the last week. Back in July 2007, there were floods in Croydon, and in December 2010 there was enough snow to shut down the entire Southern rail network. In situations like this, shops will run low on supplies (if their deliveries can't get through) and there will be lots of people out panic buying. There have also been more serious problems elsewhere in the world, e.g. the Northern Ireland water crisis (December 2010), the 'biblical' floods in Australia (January 2011), and Hurricane Katrina in New Orleans (August 2005). The DirectGov website has information about Preparing for emergencies. There's also a lot of useful advice at the Zombie Squad website, where they use zombies as a metaphor for any kind of disaster. Quoting from their FAQ: "if you are prepared for a scenario where the walking corpses of your family and neighbors are trying to eat you alive, you will be prepared for almost anything." However, you don't need to be a crazy mountain man, planning for a cataclysm; you can just prepare for temporary problems. Some of these ideas are very easy to implement, while others either involve moving house or are something to consider while you're house-hunting.
What happens in a power cut? Your building may have a backup generator, but you should test how quickly it kicks in. You may notice that the lights flicker, if there's a half-second delay, and that would be long enough for all your computers to shut down. So, that's where a UPS is handy. Typically this will only provide power for 20-30 minutes, so it isn't a long-term alternative to a generator. However, it will provide continuous power, so it will handle any "blips" when the generator turns on. If you don't have a generator (or it runs out of fuel), the UPS will give you time to shut down your computers gracefully rather than watching them go splat.
However, this can have other effects that might not be obvious. For instance, is your ADSL router plugged into a UPS? If not, you won't be able to use it to get online, even if your laptop has battery power. (This will be less of a problem if you have an iPhone or equivalent.) Also, how about central heating? Electric heating obviously won't work, but my gas boiler uses a small amount of electricity (e.g. for the timer) so it won't work during a power cut either. If you have a prolonged power cut in the middle of winter, this could be very uncomfortable. My contingency plan is my fireplace, so I make sure that I keep a stock of matches, firelighters, kindling wood, and smokeless fuel.
As for lighting, I keep a couple of boxes of household candles in a cupboard (similar to these), with one candle stuck to a saucer on my mantelpiece. They're cheap (about 50p per candle), they don't take up much space, and they basically last indefinitely (until you use them). I also have a torch, but if you rely on that then it's prudent to keep some spare batteries for it.
You may also be able to generate electricity by other means. For instance, you could get photovoltaic (PV) solar panels attached to your roof. That's not an option for me at the moment, because I don't have a roof (just another flat above me), but I like the idea. As a fringe benefit, it should reduce your electricity bills in general, i.e. you benefit from these even when there isn't a disaster to deal with. Another option is pedal power: basically, you put your bike on a tiny treadmill (as if it was an exercise bike), then you generate electricity by pedalling it. I'm attending the London Bike Power Workshop next Sunday (23rd Jan), organised by Magnificent Revolution, so I'll be interested to see how that works.
Now suppose that you want to go to the shops. If there's a power cut affecting the local area, that means that you won't be able to pay by credit/debit card (because their machines won't work), so you'll need cash instead. However, cashpoints also need power! So, I recommend keeping a small reserve of cash at home (say £50/person). There's some more advice at This is Money: Savers warned over home safes; basically, check your home contents insurance policy, and you should be covered for relatively small amounts of cash, but don't keep your life savings at home. In a workplace, it would make sense to keep a similar reserve of petty cash in a safe.
Speaking of shops, they often order stock on a "just in time" basis. For instance, I was at Sainsbury's recently, and there was a sign up saying that they didn't have any milk because that day's delivery hadn't arrived. So, it's useful to keep some food supplies at home, in case you can't get new stuff from the shops. The tricky issue is expiry dates, but some things can be frozen. There are also longlife versions of milk and orange juice, and canned food is useful (e.g. soup or baked beans). If you bake your own bread then flour/yeast/salt/sugar all last for ages in a cupboard; butter is the only awkward ingredient.
As well as food, we also need water. As a rough guide, I'd say that each person needs to drink 2 litres a day, although it will vary based on exercise and weather, e.g. you'll get thirstier if you do a long cycle ride in the summer. However, there are various things that could cause your water to get cut off, e.g. burst pipes (where water has frozen). Looking at Oxfordshire in 2007, and Northern Ireland more recently, the local authorities set up standpipes in the streets, so people would queue up to fill bottles with water. However, those amounts are rationed, so it will make life easier if you have your own supply. Normally I carry a water bottle around that I refill from a tap, rather than buying bottled water from shops. However, if you're putting water aside for an emergency then tap water may get a bit manky after a while (stagnant?), so pre-bottled water is probably better. It's pretty cheap too, e.g. 20p/litre from Sainsbury's. I keep 4 x 2L bottles in my kitchen, which doesn't take up a huge amount of space, but it would last me for a long weekend if my water was cut off. However, this doesn't include washing or flushing the toilet. Ideally I would keep a rain butt for that, since washing water doesn't need to be drinkable; however, that's not practical where I live at the moment.
