Corrected entry: The generator in the basement works on petrol but they are almost out of it. However the streets are filled with cars with petrol in them, all they have to do is pump it out of the tanks. They could have power (and thus light) for months with that generator and so take a lot more time figuring out what to do next.
lionhead
26th May 2011
Correction: The generator was never dying due to the lack of petrol, the reason was because the electric charge was fading. This is confirmed in the scene where Paul takes a look inside it to identify the problem. After Rosemary points out that there actually is plenty of petrol (showing her with a bunch of cans with petrol on a shelf), Paul says that it doesn't matter, as the electric charge is fading anyway.