If we take World Energy Use in 2006 as W2006, the amount of energy we can use sustainably(Se) - i.e via renewable energy (RE), traditional energy generation (TEG) and harshly reduced use levels, then
Se = RE + TEG
W2006 = RE X 10
Renewable energy - hydro electric, wind and solar for example, are usually seen as supplying around 10% of present energy needs. I don't think there's a way to supply 100%, so we have to reduce our consumption by whatever it takes to get to Se.
A way to understand how to do this could be looking to the past and seeing what per capita energy consumption(Epcx) was, multiply this by however many people are living now in comparison(POP2006), and there you have it. I think we're going to get blasted into the past.
Epcx : POP1XXX = Epc : POP2006
W2006 = Epc . POP2006
Or in other words, we're fucked, but how can we stabilise as well as possible, with the least disruption? See individual houses for example. What do we have to give up until taking anything else away would take away our health? What is the minimum a person needs to survive? This is the starting point, the end point being our situation now - in the western world that is, where we're really oil and energy dependent.
I think a big problem will be getting water to people, keeping them sheltered, and growing enough food in a small space to feed everyone. Another problem will be access to healthcare and medicine. Finally, violence and security.
We need close personal ties with people living close to us, so that we help each other, a spirit of getting things done and working together. We need skilled people in older traditional trades such as farming and preservation of food. We need to get what knowledge we can now of what various traditions and cultures have done in hours of need, but also to do simple tasks that we might do at a great expense of energy.
With a few old computers you can manage a barter economy and maintain medical records and census info within a local area of even thousands of people living in very low energy conditions, with engineering you can produce working bicycles from discarded ones, or even better make them even more favourable and able to use geometric properties to be more efficient. We can use junk and rubbish we've thrown away until now as something that we can convert back into energy or simply renovate and re-use. We can use mass transport systems and bio-fuel for sporadic long distance travel, although that will become something for the rich. There may be more of a division between rich and poor, but women are now free of cultural ties they had in the past, and able to work and function in society much more than before.
On the other hand, we've forgotten a lot of old world skills, and most of all, there's no clear sign people are realising the seriousness of this situation.