Aside from food and drink, you should also consider toilet paper. It sounds silly, and it's obviously not essential to life, but I think it would be unpleasant if you had to go for 3 days without it. In the past, I used to buy a multipack, then go out to buy another one when I was down to my last roll. A few months ago, I bought 2 multipacks when I was at the shops, so now I always make sure that I have a complete multipack in reserve (4 rolls). In other words, I started out with 2 multipacks. I opened the first one, and eventually got down to the final roll, with the other multipack still unopened. I then bought a new multipack, and that became my new reserve. (I rotate them, rather than keeping the original one indefinitely.) I stack the multipacks against the wall, so this doesn't take up any extra floor space. On that first shopping trip, I paid an extra £2 to buy an extra multipack, which isn't too expensive; after that, I've just replaced them at the same rate as before, so there's no extra ongoing cost. I've kept track of how long each roll lasts, i.e. I record the date each time I throw an empty tube away, and it averages out at about 3 weeks. So, a multipack of 4 rolls should last me for 2-3 months, which is plenty of time. However, that applies to me living alone; also, I'm out at work every day, so I use the toilets in the office. If I was stuck at home every day then I'd get through it more quickly. Similarly, if there are several people sharing a house then you'll use it more quickly. So, I'd advise you to record your own usage, e.g. stick a pencil and piece of paper to the toilet door and ask your flatmates to write down the date each time they change a roll. This will get a bit more fiddly if you have more than one toilet in a house, but you should be able to get a general idea. You can then work out how many rolls to keep in reserve.
I realise that some of this sounds like hoarding, which has a bad reputation in emergency situations. However, I don't actually see a problem with it. If I wait until the blizzard hits, then go and empty the supermarket shelves, that's selfish because nobody else can get any food. However, if I buy extra stuff in advance, there's still plenty to go round for everyone else, and it's good for the shops because they're selling extra. Also, when things do go wrong, I'll be able to stay at home and use my supplies rather than competing for scarce supplies with other people, i.e. there's more left for everyone else.
The bigger question is: "How long do you prepare for?" Up to a point, I'd say that longer is better. For instance, if you have a week's worth of supplies, and you get stuck in a blizzard for 3 days, you'll be fine. On the other hand, if you have 3 days of food, then you're isolated for a week, you'll be in trouble. However, this will depend on your resources. As an extreme example, the Mormons (aka the Church of Jesus Christ of Latter-day Saints) are supposed to store a year's worth of food, which is fine if you've got a huge barn, but I wouldn't have space for all of that in my flat.
I also think that stockpiling has its limits; if the infrastructure has been so badly affected that it's not back to normal within a year, it's probably going to take a lot longer. At that point, it's better to look at being (relatively) self-sustaining. For instance, keeping chickens is a better bet than freezing hundreds of eggs. Over at Zombie Squad, people use terms like PAW (Post-Apocalyptic Wasteland) or TEOTWAWKI (The End Of The World As We Know It); similarly, pozorvlak has referred to the Grim Meathook Future. However, the Transition Network has a somewhat more optimistic view: the basic idea is to make lifestyle changes now so that there won't be any major upheaval when (not if) the oil runs out. There's a bit of overlap between them and the WNBR (e.g. Transition Southampton), which is how I've heard of them; I haven't investigated their ideas in detail yet, but I intend to.
As I understand it, one simple idea is to "buy local". This refers to the origin of the food (or whatever) rather than just the retail outlet. For instance, it's better to buy honey from a local apiary, rather than going to the local supermarket where it's been imported from other countries. As I mentioned above, there are some things that I'd like to do which aren't practical at the moment, so I'd like to move somewhere with my own roof and a garden. Actually, I quite like the idea of a smallholding, so that I could keep bees without upsetting my neighbours. However, I don't really aspire to being a farmer, and I don't think that every household needs to keep their own chickens; it's fine for people to specialise. The point is that if I can buy what I need from somewhere that's within walking/cycling distance then it won't matter if long distance travel gets prohibitively expensive. Similarly, there were people who got caught out before Christmas because they'd ordered gifts from Amazon etc. which had been delayed due to the snow; if you buy locally then this won't be a problem, so this fits back into the general theme of contingency planning.