In a post apocayptical world, you would have to have amniesia to not get a working electricity supply back fairly quickly. The obvious renewable is wood and with water that gets you steam which can drive a turbine which drives a generator. Would it really be so hard to ramp up to modern civilisation? Its not like we would have to invent things more reimpliment the things that already have existed in one form or another.
While i happen to have a plc that would run off 12 volts. its basically relay logic anyway so scavange from a few cars and i can automate from that basis. probably the two most important things needed are electricity and refridgearation. computers would be one of the best accelerators of recovery.